A2 Geography 4.1 Atmospheric Systems Student Notes Development of Depressions (Formation, Movement and Change) Depressions form at wave-like disturbances in the polar front below a diverging section of the polar jet stream. This induces a centre of low pressure near the surface along the line of the polar front. The polar front is a boundary between warm, moist tropical air (Tm) and cooler polar maritime air (Pm). As the two air masses converge, the warms, less dense Tm air is forced to rise in the spiralling vortex of the newly forming embryonic depression. The Earth s coriolis force induces the vortex. The rising air causes the pressure at the centre of the depression to become lower. Within the depression will be a warm front (the leading edge of the Tm air) and a cold front (the leading edge of the Pm air), separating the two air masses into a warm sector and a cold sector. As most (but not all) depressions develop they drift in a north-easterly direction under the influence of the westerlies, i.e. the polar jet stream. As the depression matures, its pressure deepens and the warm, moist air of the warm sector is forced to rise and will cool to its dew point. Some vapour will condense and release latent heat, leading to cloud formation and eventually precipitation along both fronts (about 150 km band or rain along the warm front and about 50-60km band along the cold front). The pressure gradient increases, increasing the wind speed, which circulated in an anticlockwise direction around the centre of low pressure. The depression begins to decay when the cold front catches up the warm front to form an occlusion. By this time the Tm air will have been squeezed above the colder Pm air, so that none of it touches the ground. As the uplift of air is reduced, there will be less condensation, less latent heat and the rain will become less intense or patchy. Cloud cover decreases, the depression will fill and pressure gradients fall leading to lighter winds. Weather Associated with Depressions 5. Behind the cold front 4. At the cold front 3.warm sector 2. At the warm front 1 Approaching Depression Pressure Rise continues more slowly Sudden rise Steady Fall ceases Steady fall Wind direction NW Veers from SW to NW SW Veers from SSE to SW SSE Wind speed (Beaufort Scale) Squally; speed slowly falling (3-6) Strong to gale force (6-8) Decreasing (2-4) Strong (5-6) Slowly increasing (1-3) Temperature (e.g. winter) Cold e.g. 3 o C Sudden decrease Mild e.g. 10 o C Sudden rise Cool e.g. 6 o C Relative Humidity Rapid fall (except in showers) High Steady and high High Slowly rising Cloud Decreasing Cb and Cu Towering Cb Low / clear St, Sc, Ac Low thick Ns High thin Ci, Cs,Ac,.As Precipitation Heavy showers Short spell of heavy rain Dry or drizzle Long spell of steady rain None Visibility Good (poor in showers) Poor Variable (often poor) Decreasing rapidly Good bur decreasing 1
a) Ana fronts depression: warm air rises relative to the frontal surfaces to give thick frontal clouds. 2
b) Kata fronts depression: upper air sinks relative to the frontal surfaces and the vertical extent of cloud is limited by surface a subsidence inversion. Impact of Depressions on Human Activity AGRICULTURE Depressions bring regular rainfall to Britain so that the use of irrigation is usually unnecessary (except on intensive arable farms in eastern Britain). The high rainfall and mild winter conditions associated with depressions are also beneficial to dairy and livestock farms as the growing season for pasture is long, particularly in the south west (such as Devon and Cornwall). Livestock can be left in fields for longer, reducing winter feed costs. Heavy winter rainfall (such as in 2000/2001) can cause flooding and damage to crops such as winter wheat, potatoes or sugar beet. Strong winds associated with depressions can be damaging to grain crops. In extreme cases, hail, lightning and tornadoes, associated with depressions can cause localised damage. Summer rainfall can damage crops such as wheat or lower their economic value if it falls before harvest. WATER MANAGEMENT With frequent depressions, Britain can meet its water demands without the large scale storage and transfer schemes that are necessary in some parts of the world such as California. Regular rainfall also helps to maintain river water quality both for human consumption and for wildlife. When depressions are too dominant in Britain s weather pattern, saturation of soils and flooding can cause damage to property and, on rare occasions, injury and loss of life. For example, the winter floods of 2000/2001. TRANSPORT 3
There are few advantages for transport although, with the exception of mist or fog at warm fronts, the visibility usually remains clear. The risk of frost and icy roads is also much lower in a depression. In addition to problems created by flooding, depressions can lead to heavy spray on motorways and winds that are hazardous for high sided vehicles. Bridges such as the Severn Bridge, may have to be closed to such vehicles in intense depressions. Air and sea traffic can also be disrupted by strong winds. Hovercraft, catamarans and jetfoils cannot operate in heavy seas caused by the strong winds in a depression. The wet snow from Atlantic depressions in winter can create hazardous conditions on roads. ENERGY DEMAND AND SUPPLY Heavy frontal rainfall in the west of Britain provides water for hydro-electric power stations. Most of Britain's HEP plants are in Scotland or Wales. Winds associated with depressions are important for wind power turbines. Wind could supply up to 20% of Britain's energy needs in future. The milder weather associated with Atlantic depressions in winter, reduces the demand for energy for winter heating. Strong winds can bring down power cables and damage wind turbines. Oil tankers cannot leave or enter ports in heavy seas. RECREATION AND TOURISM The weather associated with depressions can benefit a few specialist leisure activities such as sailing, surfing and canoeing. Britain s weather is unreliable and very variable, largely as a result of depressions. Many people from the UK choose to holiday abroad to avoid such weather. Depressions are the main cause of disruption to sporting events such as the Wimbledon Lawn Tennis Championships and cricket matches. Development of Anticyclones An anticyclone is a large mass of subsiding (descending) air, which produces high pressure on the Earth s surface. The source air is in the upper troposphere where the amount of water vapour is limited. The subsidence is often part of the global circulation of air, although some ridges of high pressure (small, short-lived high pressure areas) are induced by accelerating jet streams following the Rossby Waves along the polar front. As the air descends, it warms and water evaporates, so dry conditions prevail. 4
Pressure gradients are gentle so winds are weak or calm. On the edges of the system, the Coriolis force and centrifugal forces combine to cause outward clockwise flowing air. Anticyclones may be 3000km in diameter (compared with about 1000km for a depression) and, once established, can give days or even weeks of settled weather. When cells of high pressure detach themselves from major high pressure areas, such as the Azores, sub-tropical high, they can cause blocking anticyclones. These block the easterly movement of depressions over the British Isles. In 1995, such an anticyclone resulted in a prolonged summer drought. Weather Associated with Anticyclones Characteristics of anticyclones include: Stable air with limited cloud formation and precipitation. Some limited convectional cloud may form in summer. (If fronts occur within anticyclones, they will produce limited cloud cover and little precipitation, if any.) Circulation is in a clockwise direction around the whole weather system (in the northern hemisphere). Light wind speeds or calm, with land and sea breezes in coastal areas in summer and anabatic and katabatic winds in mountainous areas. Warm, dry days and mild nights in summer. Increasingly humid. Cool, dry days and frost and/or fog and mist at night in winter. In hilly or mountainous areas, frost or fog may accumulate in vales, valleys, or hollows. Decreasing air quality, particularly in urban areas (due to limited circulation). Visibility is usually good if the humidity is low. If the humidity is high, visibility may be impaired. It may become hazy or even overcast with low stratus cloud. In winter, high humidity can lead to fog or overcast conditions: so called anticyclonic gloom. A Comparison of Summer and Winter Anticyclones Wind: in both summer and winter, wind speeds are light or calm towards the centre of the anticyclone. General circulation is in a clockwise direction around the edges of the system. In summer land and sea breezes may develop on coasts and anabatic and katabatic wind may develop in mountainous regions. Temperature: lack of cloud cover leads to high insolation levels and high summer daytime temperatures of 20-30o in the UK. Night time temperatures will fall to 10-15oC in the UK. Winter temperatures will be significantly lower (5-10oC) as, in spite of clear skies, the altitude of the sun is low so insolation is limited. At night, with lack of insulating cloud cover, there will be loss of longwave radiation to space leading to sub-zero temperatures of 0oC to -10oC Moisture and Visibility: in summer skies are generally clear but some localised convection can lead to some fair weather cumulus cloud in the daytime. Dew may form on the ground at night and patchy mist can form in valleys. In winter the lack of convection can lead to completely clear skies, but lower temperatures can lead to higher relative humidity. This increases the risk of stratus cloud formation. Cold nights may lead to frost and fog formation, particularly in frost hollows and valleys. These may linger well into the following day, particularly if daytime insolation levels are low. 5
LAND AND SEA BREEZES Anticyclones generally produce calm clear conditions. Under these clear skies, insolation levels (short wave radiation from the sun) are high in the day time and loss of longwave terrestrial radiation at night is also high. Water has a higher specific heat capacity than land, so the land surface warms more in the day time and cools more at night. This can lead to localised coastal land and sea breezes at times when there is little general circulation. MOUNTAIN WINDS Anabatic and Katabatic winds are also more pronounced under anticyclonic conditions in mountainous areas. On hot still days, air over a heated valley floor will expand and rise causing anabatic draughts to move up the valley side. Under humid conditions, clouds may form over the valley sides. On cold still nights, upper mountain surfaces rapidly lose heat. The air becomes chilled and denser and flows down the valley sides as a katobatic wind. As the cold air accumulates in the valley, fog or frost may form. Similar circumstances can lead to frost and fog hollows in hilly regions such as the Chilterns. URBAN CLIMATES Urban heat islands and distinctive urban micro-climates are much more likely to occur under anticyclonic conditions. (See later notes). Impact of Anticyclones on Human Activity AGRICULTURE In summer anticyclonic conditions lead to long hours of sunshine. This is important for ripening grain and fruit crops before harvesting. In winter, frost can help to break up ploughed soils and to kill soil based diseases. Some root crops such as parsnips benefit from frost as it improves flavour. Late frosts, associated with anticyclones can damage fruit blossom and prevent pollination. Long dry periods in summer can increase the need for the irrigation of salad crops, vegetables, soft fruit, potatoes or sugar beet. WATER MANAGEMENT Prolonged periods of drought, associated with anticyclones, can both reduce supply and lead to an increase in demand for water for domestic and agricultural uses. In warm summer weather there is an increased risk of eutrophification and the growth of poisonous algal blooms in rivers, reservoirs, lakes and canals. Penetrating winter frosts can lead to burst mains when the frozen pipes thaw. 6
TRANSPORT In summer and winter, settled dry conditions usually favour most forms of transport from cross channel ferries to road users. Winter frost and fog can be particularly hazardous for road transport and light aircraft. (large airports in the UK usually have ground radar, which allows the use of runways even in thick fog). Fair summer weather can encourage a higher frequency of road use leading to higher risks of traffic congestion. ENERGY DEMAND AND SUPPLY Warm dry conditions lead to a fall in the demand for energy in summer anticyclones. In winter anticyclones, cold conditions lead to an increase in demand for energy. Low wind speeds reduce the potential for wind power. RECREATION AND TOURISM Demand for outdoor recreational facilities and activities increases with anticyclonic weather in summer. Economic activity in the leisure and tourism industries increases under anticyclonic weather in summer. In winter, frost and fog may be hazardous to outdoor recreational and leisure activities. Some soccer clubs have heated pitches to overcome the problem of frozen soil. 7