A Handbook of Climate Trends Across Scotland

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Claire Barnett and Matthew Perry Met Office, FitzRoy Road, Exeter, Devon, EX1 3PK, United Kingdom; and

A handbook of Jo Hossell, Greg Hughes and Chris Procter Woodthorne, Wergs Road Wolverhampton, WV6 8TQ climate trends across Scotland United Kingdom.

The report should be referenced as: Presenting changes in the climate across Scotland over the last century Barnett, C., J. Hossell, M. Perry, C. Procter and G. Hughes (2006) A handbook of climate trends across Scotland. SNIFFER project CC03, Scotland & Northern Ireland Forum for Environmental Research, 62pp.

SNIFFER’s project manager SNIFFER’s project manager for this contract is: Noranne Ellis, Scottish Natural Heritage

SNIFFER’s project steering group members are: June Graham, Scottish Environment Protection Agency (SEPA) Helen McKay, Forestry Commission Peter Singleton, Scottish Environment Protection Agency (SEPA) Guy Winter, Scottish Executive

SNIFFER First Floor, Greenside House, 25 Greenside Place, EDINBURGH EH1 3AA Company No: SC149513 Scottish Charity: SCO22375 www.sniffer.org.uk

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND © Crown Copyright, Met Office 2006

Dissemination status Unrestricted

Research contractor This document was produced by:

Claire Barnett and Matthew Perry Met Office, FitzRoy Road, Exeter, Devon, EX1 3PK, United Kingdom; and

A handbook of Jo Hossell, Greg Hughes and Chris Procter Woodthorne, Wergs Road Wolverhampton, WV6 8TQ climate trends across Scotland United Kingdom.

SNIFFER’s project manager SNIFFER’s project manager for this contract is: Noranne Ellis, Scottish Natural Heritage

SNIFFER First Floor, Greenside House, 25 Greenside Place, EDINBURGH EH1 3AA Company No: SC149513 Scottish Charity: SCO22375 www.sniffer.org.uk

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 1

CONTENTS

Introduction 2

Chapter 1 Temperature related variables 6 1.1 Average temperature 7 1.2 24-hour maximum temperature 10 1.3 24-hour minimum temperature 12 1.4 Daily temperature range 14 1.5 Heating degree days 15 1.6 Growing degree days 17 1.7 Length of the growing season 18 1.8 Growing season start and end dates 20 1.9 Extreme temperature range 22 1.10 Length of heat-waves in summer and cold spells in winter 24 1.11 Air frost 26 1.12 Ground frost 28 1.13 Early and late season frosts 30

Chapter 2 Precipitation related variables 32 2.1 Average precipitation totals 33 2.2 Snow cover 36 2.3 Days of heavy rain each year 38 2.4 Number of consecutive dry days 40 2.5 Average rainfall intensity 42 2.6 Maximum five-day precipitation total 44

Chapter 3 Air-pressure related variables 46 3.1 Average air-pressure at sea level 47 3.2 Average wind speed each year 49 3.3 Days of gale each year 50

Chapter 4 Sunshine related variables 51 4.1 Sunshine hours 51 4.2 Cloud cover 54

Appendix 1 Where the information comes from and the techniques we have used to analyse it 55 Analysing the information 57

Appendix 2 References 58 2 Introduction

This handbook presents the changes in climate across Scotland in the last century. It provides a benchmark against which we can measure future climate change. This should help with the development of strategies to adapt to the impacts of climate change. The handbook presents the highlights in a simplified way. For a more detailed technical analysis please see the accompanying SNIFFER publication “Patterns of Climate Change across Scotland: Technical Report” at www.sniffer.org.uk.

Table 1 presents a summary of the results The Met Office datasets we have used in this showing what we have analysed, and the changes analysis cover temperature and precipitation for Scotland, by region and by timescale (in other from 1914 – 2004, sunshine from 1929 to 2004, words, annual or seasonal). Figure 1 shows the and a range of other variables from 1961 up three regions for which we describe the results. to 2004, including pressure, derived temperature The other sections provide the results of indices, snow cover, wind speed, and the analyses for each of the variable groups (related intensity of rainfall. Appendix 1 provides a brief to temperature, precipitation, atmospheric summary of the techniques we have used. pressure and sunshine and clouds). Figure 1 - Map of Scotland showing boundaries For each group we present the results as: of the three regions as defined in this study • a graph (time series) showing changes each (North, West and East Scotland). year since 1914 or since 1961; • a table summarising average change over the period by each region (see the note below table 2 for a worked example); and • a map of the patterns of change over the period for the whole of Scotland.

In presenting the results, we have highlighted changes that are statistically significant at the 5% level. These show that we are 95% confident that we can measure a statistically significant trend in the changes.

At the end of each analysis we compare these recent trends with the future climate, as estimated by the UK Climate Impacts Programme Scenarios published in 2002 (UKCIP02). The UKCIP02 scenarios provide a snapshot of future climate for three 30-year periods, the 2020s, 2050s and 2080s. The comparison considers whether or not the trend we have seen already agrees with the future direction of change for that variable and whether the pattern across Scotland is similar or not. When we describe areas of Scotland in this Handbook, a capital letter is used when any of the three regions are being discussed, that is to say ‘North’ Scotland rather than ‘north’ Scotland.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 3

Table 1 - Summary of the main findings by variable, region and season and a comparison of the trend for 1961 to 2004 with expectations for the climate in the future as presented by the UKCIP02 scenarios (Hulme et al., 2002).

upward trend, downward trend, blank means no trend detectable Variable Change Region Expected future trend by season North East West (from UKCIP02 scenarios) Annual Average Spring Possible increase in all seasons, greater in Temperature Summer south than north. This matches the trends already seen. Autumn Winter Annual 24-hour Spring Maximum temperatures may increase in all maximum Summer seasons. The range of possible increases is temperature smallest in winter and greatest in autumn. Autumn This matches trends we have already seen. Winter Annual 24-hour Spring The minimum temperatures may increase minimum Summer more in winter than summer. This generally temperature matches the trends already seen. Autumn Winter Annual Daily Spring This range may increase most in summer. temperature Summer We cannot find a trend in the information range we have for summer, but some regions Autumn are showing an increasing trend in other Winter seasons. Annual Heating Annual This may reduce in the future, which is in degree days line with the trend already seen, but the possible reduction by the 2080s is 50% to 300% greater than that experienced so far. Growing Annual No estimates have been made in the degree days UKCIP02 scenarios for this measure. Length of the Annual A possible increase in the length of the growing season growing season of 20 to 60 days by the 2080s. We have seen a similar trend but the spatial pattern is different. Growing season Start Estimates for the start of the growing start and end season are similar to those we have already dates End seen, but suggest a later end to the growing season than has been seen already. Extreme Annual No estimates have been made in the temperature UKCIP02 scenarios for this measure. range Length of Summer No estimates have been made in the summer heat UKCIP02 scenarios for this measure. waves and winter Winter cold spells 4 Introduction

Table 1 Continued

upward trend, downward trend, blank means no trend detectable Variable Change Region Expected future trend by season North East West (from UKCIP02 scenarios) Air frost Spring No estimates have been made in the Summer UKCIP02 scenarios for this measure, but the reductions in minimum temperatures Autumn expected should mean some reduction in Winter the number of air frosts. This matches the Annual trend already seen. Ground frost Spring No estimates have been made in the Summer UKCIP02 scenarios for this measure, but the reductions in minimum temperatures Autumn expected should mean some reduction in Winter the number of ground frosts. This matches Annual the trend already seen. Early and late Early No estimates have been are made in the season frosts Late UKCIP02 scenarios for this measure, but (based on the reductions in minimum temperatures individual Frost-free expected should mean some reduction in weather stations.) period the number of early and late season frosts. This matches the trend already seen. Average Spring Winter months may become wetter while precipitation Summer summer months may be drier than at total present. The spatial pattern of change Autumn expected is the opposite of the trend that Winter has already been seen. Annual Snow cover Spring The UKCIP02 scenarios present a different measure but winter snowfall may reduce by Autumn 50% or more across Scotland by the 2080s Winter Medium High scenario. The spatial pattern of possible change is again different from Annual the trend already seen. Days of heavy Spring No estimates have been made in the rain each year Summer UKCIP02 scenarios for this measure. Autumn Winter Annual Number of Annual No estimates have been made in the consecutive UKCIP02 scenarios for this measure. dry days Average rainfall Annual The intensity of rainfall may increase in intensity winter months. A contrasting change between the east and west, with most extreme changes taking place in eastern Scotland, is expected. Maximum Annual No estimates have been made in the five-day rainfall UKCIP02 scenarios for this measure.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 5

Table 1 Continued

upward trend, downward trend, blank means no trend detectable Variable Change Region Expected future trend by season North East West (from UKCIP02 scenarios) Average pressure Spring Changes in spring and autumn may at sea level Summer be small, with little change in winter. We cannot see a trend in the information Autumn we have from observations. Winter Annual Average wind Annual A possible change in the average wind- speed each year speed in the future is highly uncertain. (based on one In the High emissions scenario a change weather station of less than 7% for the 2080s is expected. in each region.) We cannot see a trend in the information available from observations. Sunshine hours Spring No measure is included in the UKCIP02 Summer scenarios but we could use cloud cover instead to estimate the number of Autumn sunshine hours. We cannot see a trend Winter in the information available from Annual observations. Cloud cover Spring By the 2080s cloud amounts are expected Summer to increase slightly in winter, particularly in the northern half of Scotland, and to Autumn reduce in all other seasons. Also, the Winter greatest changes are expected in summer Annual months in southern and eastern areas. We cannot see a trend in the information available from observations.

Throughout this handbook, unless stated otherwise, where data are separated into seasons: spring means March, April May; summer means June, July, August; autumn means September, October, November; and winter means December, January, February. 6 Chapter 1 Temperature related variables

CHAPTER 1 TEMPERATURE RELATED VARIABLES

This section presents the analysis for: • Growing degree days (GDD). This is the sum of daily average temperatures above 5ºC. 1.1 AVERAGE TEMPERATURE This represents the temperature above which 1.2 24-HOUR MAXIMUM TEMPERATURE grass grows. Typical values in the early 1960s 1.3 24-HOUR MINIMUM TEMPERATURE were 950 GDD a year in North Scotland, 1000 1.4 DAILY TEMPERATURE RANGE GDD a year in East Scotland and 1150 GDD a 1.5 HEATING DEGREE DAYS year in West Scotland. 1.6 GROWING DEGREE DAYS 1.7 LENGTH OF THE GROWING SEASON • Growing season length (GSL). This is the 1.8 GROWING SEASON START AND END DATES number of days between the start and end of 1.9 EXTREME TEMPERATURE RANGE the growing season (GSE-GSS). In the early 1.10 LENGTHS OF HEAT WAVES IN SUMMER 1960s typical values for the length of the AND COLD SPELLS IN WINTER growing season were 213 days in East 1.11 AIR FROST Scotland, 217 days in North Scotland and 1.12 GROUND FROST 237 days in West Scotland. 1.13 EARLY AND LATE SEASON FROSTS • Growing season start date (GSS). This is the start date for the growing season (calculated Records of temperature are probably the most from 1 January). We assume that the growing frequently analysed of all information linked to season starts on the fifth day in a row which meteorology. A Met Office gridded dataset covers has an average daily temperature of 5ºC or the period 1914 to 2004 and includes average, greater. During the 1960s the typical start date maximum and minimum daily temperatures. for the growing season was 12 April in East Figure 2 shows where the temperature recording Scotland, 10 April in North Scotland and 29 stations are in Scotland. March in West Scotland.

Calculated temperature variables • Growing season end date (GSE). This is the We can work out a number of other measures end date for the growing season (calculate from the information we have on temperature. from 1 January). We assume that the growing These measures are particularly significant for season ends on the fifth day in a row with an several sectors and can be better related to average temperature of 5ºC or less. During direct impacts of climate. the 1960s the typical end date for the growing season was 10 November in East Scotland, • Heating degree days (HDD). This is an indicator 12 November in North Scotland and 20 of how much heat energy households use November in West Scotland. and represents the energy needed to keep a building at a constant temperature. The base • Extreme temperature range (ETR) is the range temperature for working out a heating degree between the highest maximum and lowest day is 15.5ºC. This means that if the average minimum temperature within a year. temperature is below 15.5ºC, the value of the HDD for that day would be 15.5ºC minus the • We work out cold spells for the winter half-year average temperature. For example, if a day (October to March). In this study we define it as has an average temperature of 13.5ºC, this the total length of any period, lasting six days is equal to 2 heating degree-days (15.5-13.5). or more, where the minimum temperature each Typical figures at the start of the 1961 to 2004 day is at least 3ºC lower than the 1961 to 1990 period were 3200 HDD a year for North and average temperature for those days. East Scotland, and 2900 HDD a year for West Scotland.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 7

CHAPTER 1 TEMPERATURE RELATED VARIABLES Continued

• Heat-waves are worked out for the summer Figure 2 – Met Office sites for observing half-year (April to September). We define precipitation and temperature (January 2001). a heat-wave as the total length of any period, lasting six days or more, where the maximum temperature each day is more than the 1961 to 1990 average temperatures for those days by at least 3ºC.

• Air frost happens on a day with a minimum air temperature of less than 0ºC. We only analyse the seasons where air frost is likely to happen (in other words we do not analyse summer temperature information).

• Ground frost happens when the minimum temperature at grass level reaches 0ºC or below. This is a common event in Scotland, even in the summer.

• We have assessed the dates of first and last ground frosts from the records from four individual sites. The date of the first frost considers temperatures from 1 August and the last frost examines information before the end of the following July.

1.1 AVERAGE TEMPERATURE

Figure 3 - The average temperature (in ºC) each year for Scottish regions, from 1914 to 2004, with smoothed curve to show a running average. The vertical dashed line marks 1961. 8 Chapter 1 Temperature related variables

1.1 AVERAGE TEMPERATURE Continued

Table 2 – The changes in average temperature (in ºC) between 1914 and 2004 (left) and between 1961 and 2004 (right). Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

1914 to 2004 1961 to 2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring 0.59 0.83 0.66 0.69 1.03 1.23 1.20 1.14 Summer 0.50 0.59 0.43 0.51 1.06 1.12 1.08 1.08 Autumn 0.46 0.85 0.68 0.64 0.64 0.68 0.66 0.66 Winter 0.02 0.45 0.33 0.24 1.03 1.39 1.31 1.22 Annual 0.37 0.66 0.51 0.50 0.92 1.08 1.04 1.00*

For example, the figure at * in the table above means that using statistical analysis (see Appendix 1) the average annual temperature increased by 1ºC between 1961 and 2004. This figure is in bold because analysis indicates that we can be 95% confident that measurements showed a genuine change over this time period. Similar analysis is used for each variable in tables throughout the handbook.

Regional trends Spatial trends • There is a great deal of difference in the • The increases are smallest during autumn and average annual temperatures between years there has been some slight cooling in Highland for all regions. areas during autumn. • The average annual temperatures in each • The greatest increases have taken place during region are now higher than at any other time spring and winter and the largest in southern since 1914. and eastern Scotland in winter. • The analysis from 1914 to 2004 shows a trend • The northern Outer Hebrides, Shetland and of increases in annual temperature, particularly Orkney are warming at a similar level in all in East and West Scotland. This matches seasons. information about temperature increases in the UK and around the world. Future Trends • Temperature increases have been greater • Temperatures are expected to rise over since 1961 than between 1914 and 1961. Scotland, no matter which scenario of future • The increases in temperature we have seen emissions is used, with increases being since 1961 also are part of a trend in each greatest during summer and autumn months region and every season, apart from winter (up to 4ºC for the UKCIP02 medium-high in North Scotland. emissions scenario by the 2080s). • In line with the trends we have seen, southern Scotland is expected to warm at a faster rate than the north. • The warming expected in autumn months is not reflected strongly in the mapped trends for the 1961 to 1990 period. However, the longer 1914 to 2004 period does show that some of the largest temperature trends have been during autumn.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 9

1.1 AVERAGE TEMPERATURE Continued

Figure 4 - Patterns of change in average temperature (in ºC) between 1961 and 2004 for each season. 10 Chapter 1 Temperature related variables

1.2 24-HOUR MAXIMUM TEMPERATURE

Figure 5 - The average 24-hour maximum temperature (in °C) each year for Scottish regions, from 1914 to 2004, with smoothed curve to show a running average. The vertical dashed line marks 1961.

Table 3 – The changes in average 24-hour maximum temperature (in °C) between 1914 and 2004, and between 1961 and 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

1914 to 2004 1961 to 2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring 0.70 0.71 0.48 0.64 1.16 1.41 1.35 1.29 Summer 0.58 0.37 0.20 0.40 1.11 1.14 1.12 1.12 Autumn 0.58 0.66 0.46 0.57 0.85 0.83 0.83 0.84 Winter 0.41 0.58 0.42 0.47 1.16 1.51 1.47 1.36 Annual 0.59 0.60 0.41 0.54 1.14 1.29 1.25 1.21

Regional trends • The maximum temperatures in North Scotland • In the 1961 to 2004 period every change is part have increased at a faster rate than average of a trend of increasing maximum temperatures. temperatures since 1914. • But for the 90-year record, while we can see a • Since 1961 increases in the maximum trend in the average annual figures, it happens temperature have consistently been greater less frequently in the seasonal averages (only than the increases in average temperature spring in North and East Scotland and autumn in all regions. in East Scotland).

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 11

1.2 24-HOUR MAXIMUM TEMPERATURE Continued

Spatial trends Future trends • The greatest rises in temperature have taken • Maximum temperatures are expected to place in the winter rather than summer increase in all seasons. season, particularly in southern Scotland. • A maximum temperature that currently • Winter patterns of change are similar to that happens once in 10 years around Inverness of the average temperature may be 6ºC warmer by the 2080s. • The rise in daily maximum temperatures in • The analysis in the UKCIP02 report suggests summer has been relatively steady across the that the range of possible increases is smallest country except for the Shetland Islands, which in winter and greatest in autumn. have become cooler.

Figure 6 - Pattern of change for average 24-hour maximum temperature (in ºC) for summer and winter, from 1961 to 2004. 12 Chapter 1 Temperature related variables

1.3 24-HOUR MINIMUM TEMPERATURE

Figure 7 - Average 24-hour minimum temperature (in ºC) each year for Scottish regions, from 1914 to 2004, with smoothed curve to show a running average. The vertical dashed line marks 1961.

Table 4 – The changes in average 24-hour minimum temperature (in ºC) between 1914 and 2004, and between 1961 and 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

1914 to 2004 1961 to 2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring 0.46 1.00 0.94 0.76 0.89 1.08 1.07 1.00 Summer 0.30 0.80 0.65 0.55 1.07 1.18 1.11 1.12 Autumn 0.39 1.12 0.99 0.79 0.55 0.64 0.56 0.58 Winter -0.37 0.35 0.22 0.02 0.96 1.32 1.22 1.15 Annual 0.22 0.84 0.73 0.56 0.94 1.13 1.06 1.03

Regional trends • In contrast to this, the minimum temperatures • Minimum temperatures in North Scotland are in winter have reduced in North Scotland since increasing at a slower rate than in the other 1914, but do not show a clear downward trend. two regions. • Maximum temperatures in the day have • East Scotland has moved from a climate with increased at a faster rate than night-time lower night-time minimum temperatures than minimum temperatures. This goes against North Scotland, to one with very similar annual a worldwide average trend, identified in the averages. Intergovernmental Panel for Climate Change • Since 1914 there has been an upward trend in Third Assessment Report (IPCC, 2001), night-time minimum temperatures in the East which suggested that, on average, night- and West of Scotland in all seasons except for time minimum temperatures increased at winter. These trends are for warming at a rate about twice the rate of daytime maximum faster than the average temperature. temperatures. However, some areas, including • All increases since 1961 may be ascribed to a parts of Scotland, were identified as having a measurable trend, although some increases are trend going in the opposite direction. at a slower rate than for average temperatures.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 13

1.3 24-HOUR MINIMUM TEMPERATURE Continued

Spatial trends Future trends • Parts of northern Scotland have seen relatively • Minimum temperatures are expected to little increase in minimum temperatures over increase but the UKCIP02 scenarios do not the period 1961 to 2004. give values for these changes. • Minimum temperatures have increased during • A minimum temperature of –5ºC that currently summer months at a faster rate in northern, happen on 15% of days in winter would only eastern and southern Scotland than in central take place on 4% of days in winter by the 2080s. areas or the Highlands. • Minimum temperatures are expected to increase more in winter than in summer.

Figure 8 - Pattern of change for 24-hour average minimum temperature (in ºC) for summer and winter from 1961 to 2004. 14 Chapter 1 Temperature related variables

1.4 DAILY TEMPERATURE RANGE

Figure 9 - The average daily temperature range (in ºC) each year for Scottish regions, from 1961 to 2004, with smoothed curve to show a running average.

Table 5 – The changes in average daily temperature (in ºC) between 1961 and 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

1961 to 2004 North Scotland East Scotland West Scotland Scotland Spring 0.30 0.24 0.27 0.27 Summer 0.09 -0.06 0.01 0.02 Autumn 0.46 0.24 0.32 0.35 Winter 0.45 0.30 0.37 0.38 Annual 0.33 0.19 0.23 0.26

Regional trends Future trends • With maximum daytime temperatures rising • In the UKCIP02 scenarios they expect the daily faster than night time minimums, daily temperature range to increase the most in temperature range is also increasing. summer with a complicated pattern of smaller • Apart from summer in East Scotland, the daily increases and reductions in other seasons. temperature range has increased in all regions • We do not see this pattern of change when and seasons. analysing temperatures. Over the last forty • A trend of an increase in daily temperature years there has been little change to the daily range has taken place in winter across the temperature range in summer. And, it has country. We can also see an upward trend in actually increased in all other seasons, North Scotland during autumn. particularly winter. • Increases in summer are modest for all regions and are not part of a clear trend.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 15

1.5 HEATING DEGREE DAYS

Figure 10 - Heating degree days for Scottish regions each year from 1961 to 2003, with smoothed curve to show a running average.

Table 6 – The changes in heating degree days (HDD) between 1961 to 2003. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Heating degree days -9.2% -10.7% -11.3% -10.2%

Regional trends • Typical figures at the start of the 1961 to 2004 period were about 3200 HDD a year for North and East Scotland, and 2900 HDD a year for West Scotland. • In all areas of Scotland, the number of heating degree days has reduced and this appears to be part of a clear downward trend. • These trends are consistent with the temperature trends we have already described. 16 Chapter 1 Temperature related variables

1.5 HEATING DEGREE DAYS Continued

Spatial trends Figure 11 - Pattern of change in heating degree • The pattern within the HDD map is similar days (as a percentage), from 1961 to 2003. to that for average temperatures with the southerly, coastal and lower lying areas showing the greatest change. • There are areas in the Shetlands and the Outer Hebrides showing large reductions.

Future Trends • In the UKCIP02 scenarios, heating degree days are expected to reduce by 15% to 40 % by the 2080s. Changes are expected to be greatest in southern and eastern Scotland. • The pattern of change we see from analysing different variables is consistent with this. However, the changes so far (10.2% for Scotland as a whole) are not as great as those expected for the future.

1.6 GROWING DEGREE DAYS

Figure 12 - Growing degree days for Scottish regions each year, from 1961 to 2003, with smoothed curve showing a running average.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 17

1.6 GROWING DEGREE DAYS Continued

Table 7 - Changes in growing degree days (as a percentage) from 1961 to 2003. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Growing degree days 23.7% 22.5% 21.1% 22.5%

Regional trends Figure 13 - Pattern of change in growing degree • Typical values in the early 1960s were about 950 days (as a percentage) from 1961 to 2003. GDD a year in North Scotland, 1000 GDD a year in East Scotland and 1150 GDD a year in West Scotland. • There is a clear trend of an increase in the length of the growing season • Increases in the North and East regions are similar, and slightly greater than that of the West • This increase is likely to be mainly due to the upward trend in both minimum and average temperatures in the spring. However, increases in autumn temperatures will also add to this.

Spatial trends • We can see areas of large increase in the Shetlands and the Outer Hebrides. • The central mountainous areas, mainly the Grampians and Trossachs, show the least change.

Future trends • There is no estimate in the UKCIP02 report for this measure. 18 Chapter 1 Temperature related variables

1.7 LENGTH OF THE GROWING SEASON

Figure 14 - Length of the growing season (in days) each year for Scottish regions, from 1961 to 2004, with smoothed curve showing a running average.

Table 8 - Changes in growing season length (in days) from 1961 to 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Growing season length (days) 31.1 32.5 36.7 33.2

Regional trends • In the early 1960s, typical values were a growing season of about 213 days in East Scotland, 217 days in North Scotland and 237 days in the West. • All regions have seen an increase of more than four weeks in the length of the growing season since 1961. • The increase in the length of the growing season is part of a clear trend in all regions. • The West has shown the greatest increase and the North the least.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 19

1.7 LENGTH OF THE GROWING SEASON Continued

Figure 15 - Pattern of change in the length of Spatial trends the growing season (in days) from 1961 to 2004 • The greatest increases in the length of the calculated from the extended UKCIP02 dataset. growing season are in coastal areas and the Shetland Islands where the season has extended by two months, or more. • The length of the growing season has changed very little since 1961 in some of the more mountainous areas. • A few upland areas show a reduction in the length of the growing season of up to eight days.

Future trends • The UKCIP02 scenarios show an increase in the length of the growing season of between 20 and 60 days by the 2080s. • This is similar to the level of change (33 days for Scotland as a whole) that we have already seen since 1961. • The UKCIP02 scenarios suggest the increase will be greater in the east than in the west, but the information from observations shows the opposite pattern. 20 Chapter 1 Temperature related variables

1.8 GROWING SEASON START AND END DATES

Figure 16 - Growing season a) start dates (days from 1 January) and b) end date (days from 1 January) for Scottish region from 1961 to 2004, with smoothed curve showing a running average.

Table 9 - Changes in growing season length (in days) 1961 to 2004 (A negative number of days shows a date earlier in the year, in other words, the growing season starting earlier in the year). Values in bold shows that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Growing season start (in days) -19.6 -20.6 -22.4 -20.7 Growing season end (in days) 11.5 12.0 14.4 12.5

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 21

1.8 GROWING SEASON START AND END DATES Continued

Regional trends • We have already seen the longer growing • The growing season is starting almost three season, and in particular the earlier start, weeks earlier in North and East Scotland, and through a change in flowering dates of plants, more than three weeks earlier in the West. particularly those flowering in early spring • While the growing season is also ending later, (Roberts et al., 2004). it is by less than two weeks in the North and East and just over two weeks in the West. Future trends • The changes to the start and end of the • The UKCIP02 scenarios suggest that by the growing season are part of a clear trend in 2050s the growing season may start one to all regions. three weeks earlier and end by one to three weeks later. Spatial trends • The estimates for the start of the growing • The pattern of change for the start and end of season are similar to those we have seen, the growing season is similar to that for the but suggest a later end to the growing season length of the growing season. than we have seen already. • Coastal areas show a greater tendency towards an earlier start and later end to the growing season than central Scotland.

Figure 17 - Pattern of change in the start (left panel) and end (right panel) of the growing season (in days), from 1961 to 2004. Negative values show the season starting or ending earlier. 22 Chapter 1 Temperature related variables

1.9 EXTREME TEMPERATURE RANGE

Figure 18 - Extreme temperature range (in ºC) each year for Scottish regions, from 1961 to 2003, with smoothed curve showing a running average.

Table 10 - Changes in extreme temperature range (in ºC) 1961 to 2004.

North Scotland East Scotland West Scotland Scotland Extreme temperature range 0.0 -3.4 -2.2 -1.8

Regional trends • These changes do not appear to be part of any clear trend. They possibly reflect an increase in lowest minimum temperatures at a greater rate than highest maximum temperatures. As a result, this reduces the extreme temperature range. • As the results are not part of any trend, they may simply be a result of natural variability.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 23

1.9 EXTREME TEMPERATURE RANGE Continued

Figure 19 - Pattern of change in extreme Spatial trends temperature range (in °C) from 1961 to 2003. • Extreme temperature range (ETR) displays a complicated pattern of change, with strong increases in the islands of Islay, Jura, Skye and the Orkneys as well as the more mountainous areas surrounding Fort William and Inverness and the Great Glen between these two areas. • The Outer Hebrides and Shetlands seem to display the opposite trend along with much of the rest of the country. • Regional changes in extreme temperature range are small and we have not seen a trend. The changes shown here are also modest.

Future trends • High summer temperatures may become more frequent and very cold winters may become increasingly rare. However, the UKCIP02 scenarios report does not include any estimates of changes in this measure. 24 Chapter 1 Temperature related variables

1.10 LENGTH OF HEAT-WAVES IN SUMMER AND COLD SPELLS IN WINTER

Figure 20 - a) Length of summer heat-waves (in days) and b) length of winter cold spells (in days) for Scottish regions, from 1961 to 2003, with smoothed curve showing a running average.

Table 11 - Change in half-year heat-wave and cold spells (in days), from 1961 to 2003. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Heat-wave (summer) 6.3 6.3 4.3 5.7 Cold spell (winter) -5.8 -8.4 -8.9 -7.5

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 25

1.10 LENGTH OF HEAT-WAVES IN SUMMER AND COLD SPELLS IN WINTER Continued

Please note, in calculating heat-waves summer Spatial trends is taken as a half-year season from April to • Cold spells in winter have reduced most September, cold spells are calculated for the in southern Scotland and the Shetlands, October to March winter half-year. a pattern that correlates well with the pattern of winter temperature changes. Regional trends • Summer heat-waves have got longer in • We can see an increase in the length of areas furthest away from the coast and summer heat-waves between 1961 and 2003 in in north-eastern areas. all regions throughout the year but the change is not part of a measurable trend and so it may Future trends be a result of natural variability • There is no assessment in the UKCIP02 • The only trend we have identified is the scenarios report of future changes to the reduction in the length of cold spells during length of heat-waves or cold spells. the winter half-year in East and West Scotland.

Figure 21 - Pattern of change in the length of a) winter half-year cold spells (in days, left-hand panel) and b) summer half-year heat-waves (in days, right-hand panel), from 1961 to 2003. 26 Chapter 1 Temperature related variables

1.11 AIR FROST

Figure 22 - Days of air frost for Scottish regions each year, from 1961/62 to 2004/05, with smoothed curves showing a running average.

Table 12 - Change in days of air frost (percentage), from 1961/62 to 2004/05. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Spring -30.5 -29.0 -29.2 -29.7 Autumn -33.5 -31.3 -33.7 -32.8 Winter -20.1 -21.3 -24.7 -21.7 Annual -25.7 -25.1 -27.7 -26.0

Please note, days of air frost are calculated • Although changes in winter are the largest in according to seasons rather than a full calendar terms of total number of days (a reduction of year; they are calculated from September to the 10 days) it is the spring and autumn seasons end of the following May. that have seen the largest percentage changes and which show a clear downward trend. Regional trends This is consistent with the warming trend we • Since 1961 there has been more than a 25% discussed earlier. reduction in the number of days each year of • The time-series of annual days of air frost air frost. This is part of a downward trend that shows that the years with the highest number is clear in all three regions and nationally. of days of air frost coincide with years of low average temperature.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 27

1.11 AIR FROST Continued

Figure 23 - Pattern of change in annual days of Spatial trends air frost (in days), from 1961 to 2004. • The largest changes are usually in areas close to the coast or the Scottish islands. The closeness of an area to the sea has a moderating effect of temperatures in these regions, so days of frost will be less common than regions further inland. • Some areas show an increase in the number of days of frost, particularly in northern mainland Scotland and Orkney and Shetland.

Future trends • The UKCIP02 report does not mention air frost, but the reductions in expected minimum temperatures should mean some reduction in the number of air frosts also takes place. 28 Chapter 1 Temperature related variables

1.12 GROUND FROST

Figure 24 - Days of ground frost for Scottish regions each year from 1961/61 to 2004/05, with smoothed curves showing a running average.

Table 13 - Changes in days of ground frost (as a percentage) between 1961/62 and 2004/05. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Spring -11.4 -8.7 -7.5 -9.4 Summer -3.0 -1.8 -1.4 -2.2 Autumn -7.8 -4.1 -4.3 -5.6 Winter -8.2 -8.4 -9.8 -8.7 Annual -31.8 -25.2 -25.2 -27.8

Regional trends Spatial trends • Ground frost, which happens when the minimum • There has been a reduction in the number grass temperature falls to 0°C or below, is a of days of ground frost for most areas in all common event in Scotland, even in the summer. seasons. We can see this especially over the • Since 1961 there has been a reduction in the western Highlands and Hebrides in spring. number of days of ground frost in every season, • The number of days of ground frost has and for each of the three Scottish regions. actually increased in winter on both the • We can see a downward trend in spring, Shetland and Orkney Islands. summer and winter in all regions, as well as for autumn in North Scotland. Future trends • There is also a measurable downward trend in • The UKCIP02 scenarios report does not the number of ground frost days over the year. provide an estimate for future ground frost. • The steady reduction in the number of days of However, the expected increase in minimum ground frost each year appears to have begun temperature means that the number of frosts in the 1980s. should reduce.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 29

1.12 GROUND FROST Continued

Figure 25 - Patterns of change in ground frost (in days) between 1961 and 2005 for each season. 30 Chapter 1 Temperature related variables

1.13 EARLY AND LATE SEASON FROSTS

Figure 26 - Date of a) the first ground frost in days after the 1 August, 1960 to 2005 and b) the last ground frost before the end of July, in days from the 1 August, 1961 to 2005, at four Scottish stations, with smoothed curves showing a running average.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 31

1.13 EARLY AND LATE SEASON FROSTS Continued

Table 14 - Changes in the dates of the first and last ground frost (in days) starting from 1 August, and changes (in days) in the length of the frost-free season, for four Scottish stations for 1961 to 2005. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

Threave Blythe Bridge Kinloss Wick First ground frost 3.4 10.2 7.3 11.8 Last ground frost -22.5 -15.0 -12.1 -24.0 Frost-free season 29.1 24.5 20.0 38.8

Regional trends Future trends • The length of the frost-free season has • The UKCIP02 scenarios report does not give any extended, by between 20 and 40 days. estimates for changes in frost dates. But the • At each of the stations, the first frost is expected increases in minimum temperatures happening later but we can see no clear trend. should also mean a reduction in the number However, there is a trend towards an earlier of frosts. last frost by between 12 and 24 days since 1961. • The expected change in length of the growing • There are still big differences each year in the season also suggests that there may be a longer dates of the first and last frost and summer frost-free period. frosts are still common at each of the four sites. 32 Chapter 2 Precipitation related variables

CHAPTER 2 PRECIPITATION RELATED VARIABLES

Precipitation includes rain, snow, hail and sleet. • Consecutive dry days (CDD). The maximum This section presents the analysis for: number of dry days in a row, where a dry day is a day with no more than 0.2 millimetres of 2.1 AVERAGE PRECIPITATION TOTALS rainfall. This does not really give us an idea of 2.2 SNOW COVER drought. Even in drought conditions there may 2.3 DAYS OF HEAVY RAIN EACH YEAR be an occasional day of rain. 2.4 NUMBER OF CONSECUTIVE DRY DAYS 2.5 AVERAGE RAINFALL INTENSITY • Rainfall intensity. This is the average amount 2.6 MAXIMUM FIVE-DAY PRECIPITATION TOTAL of rainfall that falls in a day. This only includes days when rainfall is greater than or equal to one millimetre and represents the average Calculated Precipitation Variables rainfall on that day. As with temperature, there are a number of other measures that we can calculate from • Maximum five-day precipitation amount. This is the information we have on precipitation. a measure of the heaviest rainfall in a five-day These usually describe something about the period for a year. nature of precipitation within a year and relate to frequency (in other words how often it happens), the intensity and the volume of Figure 27 shows the rainfall climatology and rainfall happening over a period figure 2 (in chapter 1) shows the network of rainfall stations from which we gather this • Snow cover. Meteorological stations record the information. The rainfall climatology is the state of the ground at 0900 hrs, that is to say pattern of average rainfall for Scotland over 9 o’clock Greenwich Mean Time, and from this the 30-year period 1961 to 1990 (the standard we can measure the number of days with snow length of time over which we assess patterns lying on the ground. It is recorded that snow in climate). As the map shows, North and is lying on the ground if more than 50% of the West Scotland have a wetter climate than ground is covered with snow. This index is not eastern areas. This is to be expected given the same as the length of the snow season but that the Highlands are in this area and they it gives us a good idea of it. provide a rain-shadow effect for eastern areas, because of the prevailing westerly winds which • Days of intense or heavy rainfall, in other pass over Scotland. The map allows you to words we count of the number of days with compare the patterns of change presented on rainfall of more than 10 millimetres. the following pages.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 33

CHAPTER 2 PRECIPITATION RELATED VARIABLES Continued

Figure 27- Climatology of average amount of rainfall in a year (millimetres) for Scotland between 1961 and 1990. Source: www.metoffice.gov.uk/climate/uk/averages/19611990/mapped.html.

2.1 AVERAGE PRECIPITATION TOTALS

Figure 28 - Precipitation total (in millimetres) each year for Scottish regions, from 1914 to 2004, with smoothed curve to show a running average. 34 Chapter 2 Precipitation related variables

2.1 AVERAGE PRECIPITATION TOTALS Continued

Table 15 - Changes in average precipitation totals (as a percentage), from 1961 to 2004 and 1914 to 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

1914 to 2004 1961 to 2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring 13.9 6.1 22.0 14.3 16.2 9.4 17.3 14.8 Summer -12.7 -18.9 -7.5 -12.7 -7.0 0.2 7.3 -0.6 Autumn 13.6 0.7 15.6 11.1 5.3 22.2 5.9 9.1 Winter 20.9 -0.8 9.0 11.6 68.9 36.5 61.3 58.3 Annual 9.6 -3.5 9.5 6.2 21.0 18.4 23.3 21.1

Regional trends • The pattern of change is completely reversed • In each region, and across the country, the in autumn, with eastern areas being the only change in winter precipitation since 1961 widespread region to become wetter, with shows a clear upward trend. We can see an increases of more than 20%. increase of almost 70% in winter precipitation • In summer, northern areas of Scotland have in North Scotland. become drier since 1961, particularly the • The average precipitation each year also shows north-west. This reduction in summer a trend towards much higher totals over the precipitation is more than 20% in some areas. same period. Scotland has become 20% wetter between 1961 and 2004. Future trends • But there has been little or no change in • The UKCIP02 scenarios show relatively average summer precipitation totals in each little change to average precipitation amounts region. Changes in summer precipitation show each year (the trend we have measured shows no clear trend over the 1961 to 2004 period. increasing totals) but winter months may • Looking at the 1914 to 2004 period, the pattern become wetter (as already seen) while summer of change is less clear and we can see only two months may be drier than at present (we have trends - a reduction in summer precipitation seen little change so far). in East Scotland and an increase in spring • The pattern of change may not be the same precipitation in West Scotland. across Scotland. UKCIP02 estimate eastern • The average precipitation each year has Scotland may experience the most extreme increased across most of Scotland since 1914, percentage changes in precipitation (going but there has been a slight reduction in both against the trend we have seen already), average annual and winter precipitation in with an increase in winter and a reduction East Scotland. This is the opposite of the in summer. trend measured over the period 1961 to 2004. • As with the trends of temperature change, there are similarities between the precipitation Spatial trends trends over the longer 1914 to 2004 period and • The largest changes have taken place in winter the expected changes in the future. Over the months across all but the most eastern areas of longer period, the summer months have Scotland. In some areas of the west Highlands become drier and there has been relatively and the Hebrides, winter precipitation has more little change to the average values each year. than doubled since 1961.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 35

2.1 AVERAGE PRECIPITATION TOTALS Continued

Figure 29 - Patterns of change in precipitation totals (as a percentage) between 1961 and 2004 for each season. 36 Chapter 2 Precipitation related variables

2.2 SNOW COVER

Figure 30 - Days of snow cover each year for Scottish regions, from 1961/62 to 2004/05, with smoothed curve showing a running average.

Table 16 - Changes in days of snow cover (as a percentage), from 1961/62 to 2004/05. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Spring -28.0 -27.5 -44.6 -31.0 Autumn -70.9 -66.8 -82.6 -71.7 Winter -25.9 -31.8 -36.9 -30.2 Annual -28.8 -31.6 -40.7 -32.1

Please note, days of snow cover are calculated • In autumn we can clearly see a downward according to seasons rather than a full calendar trend with large reductions in each region - year; they are calculated from September to the greater than 70% in North and West Scotland. following May. • The average number of days of snow cover in autumn is low, so even a large percentage Regional trends change results in a relatively small reduction • The number of days of snow cover has reduced in the number of days. in each region and in all seasons. • In spring there are also large percentage • In winter, the decreases are greater than 25%, decrease. However, the average number of and are the largest changes (in terms of total days of snow cover in spring is greater than number of days), a decrease of 7 days. in autumn. • The largest percentage changes have taken place in spring and autumn, which shows that the snow season is getting shorter.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 37

2.2 SNOW COVER Continued

Figure 31 - Pattern of change in days of snow Spatial trends cover (in days) each year, from 1961 to 2004. • The west of the country and particularly the western Highlands show the greatest reduction in snow cover in terms of the number of days. However, you should note that these areas are also those that normally have the largest number days of snow cover. • In some areas there has been an increase in the number of days of snow cover, particularly in northern mainland Scotland.

Future trends • The UKCIP02 report assesses snowfall in the future scenarios. We cannot directly compare this with the information we have on snow cover, but the general reduction is what the UKCIP02 scenarios suggest for the future. • Winter snowfall may reduce by 50% or more across Scotland by the 2080s (UKCIP02, medium high scenario). • The most obvious changes are over eastern Scotland, with a possible reduction of over 90%. However, the trends we have seen currently show a greater reduction in snow cover in West Scotland. 38 Chapter 2 Precipitation related variables

2.3 DAYS OF HEAVY RAIN EACH YEAR

Figure 32 - Days of heavy rain (equal to or more than 10 millimetres) for Scottish regions each year, from 1961 to 2004, with smoothed curve showing a running average.

Table 17 - Changes in days of heavy rain (equal to or more than 10 millimetres), in days, from 1961 to 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Spring 1.8 1.0 1.6 1.5 Summer -1.4 -0.5 0.9 -0.4 Autumn -0.2 2.3 0.1 0.7 Winter 8.3 3.5 8.2 6.7 Annual 8.2 6.2 10.6 8.3

Regional trends • Most of the west has seen an increase in • We can see a trend of increasing heavy winter of more than five days of heavy rainfall. rainfall in winter. In particular, North and • Changes in summer months are small and do West Scotland have seen an increase of more not show any trend. This is consistent with the than eight days. findings of Osborn et al. (2000). They found, • In all other seasons the changes are small and for the UK during 1961 to 1995, more heavy we cannot see a trend. rainfall in winter compared to light and • By looking at the graph showing the number of medium rainfall, while the opposite happened days of heavy rainfall and the one showing the in the summer. average total precipitation in each year we can • The area around Achnashellach in Wester Ross see a link, implying that the years with highest has the largest change, with a reduction of up total rainfall are also the years with the most to 10 days in spring, summer and autumn. days of heavy rainfall, and vice versa. Future trends Spatial trends • The UKCIP02 report does not give any • The patterns of change are broadly similar estimates for this measure. to those for total precipitation with a strong east-west gradient in winter months.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 39

2.3 DAYS OF HEAVY RAIN EACH YEAR Continued

Figure 33 - Patterns of change in the number of days with heavy rain (equal to or more than 10 millimetres) between 1961 and 2004 for each season. 40 Chapter 2 Precipitation related variables

2.4 NUMBER OF CONSECUTIVE DRY DAYS

Figure 34 - The maximum number of consecutive dry days each year for Scottish regions, from 1961 to 2004, with smoothed curve showing a running average.

Table 18 - Change in maximum number of consecutive dry days (in days) between 1961 and 2004.

North Scotland East Scotland West Scotland Scotland Consecutive dry days -0.2 1.1 0.1 0.3 (number of days)

Regional trends • From 1961 to 2004 there has been very little change in the maximum number of consecutive dry days (that is to say dry days in a row) in a year. • It is also clear that there is no obvious long-term trend in this index for this period. • The years with the highest values of consecutive dry days in each region often coincide. This shows a period of widespread dry weather. However, there are also many examples of years when correlation between the regions is not high.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 41

2.4 NUMBER OF CONSECUTIVE DRY DAYS Continued

Figure 35 - Pattern of change in maximum Spatial trends number of consecutive dry days each year • There is a clear east-west contrast in the (number of days), from 1961 to 2004. change in number of dry days in a row. • This pattern does not correlate well with any of the rainfall change patterns already presented. This might possibly show that the change is unlikely to happen mainly in any one particular season.

Future trends • There are no estimates in the UKCIP02 report for this measure. 42 Chapter 2 Precipitation related variables

2.5 AVERAGE RAINFALL INTENSITY

Figure 36 - Average rainfall intensity on days with 1mm or more of rain (in millimetres a day) for Scottish regions, from 1961 to 2004, with smoothed curves showing a running average.

Table 19 - Changes in average rainfall intensity on days with 1mm or more of rain (as a percentage), from 1961 to 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Rainfall intensity (%) 7.4 7.6 7.8 7.6

Regional trends • There is a trend of increasing rainfall intensity in both East and West Scotland. • There is a similar size of increase in North Scotland but this does not appear to be part of a trend, probably because natural variability in rainfall is higher in this region. • There does not appear to be a strong link between year-to-year rainfall intensity over the three regions, although the long-term trend (smoothed curve) is very similar.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 43

2.5 AVERAGE RAINFALL INTENSITY Continued

Figure 37 - Pattern of change (as a percentage) Spatial trends in rainfall intensity each year on days with 1mm • There is an increase for most of Scotland or more of rain, from 1961 to 2004. although there is a reduction for some northern and coastal areas, including the Outer Hebrides, Orkney and Shetland Islands.

Future trends • It is very likely that the intensity of rainfall will increase in winter months. • An east-west contrast in change is estimated in the UKCIP02 scenarios, with the most extreme changes taking place in eastern Scotland. This geographical contrast is clear in each of the UKCIP02 scenarios. • However, we have not seen this pattern when analysing the intensity of heavy rainfall we have seen since 1961, although we cannot directly compare the measures we have used with those used in the UKCIP02 report. 44 Chapter 2 Precipitation related variables

2.6 MAXIMUM FIVE-DAY PRECIPITATION TOTAL

Figure 38 - Maximum five-day precipitation total (in millimetres) each year for Scottish regions, from 1961 to 2004, with smoothed curve showing a running average.

Table 20 - Changes in maximum five-day precipitation amount (as a percentage), from 1961 to 2004. Values in bold show that we are 95% confident (statistically) that the change is part of a measurable trend.

North Scotland East Scotland West Scotland Scotland Rainfall intensity (%) 16.8% 25.2% 24.5% 21.3%

Regional trends • The average increase since 1961 is over 20%. We can clearly see an increasing trend. • The value of this measure varies greatly from year to year, and, apart from a few years, there is little agreement between the regional average values in any one year. • Values of the index are very similar in North and West Scotland, although a steadily increasing trend is clear in all regions. • The lower rainfall totals of East Scotland are reflected in lower maximum five-day averages.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 45

2.6 MAXIMUM FIVE-DAY PRECIPITATION TOTAL Continued

Figure 39 - Pattern of change (as a percentage) Spatial trends in maximum five-day precipitation amount each • As with rainfall intensity there is an increase year, from 1961 to 2004. for most of Scotland, although there is a reduction for some northern and coastal areas, including the Outer Hebrides, Orkney and Shetland Islands.

Future trends • The UKCIP02 report does not provide estimates for this measure. 46 Chapter 3 Air-pressure related variables

CHAPTER 3 AIR-PRESSURE RELATED VARIABLES

This section presents the analysis for: Calculated variables These derived quantities relate to both pressure 3.1 AVERAGE AIR-PRESSURE AT SEA LEVEL at sea level and wind speed and show how often 3.2 AVERAGE WIND SPEED EACH YEAR strong winds or gales take place. One of the 3.3 DAYS OF GALE EACH YEAR quantities that we can calculate from the information we have on wind is a measure of the number of days in a year that we can consider as Large-scale pressure patterns are responsible a day of strong wind. We have recorded changes for many aspects of Scottish weather. Low- in this variable in this section. The measure we pressure systems pass across the country use is that of a “gale day”. We define this as a bringing weather that is mainly wet and windy, day with an average wind speed of 34 knots or while high pressure is associated with less more over any 10-minute period. changeable and often drier conditions. Figure 40 shows the network of air pressure and sunshine Figure 40 - Where the air-pressure and recording stations that have provided the sunshine-hour sites are based (January 2001). information we have used in this analysis. The positioning of centres of high and low pressure also affects the flow of air across Scotland and hence the wind speed and direction.

The information on wind speed and direction greatly depends on the site of the weather station. For instance, there will be a marked difference between an observation in a mountain valley compared to one taken, at the same moment in time, on nearby flat arable land. Also, if the measuring instrument is moved on the site, there are new buildings close by or if a site is moved there will be an effect on the characteristics of wind measurements. Although we have information about average wind speed it is difficult to use. Analysis has identified downward trends in average wind speeds, but we believe that these trends are likely to be a result of irregularities in the information from some of the stations (in other words trends within the information may relate to changes in the way it has been measured rather than changes in the wind speed or direction itself). For this reason we have not mapped patterns of change in wind speed. Instead, we have taken records from three observing sites, where we know the records are reliable.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 47

3.1 AVERAGE AIR-PRESSURE AT SEA LEVEL

Figure 41 - Average air-pressure at sea level each year (in hectopascals, hPa) for Scottish regions, from 1961 to 2004, with smoothed curve showing a running average

Table 21 - Changes in average sea level air-pressure (hPa), from 1961 to 2004. One hectopascal is equivalent to one millibar.

North Scotland East Scotland West Scotland Scotland Spring 0.2 0.5 0.5 0.3 Summer -0.4 -0.4 -0.6 -0.4 Autumn -0.5 -0.6 -0.9 -0.6 Winter -2.6 -1.7 -1.3 -1.9 Annual -0.8 -0.5 -0.5 -0.7

Regional trends • Given that the average sea level pressure for Scotland is about 1012 hPa, it is clear that any changes are low - much less than 1%. • Changes from year to year are high but the link between the three regions is also high. This is consistent with the large-scale nature of atmospheric pressure patterns. 48 Chapter 3 Air-pressure related variables

3.1 AVERAGE AIR-PRESSURE AT SEA LEVEL Continued

Spatial trends report. However, they do comment on it in • The summer map shows that there is no the text. consistent pattern of change, but in winter • Changes in spring and autumn may be small. months a pattern is clear. In winter the north-south pressure gradient • Average winter pressures have been falling in may increase resulting in stronger winds in northern Scotland, particularly over the Outer southern and central Britain but little change Hebrides, Orkney and Shetland Islands, from in Scotland. 1961 to 2004. • Average sea-level pressure can be used to • At the same time, there has been little change investigate possible changes in North Atlantic to average sea-level pressure in winter in storm tracks and the North Atlantic Oscillation southern Scotland. (NAO). The UKCIP02 report suggests a possible • Given that the area north of Scotland is shift southwards of the storm tracks current mainly one of low pressure (in other words, the position, which may result in stronger winter so-called Icelandic low), this suggests that low winds across southern England. pressures have become lower. It also suggests • It is expected that the NAO index may become that the average winter pressure gradient mainly positive resulting in an increased across Scotland has increased since 1961. north-south winter pressure gradient across However, we know that the change is small Britain. This is consistent with more winters and may not be part of a measurable trend. where the prevailing winds are more ‘westerly’, in other words, milder, wetter Future trends and windier. The change in average sea-level • Maps of change in average sea-level pressure pressure is broadly consistent with these were also not given in the UKCIP02 scenarios expected future changes.

Figure 42 - Patterns of change in the average sea level air-pressure (in hPa) each year, between 1961 and 2004, for the summer and winter quarters.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 49

3.2 AVERAGE WIND SPEED EACH YEAR

Figure 43 - Average wind speed (in knots) each year for three Scottish stations - Lerwick, Tiree and Leuchars (values estimated from Turnhouse before 1969) from 1957 to 2004, with smoothed curves showing a running average.

Regional trends Future trends • The graphs for both Tiree and Leuchars show a • Estimating changes in average wind speed in trend of decreasing average wind speeds in the the future is very difficult. In the High emissions last forty years. scenario the UKCIP02 report suggests a change • Lerwick has seen a trend of increasing wind of less than 7% for the 2080s. speeds over the same period. • You should treat the values with caution given the uncertainties in estimating future wind speeds. 50 Chapter 3 Air-pressure related variables

3.3 DAYS OF GALE EACH YEAR

Figure 44 - Days of gale each year for three Scottish stations - Lerwick, Tiree, and Leuchars, 1957 to 2004, with smoothed curves showing a running average.

Regional trends Future trends • The exposed island sites of Lerwick and Tiree • The UKCIP02 scenarios report does not experience a higher number of gale days in a give estimates for this specific measure, but year than Leuchars. estimates for changes to average wind speeds • The year to year changes are also higher for are small. the island sites but there is no clear trend in • Confidence in any estimate for this variable is any of three records. very low.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND Chapter 4 Sunshine related variables 51

CHAPTER 4 SUNSHINE RELATED VARIABLES

This section presents the analysis for: only need to see a small change to result in a relatively large percentage change. 4.1 SUNSHINE HOURS 4.2 CLOUD COVER We have also analysed information on cloud cover in this handbook. Recording methods for hours of sunshine and cloud cover mean that Sunshine records are some of the longest duration there is no direct relationship between the two. meteorological data in the UK. Figure 40 (chapter However, it is reasonable to assume some 3) shows where the recording sites are located. kind of correlation between the two. But the Records of the number of hours of sunshine in a information on cloud cover should be treated day have been kept since 1929. We present this with some caution. The large-scale move to information alongside the assessment of trends automated observing methods over recent years over the 1961 to 2004 period that we are using means that we cannot assume that the values as the benchmark for this study. The maps have been consistently measured. Any trend may show change in sunshine hours as a percentage. be due to the change in recording method rather Because daylight hours are lowest in winter we than any real change in cloud cover.

4.1 SUNSHINE HOURS

Figure 45 - Total sunshine hours for each year for Scottish regions from 1929 to 2004, with smoothed curves showing a running average. The dashed vertical line marks 1961. 52 Chapter 4 Sunshine related variables

4.1 SUNSHINE HOURS Continued

Table 22 - Changes in total sunshine hours (as a percentage), from 1929 to 2004 and 1961 to 2004.

1929 to 2004 1961 to 2004 North East West North East West Scotland Scotland Scotland Scotland Scotland Scotland Scotland Scotland Spring -5.6 0.5 -4.4 -3.3 4.5 6.9 2.7 4.7 Summer -3.1 1.1 1.9 -0.2 -2.1 0.2 -1.4 -1.1 Autumn -3.0 4.5 8.3 2.8 17.9 12.1 11.0 13.8 Winter -13.8 -0.4 -0.3 -5.1 -4.6 12.8 -0.6 2.6 Annual -5.6 1.2 0.2 -1.6 2.7 5.5 1.6 3.3

Regional trends Spatial trends • The average number of hours of sunshine • There is wide variation in the patterns of recorded each day is lowest in North Scotland change in sunshine across Scotland for but this is also the region that has seen some each season for the 1961 to 2004 period. of the largest changes. • There has been only slight change over this • Only North Scotland shows any obvious period in either spring or summer and the trends (winter and annual sunshine hours main changes have taken place in the second have reduced) and then only over the longer half of the year. analysis period, in other words, since 1929. • In some areas the changes are large, for • Since 1961, the average number of sunshine example, up to a 40% reduction in sunshine hours in a day has increased by a small hours in winter (December to February) in percentage each year in all three regions. parts of North and West Scotland. As the This seems largely due to a large increase patterns are in small areas only these changes in sunshine in the autumn months. Since are not apparent in the regional and national we have not seen a trend any change may averages. be simply be part of natural variability. • East Scotland has become sunnier in all Future trends seasons since 1961 but again there is no • The UKCIP02 scenarios report did not include obvious upward trend. an estimate for future sunshine hours, so it is not possible to compare trends with those expected for the future. • However, sunshine hours may be related to cloud cover. By the 2080s, the UKCIP02 scenarios estimate cloud cover may increase slightly in winter, particularly in the northern half of Scotland, and that it may reduce in all other seasons, with the greatest changes in summer in the southern and eastern areas. • The reduction of winter sunshine hours we have already seen is, to some extent, consistent with this.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 53

4.1 SUNSHINE HOURS Continued

Figure 46 - Patterns of change in sunshine hours (as a percentage), between 1961 and 2004, for each season. 54 Chapter 4 Sunshine related variables

4.2 CLOUD COVER

Figure 47 - Percentage cloud cover each year for Scottish regions, from 1961 to 2004, with smoothed curves showing a running average.

Table 23 - Changes in percentage cloud cover, from 1961 to 2004.

North Scotland East Scotland West Scotland Scotland Spring -0.73 -0.69 -0.56 -0.67 Summer -0.02 0.06 -0.64 -0.17 Autumn -0.57 0.55 1.00 0.24 Winter -1.98 -2.15 0.02 -1.45 Annual -0.90 -0.43 0.26 -0.42

Regional trends Future trends • None of the changes show an obvious trend in • The UKCIP02 scenarios estimate cloud cloud cover. amounts may increase slightly in winter by • The cloud cover changes do not appear to the 2080s, particularly in the northern half of be linked to sunshine hours. It may be that Scotland, and may reduce in all other seasons. natural variability is hiding any underlying The greatest changes may take place in trend but it is much more likely that the summer in the southern and eastern areas. difficulties in comparing records based on • The patterns of change we have already seen different observing methods is the cause. do not match these predictions. • The large reduction in recent years is likely to be due to the change in observing methods.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND Appendix 1 55

WHERE THE INFORMATION COMES FROM AND THE TECHNIQUES WE HAVE USED TO ANALYSE IT

Most of the information used in this study is from as altitude, terrain shape and proximity to sea 1961 until the end of 2004, although there are or urban areas. The gridded results were also exceptions to this. In particular, we have much checked with values we have from observations. longer records for temperature, rainfall and The full method is described in Perry and Hollis sunshine. To assess whether we can see trends (2005a, 2005b). Although the gridded datasets in records of measure of climate for Scotland, have been checked using independent we try to use as long a record as possible. observations, they are the result of interpolation On the other hand, using a consistent period of techniques, and so you should treat the maps of history for all variables allows us to produce a trends from this dataset only as an estimate of comprehensive picture of change, and to identify local patterns of change. any links between variables. For this reason, we analysed all information for the period 1961 The Met Office dataset is notable for the range of to 2004 (or less if the records are not available elements included. The Met Office have produced throughout the full period). However, if we gridded datasets at 5km by 5km resolution over have records covering a longer period, we also the UK for 36 monthly or annual climate variables, analysed these and present them alongside for the period 1961 to 2000. The Met Office the 1961 to 2004 analysis. If we did not have chose the start date of 1961 because there is a information or it is difficult to interpret because significant increase in the availability of digitised of irregularities in the information (for example, information from this point. A number of these a change in instrumentation or observing site) variables are routinely updated with the latest we used individual site records. month while we have updated several other variables to 2004 for this study. The Met Office has a historical database containing observations of different weather The Met Office have used all available monthly elements. These observations come from a meteorological information to make best use network of meteorological stations across the of the information available and make sure United Kingdom which is constantly evolving. that we can make the most accurate possible From this information we have produced a representation of the climate for each month. consistent series of climatic statistics which As a result, the network of stations used changes allows us to compare weather and climate slightly each month, and the methods we have across space and time. In order to do this, used are designed to reduce, as far as possible, methods were developed to create gridded the effect of these changes on the consistency of datasets from the information gathered at the datasets through time. Table A1 shows, by each station. variable, the average number of stations included in a month of the dataset. We also give the The process for analysing this information average number of stations for the variables applied geographical information systems (GIS) available before 1961, clearly showing the capabilities and included a range of factors such increase in digitised information since this time. 56 Appendix 1

WHERE THE INFORMATION COMES FROM AND THE TECHNIQUES WE HAVE USED TO ANALYSE IT Continued

Table A1 - Average number of stations included each month in the gridded dataset.

Climate variable Before 1961 1961 onwards Precipitation 102 740 Days of 10 mm or more rain n/a 693 Rainfall intensity / greatest five-day rainfall n/a 458 Air temperature 76 158 Consecutive dry days each year n/a 145 Extreme temperature range each year n/a 141 Heating and growing degree-days n/a 126 Days of snow cover n/a 106 Sunshine 59 75 Length of heat-waves and cold spells n/a 52 Average sea level pressure / cloud n/a 19

We can estimate the values of climate variables between observing stations to a good degree of accuracy, producing detailed and representative maps of the Scottish climate. We can investigate changes in patterns over time as well as trends in climate using the processed information. However, the accuracy depends on the nature of the variable plus how many stations there are providing information for an area. Inaccuracies occur most often in areas where there are few stations, particularly the highlands of Scotland, which are also areas of complicated mountainous terrain. The process does not take account of localised effects on climate such as frost hollows, and effects caused by the type of soil and forests.

A HANDBOOK OF CLIMATE TRENDS ACROSS SCOTLAND 57

ANALYSING THE INFORMATION

We have presented our analysis in three ways. We analysed the information using linear regression, and tested the significance of the • Firstly, we calculated linear trends for each trends using the non-parametric Mann-Kendall variable for Scotland as a whole and for three tau test (Sneyers, 1990). The Mann-Kendall test regions (see figure 1). We did this for annual is a rank-based non-parametric test. For each and seasonal average periods. Each region value in the series, we work out the number of encircles an area of similar characteristics values before it which are higher than it to gain in terms of climate. For the purposes of this evidence of a trend in the series. You need to report we have been termed them North, West be careful when interpreting results from linear and East Scotland, using the capital letter to trends as the assumption that the trend is linear indicate one of the three regions rather than is not always valid. However, the trends are often an area of Scotland. We used a method called close to linear, and the combination of linear linear regression to work out the trend in each trends with the Mann-Kendall significance test variable. We then used the average rate of has been widely used when analysing of climate change from the linear regression multiplied trends (for example, Domroes and El-Tantawi, by the length of the data period to provide a 2005; Shen et al, 2005). clear measure of change since the start of the period. We used a statistical test, known as the In this study the spatial analysis of growing Mann-Kendall test, to show whether significant season length, start and end dates is slightly changes had taken place. If these trends are different to that of the other calculated variables, statistically significant we show them in bold in that it is based directly on the baseline (significant at the 5% level). recorded climate of the UK dataset provided by UKCIP with the UKCIP02 scenarios. • Secondly, we produce graphs of the time-series This information provides monthly average for each variable for each of the three regions temperatures rather than the daily values of Scotland. These show the year to year needed to work out growing season, so we variability for each variable. We also include a used a method called sine curve interpolation smoothed version to show the running average to estimate daily values based on the method as an indication of the longer-term trends and of Brooks (1943). We then used this daily variations (see figure 3 for an example). information were then used within the calculations for growing season length, start • Finally, where appropriate, we produced a and end dates. Also, the standard dataset map of trends we have already seen showing the UKCIP02 is for 1961 to 2000, but for this spatial variation which was not easy to see from application we have updated it to 2004. the national average figures often presented elsewhere. (See figure 4 as an example). For a more detailed technical analysis please see the accompanying SNIFFER publication “Patterns of Climate Change across Scotland: Technical Report” at www.sniffer.org.uk 58 Appendix 2

REFERENCES

Barnett, C., J. Hossell, M. Perry, C. Procter and G. Hughes (2006) Patterns of Climate Change across Scotland: Technical Report. SNIFFER Project CC03, Scotland & Northern Ireland Forum for Environmental Research, 102pp.

Brooks (1943) Interpolation tables for daily values of meteorological elements. Q. J. Roy. Meteor. Soc., vol 69, no 300, 160-162.

Domroes M. and A. El-Tantawi (2005). Recent temporal and spatial temperature changes in Egypt. International Journal of Climatology 25: 51-63.

Hulme, M., G.J. Jenkins et al. (2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Published by the Tyndall Centre, UEA Norwich, April 2002.

Osborn, T.J., M Hulme, P.D. Jones and T.A. Basnett (2000) Observed trends in the daily intensity of United Kingdom precipitation. International Journal of Climatology 20: 347-364.

Perry, M.C. and D.M. Hollis (2005a) The development of a new set of long-term climate averages for the UK. International Journal of Climatology 20: 1023-1039.

Perry, M.C. and D.M. Hollis (2005b) The generation of monthly gridded datasets for a range of climatic variables over the UK. International Journal of Climatology 20: 1041-1054.

Shen, S.S.P., H. Yin, K. Cannon, A. Howard, S. Chetner, T.R. Karl (2005). Temporal and spatial changes of the agroclimate in Alberta, Canada, from 1901 to 2002. Journal of Applied Meteorology 44: 1090-1105.

Sneyers R. (1990) On the statistical analysis of series of observations. WMO Technical Note No. 143.

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