Fergal Tierney writes:
Depressions / Lows / Cyclones - there are many names used to describe the kind of unsettled Atlantic weather systems we get in Ireland, but they all mean the same thing - an area of low atmospheric pressure, bringing wet and windy weather. But how do they form?
Firstly, it may be of help to review the articles on
Atmospheric Pressure and Pressure Levels and
Airmasses. Depressions normally form along what is called the Polar Front, the boundary between warm, humid tropical airmasses from the south and cold, dry polar airmasses from the north. There is a marked temperature difference (or
temperature gradient) along this front, not only at the surface but also extending throughout the whole depth of the troposphere, up as far as the stratosphere. The presence of this temperature gradient in the upper troposphere is what causes the
Jetstream, a fast moving westerly wind that meanders around the globe between around 40 - 70° North and South, and at an altitude of around 30,000 - 40,000ft. In wintertime, the average location of the jetstream shifts equatorward, and in summertime it shifts poleward. The jetstreams strength and direction is not constant, however, meandering north and southwards in places, and varying in strength from between 0 - 250mph. This variation can be seen in an upper chart as a series of
Troughs (southward-pointing kinks containing cold air and hence low geopotential heights) and
Ridges (northward-pointing kinks containing warm air and high geopotential heights), forming a series of so-called
Rossby Waves around the globe.
We can see that the jetstream flows southeastwards along the western flank (edge) of a trough, curves around the base of it, and then flows northeastwards along its eastern flank, up along the western flank of the neighbouring ridge. It then flows over the top of this ridge and back down southeastwards on its eastern flank. This pattern repeats around the globe (though in some cases the jetstream may become detached or split and flow in two directions, or even dissapear completely in some areas).
Areas where the jetstream is strongest are referred to as
Jetstreaks, as shown below. These jetstreaks are key factors in the formation of surface low pressure systems. Jetstreaks are divided into four regions, or Quadrants, as illustrated. Both the Left Front Quadrant (LFQ) and the Right Rear Quadrant (RRQ) are areas where the upper level flow is spreading apart, or
diverging. This upper divergence causes a "void" to develop at the top of the atmosphere, and air from below must rise and "fill this void". In turn, air at the surface must
converge and rise in order for this compensation to occur, a bit like a vacuum cleaner, although the speed of this rising motion is only in the order of cms/second. This rising motion causes condensation to occur, leading to clouds and precipitation. This divergence/convergence process is called
Quasi-geostrophic (QG) Forcing, and causes the pressure at the surface to drop. The stronger the upper divergence, the stronger the surface convergence, and faster the drop in pressure. This is one reason why depressions are stronger in winter than they are in summer, as the wintertime jetstream is a lot stronger due to a stronger temperature gradient along the Polar Front. Another reason is due to the cold polar airmasses over relatively warm seas.
10th November 2010. Storm Carmen develops in the western Atlantic, underneath the Left Front Quadrant of a strong 80 m/s (180 mph) jetstreak east of New Foundland. Yellow lines are isotachs (lines of equal speed), in m/s. Base image from Satreponline.org
An average depression has a lifetime of around 5 days. On a surface synoptic chart it starts off as a kink in the polar front, as shown in (A) below. As it deepens, it starts to circulate around this kink (anticlockwise in the northern hemisphere, clockwise in the southern hemisphere) and forms a closed circulation centre. To the east of the centre, warm air flows northwards, marking the Warm Front. To the west, cold air flows southwards, marking the Cold Front, as in (B). As the upper divergence continues to cause the pressure to drop, the circulation tightens and the windspeeds increase. The cold front moves southeastwards and starts to catch up with the slower-moving warm front. Eventually it catches and starts to undercut under the warm front, lifting it off the surface. This process is called
Occlusion, and marks the peak stage of the depression's life, (C). After occlusion has started, the pressure usually stops falling, and the depression will start moving to the left and slow down. It will have done its primary job of dispersing the strong temperature gradient along the polar front (D). It will start to fill, and may even set the scene for one or more new lows to develop, possibly leading to a
Family of Lows.
As with most things in weather, things don't always go as smoothly as this in reality, but you now have a general idea of how one of our more common (but probably more unwelcome, especially in the summer) vistors works. Depressions can, as the name implies, be depressing, and lead to seemingly endless spells of unsettled weather, like in the recent wet summers, which were due to the jetstream taking up a more southerly position than normal. They can give abundant rainfall, as in November 2009, or strong winds, as in early November 2010, but they are also the reason why we live in an Emerald Isle, and do not suffer from the serious droughts of other countries.
