Weather Hazard Revision Key Idea Global atmospheric circulation helps to determine patterns of weather and climate. What you should know • General atmospheric circulation model: pressure belts and surface winds. The General Atmospheric Circulation Model General Atmospheric Circulation Model • To understand the General Atmospheric Circulation model you first of all need to understand: ​ • Why variations in solar energy exist ​ • How variations in solar energy cause high and low air pressure ​ • How areas of high and low air pressure cause wind ​ • The impact of the Coriolis Effect on wind ​ Why variations in solar energy exist The Earth’s surface receives the highest amount of solar radiation at the Equator. This is because the energy is spread over a smaller area and travels a shorter distance through the atmosphere at the equator. These are both as a result of the curvature of the Earth and its atmosphere • So how do the different temperatures at the Earth’s surface create different air pressures around the earth?​ ​ To understand this think about the following… Low Pressure Moist rising air • Air is rising as it is warmer than surrounding air and warmer air is less dense (lighter) ​ • This leads to less air particles resting on the earths surface and so creates an area of low pressure ​ • As the air rises it cools, so water vapour condenses and causes cloudy, rainy conditions. ​ • It creates unsettled weather conditions High pressure • Air is sinking as it is cooler than surrounding air and cooler air is denser (so is heavier)​ • This leads to more air particles resting on the earths surface and so creates an area of high pressure​ • This means condensation does not occur and so leads to clear skies and settled conditions Dry sinking air How areas of high and low air pressure cause wind ​ • The Earth's atmosphere exerts a pressure on the surface of the Earth. ​ • Pressure is measured in millibars. Standard pressure at sea level is defined as 1013mb, but we can see large areas of either high or low pressure Wind always moves from high to low pressure • The earth tries to balance air pressure, so air from higher pressure areas moves into low pressure areas. This movement of air is what we know as wind. ​ • Areas of low pressure have higher wind speeds as wind rushes in from surrounding areas. So, not only is it cloudy and wet in areas of low pressure it is windy too. ​ ​ Global cloud patterns are linked to the distribution of low and high pressure systems that separate the convection cells What is the Coriolis effect? • The effect on wind caused by the spinning of the Earth • On a non-rotating earth air flow between high and low pressure areas would generate one convection cell per hemisphere ​ ​ ​ • On a rotating earth the ‘Coriolis Effect’ causes atmospheric circulation to be divided into three cells per hemisphere (Hadley, Ferrel, Polar Cells) ​ ​ ​ The link between High and low pressure and surface winds How does the global circulation influence surface wind patterns • The three cells Hadley ,Ferrel and Polar cells influences surface and high level winds as they transfer the energy received from the sun in the atmosphere. Sub tropical high pressure • The sub tropical high pressure areas move toward the low pressure areas near the equator. • The rotation of the earth produces the Coriolis effect which deflects the winds to the right in the northern hemisphere and left in the southern. • This leads to prevailing winds ie westerlies in the uk. Relationship between Tropical Storms and the General Atmospheric Circulation • Tropical storms originate (begin) in warm ocean waters close to the equator because this area receives the greatest concentration of energy from the sun, this energy warms the oceans to 27°c that’s needed for tropical storms to form.​ • Tropical storms occur between 5° and 20 ° North or south of the equator because the Coriolis Effect is great enough to cause the storm to spin​.to close to the equator not enough spin • They begin at an area of low pressure, that means the air is rising, cooling and condensing to form clouds. As more and more air rises and condenses the clouds get thicker and lower in the sky and a storm will begin to develop.​ • Air blows into the low pressure systems from the Tropic of Cancer and Tropic of Capricorn which are 30° north and south of the equator, this is where the high pressure belts lie.​ ​ • The air moving from high to low pressure generates winds called the trade winds, these converge (come together) and meet which causes the storm to spin​ Key Idea Tropical storms (hurricanes, cyclones, typhoons) develop as a result of particular physical conditions. What you should know • Global distribution of tropical storms (hurricanes, cyclones, typhoons). • An understanding of the relationship between tropical storms and general atmospheric circulation. • Causes of tropical storms and the sequence of their formation and development. • The structure and features of a tropical storm. • How climate change might affect the distribution, frequency and intensity of tropical storms Causes of tropical storms • light winds aloft (above) • winds near the ocean surface blowing from different directions converging • low-wind-shear winds that do not vary greatly with height (so storm not broken up as it forms) Global distribution of tropical storms (hurricanes, cyclones, typhoons). Global distribution of tropical storms (hurricanes, cyclones, typhoons). • Could be asked to describe the distribution • Start large work in finish with anomalies • Most tropical storms occur between 5°and 20° north and south of the equator • Tropical cyclones occur more frequently in the northern hemisphere where 69% occur • Where as in the southern hemisphere 31% occur this is 25% less than the northern hemisphere • 33% of all tropical cyclones occur in the western Pacific the where they receive 16 % more than in the East Pacific which has the second highest frequency at 17% sequence of tropical storms formation and development. How tropical storms are formed • They contain enormous amounts of energy captured from the atmosphere and moisture from the oceans • Areas they form are characterised by low pressure and rising air • Results in a strong up lift of very moist warm air • Warm air rises in a spiralling motion drawing up huge amounts of evaporated water which cools and condenses quickly to form tall towering banks of clouds • Vast amounts energy is produced • This energy powers the storm • As storm builds more water vapour is drawn in and upwards • Clouds grow larger wind speeds increase heavy rain fall increases • Storms increase their power /energy as they move over water • Lose energy as its moves over land How do storms lose their energy • The frictional effect of moving over the land, eg they pass over land which slows their movement due to friction. • Loss of energy due to cooling effect of passing over water (or land) at higher latitudes, eg they move into areas of cooler water, where there is less energy. The structure The Features of a Tropical Storm How can you tell the winds are anticlockwise • Figure 4 shows that the pattern of winds moving around the hurricane centre was anticlockwise because the clouds show an anticlockwise pattern/the clouds spiral inwards/of the way the clouds are arranged (1) How climate change might affect the distribution, frequency and intensity of tropical storms Changing DISTRIBUTION of tropical storms (where they happen) 1. How much have the surfaces of our seas warmed by? . Over the last few years, as our climate is warming, sea surface temperatures have increased by 0.25-0.5°C 1. What impact might this have on the distribution of storms? . Cold ocean temperatures keep waters below the temperature needed (27°C) for tropical storms to develop. As ocean temperatures are warming tropical Storms may begin affecting areas further north and south of the equator, areas that were once too cold for tropical storms to form. 1. Is there any evidence to suggest this has already started to happen? Hurricane Catarina moved into the Brazilian state of Santa Catarina on 28 March 2004. Hurricanes do not normally form in the South Atlantic, it was the first ever recorded here, and coastal communities were taken by surprise. Increasing INTENSITY of Tropical Storms (how strong they are) 1. Describe the overall pattern of hurricane intensity in the Graph C North Atlantic, shown by Hurricane Intensity in the North Graph C Atlantic, 1951- 2011 . Graph C shows hurricane intensity in the North Atlantic decreased overall during the period between 1950 and the early 1980s, from just over 3 on the index scale to 1. It then increased overall in the period between about 1983-2005, reaching its peak at almost 5.5 on the index scale. However, by 2011 it was almost the same as the 1950s level again, at just under 3 on the index scale intensity 1. What reason do some scientists give for the increase in the intensity of tropical storms, which could be linked to climate change? . Some scientists say this is linked to the increase in sea surface temperature. The higher sea temperature the more water is evaporated and atmosphere above is warmed. This will increase convection (warm air rising), creating lower surface pressure, and therefore leading to stronger, more intense winds. 1. Not all scientists agree that tropical storm intensity is increasing. What do they say is needed before we can be sure? . Some scientists say that more data is needed over a long time period to say for sure that storm intensity is increasing. Frequency • Increasing FREQUENCY of Tropical Storms (how many happen) • Is there any evidence to suggest that there has been an increase in the number of tropical storms happening? • In July 2007, a survey of hurricanes in the North Atlantic over the past century (100 years) noted an increase in the number of observed hurricanes.