middle-latitude tropospheric circulations

middle-latitude tropospheric circulations People who live in latitudes between about 30° and 65° are accustomed to much greater changes in the weather from day to day than those who live in other regions. This region is characterized by weather systems which develop quickly and move quickly. Anticyclones (high-pressure systems) and depressions or cyclones (low-pressure systems) are responsible for most of these changes in weather. Most of the particularly dramatic changes in weather are associated with weather fronts, which are an integral part of depressions.

Horizontal variations in temperature are greater in middle latitudes than anywhere else in the troposphere. Bounded at low latitudes by the warm tropical air, and at high latitudes by cold polar air, there is a strong north–south temperature difference. The region of greatest temperature difference is the polar front. It is at this front that depressions usually develop. The depressions gain energy from the temperature difference. This is an unstable process. When the temperature gradient is strong and a depression begins to evolve, the development is rapid and, as it grows, it circulates air to remove the temperature gradient. When the depression is at its most vigorous the temperature gradient in that region is at its weakest. When it is past its peak the depression loses its energy gradually through friction, and the temperature gradient builds again until another depression develops. Because the temperature gradient is usually not completely removed, depressions can develop in quick succession, forming one after another. The whole process takes about five to six days (Fig. 1), during which the depression has been moving, usually from west to east, as well as developing.

At the beginning of the process the polar front separates a warm air mass from a cold air mass (day 0). A depression usually starts to develop when a region of divergence passes over the front at higher altitudes. This region of divergence removes some air from above the front and causes a drop in atmospheric pressure at the Earth's surface so that a small low-pressure area develops (day 1). Air circulating round the low-pressure area pushes the warm air ahead of the depression while the cold air sweeps in behind the depression (day 2). The front ahead of the depression is a warm front because warm air is replacing cold air. The cold front behind the depression moves faster than the warm front and eventually catches up with it. This happens first near the centre of the depression, where the fronts are already close together (day 3). As the cold front reaches the warm front it forms an occluded front. At this stage the pressure at the centre of the depression has reached its minimum value and the centre of the depression begins to move more slowly while the fronts move further ahead (day 4, Fig. 2). Now that the development processes have ceased, friction starts to dominate and the central pressure of the depression starts to increase while slowing further and turning slightly poleward (day 5). The cold front trailing behind the depression is now the only remnant of the original polar front, and further depressions can develop on this front (day 6).

This view of the life cycle of a depression is easily illustrated on a weather map but is essentially a two-dimensional view of a three-dimensional process. The fronts are not lines but sloping surfaces; the lines are the places where the sloping surfaces meet the Earth's surface. The air not only moves around the depression but moves upwards at the fronts to produce cloud and precipitation. At upper levels the wind speed increases with height. This is a result of the equator–pole horizontal temperature gradient, which requires that the wind speed increases with height. The maximum wind speed occurs close to the tropopause at about 12 km. This core of high wind speeds is the jet stream, from which commercial aircraft sometimes benefit as a tail wind on west-to-east journeys.

Depressions can sometimes develop much more rapidly than usual (explosive cyclogenesis), producing a drop in central pressure of perhaps 50 millibar (hPa) in 24 hours. A typical pressure change is about 30 to 40 millibars decrease in 48 to 72 hours.

High-pressure systems (anticyclones) form between depressions. These are part of the same dynamical system as the depressions themselves and the increase in pressure is produced by convergence aloft in the same way as the depression is produced by divergence aloft. A more significant type of anticyclone is a ‘blocking’ anticyclone, which develops in one location and moves very little, while forcing depressions to alter their track around the block.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today (Chapter 13). West Publishing Co., St Paul, Minnesota.
Palmen, E. and and Newton, C. W. (1969) Atmospheric circulation systems. Academic Press, London.