Origin and nature
Anticyclones are areas of intense high pressure (typically above 1020mb). Anticyclones can occur in both winter and summer with varying effects, but both are typified by low wind speeds due to a weak pressure gradient and stable conditions with no clouds. The absence of clouds (and hence precipitation) is due to the fact that as the air molecules descend through the troposphere they warm at the Dry Adiabatic Lapse Rate (DALR- air cools or warms at 9.8ºC for every 1000m ascent or descent through the atmosphere) meaning condensation does not occur.
In contrast to depressions, anticyclones only involve one type of air mass which usually cover large areas and do not have any fronts. They are high pressure systems in which the air moves downwards towards the earth's surface. As the air descends, the molecules become compressed, the pressure increases and it warms. When air is warming, any moisture in the atmosphere is evaporated so no clouds can form. The sky is clear. Anticyclones can be very large, typically at least 3,000 km wide which is much larger than depressions. Once they become established, they can give several days of settled weather. Winds are very gentle or even calm in an anticyclone, move clockwise, and this is shown on a synoptic chart by widely spaced isobars
Origin and nature
Anticyclones are often described as BLOCKING ANTICYCLONES, this is because these ridges of high pressure can block the passage of the Polar Front Jet Stream across the British, and this in turn means that mid latitudes pass North or South of the British Isles.
This can lead to very long periods of stable weather in the UK. Anticyclones can also result in temperature inversions. Low level temperature inversions are formed in anticyclones when air that has been heated up during the day sits up in the atmosphere, sits on top of air that is cooled during the night through the rapid loss of heat from the ground. This can cause fogs and frost depending upon the season, and can cause atmospheric pollutants to build up in the atmosphere.
Causes of anticyclones
1. Poleward movement of high pressure cells – here, were the Hadley cell meets the Ferrel cell at 30°N or S the high pressure zone can move, creating high pressure in higher latitudes. In Britain this North African High can bring heat waves
2. Thermal or cold anticyclones are typical of continental INTERIORS and hence are less likely in the British isles. In cold polar regions or during winter at lower latitudes, the land cools very rapidly (since there is no moderating influence of the sea), chilling the air above. This radiational cooling (with temperatures as low as -40°C in Siberia!) causes the air above to contract and sink to the ground, raising the air pressure and reducing the likelihood of cloud formation
3. Convergence in the Polar Front Jet Stream – The PFJS is found 9 to 12km up in the Troposphere and meanders at different latitudes in waves known as Rossby waves. Here, the air moves in balance with lots of forces and is caused by different pressures, so moves according the Pressure Gradient Force. However, the Coriolis Force deflects the wind that is caused by Pressure gradients, so the net result is an upper level wind that blows parallel to the isobars, known as the Geostrophic wind or our jet stream. This jet stream meanders from latitude to latitude which introduces another force, the Centrifugal force. Here, centrifugal force around a ridge in the Rossby waves causes the air to speed up, and in the troughs it slows down. Where the air slows down going into the Trough and just out of the ridge it builds up, and much of this air is forced to sink to the surface causing HIGH PRESSURE or an anticyclone.
Associated weather in winter
Cold daytime temperatures - from below freezing to a maximum of 5 degrees
Very cold night-time temperatures - below freezing with frosts
Generally clear skies by day and night. Low level cloud may linger and radiation fogs (caused by rapid heat loss at night) may develop in low lying areas.
high levels of atmospheric pollution in urban areas, caused by a combination of subsiding air and a lack of wind. Pollutants are trapped by a temperature inversion (when air at altitude is marginally warmer than air at lower levels.
Associated weather in Summer
- Hot daytime temperatures - above 25 degrees
- Warm night time temperatures - may not fall below 15 degrees
- Generally clear skies by day and night
- Hazy sunshine in some areas
- Early morning mists, which disperse rapidly
- heavy dew on the ground in the morning
- the East coast of Britain may have sea frets or haars caused by onshore winds
- Thunderstorms may occur when the air has a high relative humidity
Common features of anticyclonic conditions. Fog is cloud at ground level and restricts visibility to less than 1 km. It consists of tiny water droplets suspended in the atmosphere.
1) Radiation fog
- Forms under clear night skies when a moist atmosphere cools through the radiation of heat from the ground surface and is encouraged by light winds that allow slight mixing of air. The air is cooled to it's dew point and condenses.
- This type of fog is common in winter, when long hors of darknessallow maximum cooling.
- The fog can persist all day. It disperses either through an increase in wind speed or by warming of the air causing subsequent evapouration.
- This form of fog is common under temperature inversions, which often occur in the valleys. In the evening, with clear skies and high humidity, the air on the upper slopes chill more quickly than that in the valley bottom.
- Cooling increases the density of the air, and it begins to move down slope. The cooler air accumulates at the valley bottom, pushing warm air upwards.
- The cold air now in the valley bottom will cool to it's dew point , create dense fog that can last all day and cause a severe frost.
2) Advection Fog
- Advection fog forms when a mass of relatively warm air moves horizontally across a colder surface. The air is cooled to it's dew point and condenses.
- This type of fog is most common around coasts and over the sea in summer. In such areas it is sometimes called a fret or a haar.
- As the fog moves inland, it warms and evapourates