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Weather Science: What Causes Weather Understanding Weather Charts

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Weather Science: What Causes Weather Understanding Weather Charts CANNOCK CHASE U3A SCIENCE & T ECHNOLOGY GROUP Weather Science: What causes weather Understanding weather charts 13 th July 2017 THE DIFFERENCE BETWEEN WEATHER AND CLIMATE IS … The climate is what you expect; The weather is what you get. Frequently attributed to Mark Twain. So, why do we get what we get? There are loads of factors at play… TEMPERATURE The Tropics are absorbing more energy than they are losing, whilst the North and South Poles are losing more energy than they are absorbing. This would mean that the Tropics would be always warming, and the Polar regions always cooling. However this isn’t the case, something else must be transporting heat from the Tropics to the poles – Winds. Pressure is how much air PRESSURE & W IND there is above your head Tropopause acts as a lid Temperature makes hot air rise, Zero mb with cooler air replacing it, causing a flow of air or wind. If more air diverges at the top than converges (friction), we end up with less air in the column and the air pressure falls (less weight). If there is more pressure in one place, and less elsewhere, then the air moves from high to low pressure. The bigger the difference between high and low pressure, the faster winds that blow. 1000 mb The units of pressure are mb or hPa Air weighs 1.225 kg/m 3 or inches of Hg, where 1mb = 1 hPa. Water is 1000kg/m 3 Pressure is shown on a barometer and can indicate the weather approaching. WHAT ARE ALL THE WEATHER MAP MARKINGS ? Temperature, Pressure contours, Wind & Fronts all appear on various charts. Pressure is usually in the range of 950-1050mb, UK, marked by lines (isobars). Isobar contours are lines of equal pressure, usually ovals at 4mb intervals. Pressures decrease towards centre of (L)ows & viceversa (H)igh and (L)ow pressure centres marked with a cross. Close together isobars means pressure rising rapidly, so wind speed is higher and blows almost parallel to isobar lines. Spread out isobars mean little wind with slower pressure changes. Low pressure usually means cloud rain & wind (cyclone). High pressure usually means fine weather (anticyclone). [-------Cold showery weather-------] AIR MASSES AND FRONTS ON CHARTS 6 sources of air mass as in picture. Weather “Fronts” are the boundaries of two Air Masses. Northerly winds can bring cold air. Southerly winds can bring warm air. Warm fronts on charts are red semicircles and mean rising air, higher pressure (higher column of air). Cold fronts are blue triangles and mean falling air, falling pressure. Air masses/Fronts meeting will clash at occluded front in purple, causing bad storms. THE CORIOLIS EFFECT & WIND Normally the wind tries to blow from high to low pressure, but the Video Earth’s spin acts on it, diverting it. Wind then follows the pressure contours and blows around low pressure in an anticlockwise direction. And around high pressure in a clockwise direction. (True only for the Northern Hemisphere). Remember that we name winds by the direction they are blowing from DEPRESSION , F RONTS , CLOUDS & R AIN 4 stages of a depression Cold fronts are marked with triangles (reminiscent of icicles) and warm fronts with semi-circles (reminiscent of the warm sun). The lines are the boundaries of the wave of warm air. The grey shading shows the region of continuous cloud cover. The occluded front (purple) occurs when the wave breaks, and the cold front catches up with the warm front. WHAT HAPPENS AT WARM & C OLD FRONTS APPROACHING FRONT : DEPRESSION PASSING OVERHEAD 4. As the front crosses you on the ground, the temperature rises as there is a shift from cold to warm air, typically by a few °C. Cirrus 5. The air pressure falls steadily ahead of and during the passage of the warm front, but then rises Nimbostratus slowly after its passage. 2. As1. the The front first moves thing towards that reaches you, ityou gets is lowerthe and similarlywarm front, the farclouds above get your lower head. and Theheavier. You’reclouds going are to needon the your front, umbrella. so you will see 3. On thehigh warm clouds, front, typically you’ll typically wispy, ice see clouds featurelesscalled cirrus,stratus then or ‘sheets’ Cumulonimbus of cloud, becoming nimbostratusrising. or ‘rainy sheets’ with a prolonged spell of rain when the front is near the ground. Cumulus clouds are indicative of THE PHYSICS convection. They can billow upwards as the air rises within them. PV / T = constant So, if the pressure falls, the gas must cool. As the rising air cools, the rate of evaporation becomes less than the rate of condensation until eventually cloud droplets form. As water vapour condenses into liquid water, latent heat is released providing the energy source for the developing storm. The latent heat generated is significant. For example, there is enough heat released in a small cumulus cloud to power an average home for 17 years. As the storm continues to develop, some of the cloud droplets will become large enough to fall as rain. THE JET STREAM Yet another force at play. The polar front jet stream is a belt of westerly winds in the upper troposphere, with wind speeds up to 250mph. It sits at a around 10km, the height at which aeroplanes fly. Trans- Powerful example of wind flow driven by Atlantic pilots make use of it. pressure & equator/pole temperature The jet stream can split up warm differences, but shaped by earth’s spin. & cold fronts. Normal Summer UK Jet Stream Summer 2012 UK Jet Stream FRONTS & F EATURES ON WEATHER MAPS To sum up so far: Let’s look at some features of a weather map ANYONE FOR TEA? Next after tea… Will it snow? How high is a cloud? Making a cloud WILL IT SNOW ? To have snow, we need a combination of two factors: A weather front, convection clouds (cumulous) or mountains 24 Dec 2010 need to be causing precipitation. The atmosphere needs to be cold enough for precipitation to fall as snow. Are there any cold fronts or air masses on Met Office surface pressure charts bringing rain? (www.metoffice.gov.uk ) Arctic maritime or polar continental air masses can bring snow in winter to the UK. Look for “528” contours. 1. ARE THERE COLD FRONTS ? LIKELY TO RAIN ? LOOK AT MET OFFICE CHARTS . S EE 528 LINE (DOTTED ) SURFACE PRESSURE AT WWW .METOFFICE .GOV .UK 2. W ILL IT SNOW ? 2. Is the atmosphere cold enough for precipitation to fall as snow? 500mb Look at thickness of bottom half of atmosphere. Hot air rises. Colder more dense air sinks to bottom half. © Netweather.tv 1000mb So smaller height = colder air. 500-1000mb is the bottom half mass of atmosphere. Height to 500mb pressure – Look for 528 lines = 5280m. 5280m = height below 500mb, snow is possible. Weather forecast charts from netweather.tv (charts/gfs) Select ‘HGT 500-1000’ netweather.tv 3. W ILL IT SNOW ? Saturday Feb 11th 2017 It snowed! Above London (0 0) Check whether the atmosphere really is cold enough throughout its entire depth: Go to wetter3.de …Select ‘Vertikalschnitte’ (Vertical section) and select latitude & longitude. wetter3.de EXTRA TIME … Measure the height of a cloud Making a cloud MEASURE THE HEIGHT OF CLOUDS Temperature change from the ground: Approx -6 degrees C per km (-1.8C per 1000ft) Use an Infra Red non-contact temperature sensor. Point IR sensor to clouds for temp measurement (eg -16C). Point to ground for ground temp (eg +5C). Temp difference, say 5C – (-16C) = 21C Height = 21/6 km = 3.5km Making a cloud video [Copyright Reading University] THAT’S IT FOLKS See you next time References thanks to Reading University RAIN, WHEN ? What causes precipitation (rain and snow)? Precipitation forms when cloud droplets (or ice particles) in clouds grow and combine to become so large that their fall speed exceeds the updraft speed in the cloud, and they then fall out of the cloud. If these large water drops or ice particles do not re-evaporate as they fall farther below the cloud, they reach the ground as precipitation. CLOUDS.
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