And now to the formation of low pressure systems.......

Some of first people to study formation of low pressure areas were from Norway. They were the first to come up with the concept of the Polar front. The polar front is a band of cold air formed by the meeting of the cold polar winds with the equatorial warm air. This meeting spawns mid latitude depression. The position of the polar front generally coincides with the meeting of the low surface pressure in the large circulation patterns formed by the Coriolis Effect

In the Northern hemisphere for example, the polar front is where the cold air coming from the north meets warn air from the south. These different blocks of air with different temperatures are called air masses. Warm air is less dense than cold air and therefore in a polar front the warm air tends to slide over the top of the cold air. And this generally happens before any kind of a, low is formed. Through a particular combination of circumstances a disturbance may appear at some point along the front. For example the north south air temperature difference may be particularly intense at his point or there might be some influence from an external factor like sea surface temperature.

Such a disturbance is known as Baroclinic instability. if this instability is strong enough the front will develop a wave on it, which will grow and intensify. The process of warm air sliding over the top of the cold air will be particularly intense in the area of perturbation so this will lead

The fully developed low
As the warm air slides over the cold air, the baroclinic perturbation begins to grow with intensity and the whole thing turns into a huge vortex with the air being sucked in and then deflected by the Coriolis effect resulting in a n anticlockwise rotation in the northern hemisphere and a clockwise one in the southern hemisphere. This section of the polar front can now be seen to split into a system of individual fronts-The warm front behind which is warm air and the cold front behind which is cold air. The area between these two fronts is called the warm sector and area where the surface winds are strong and blowing in the same direction for some distance.

This is the fully developed low pressure system,. If all the factors are correct and the surface pressure manages to reach a very low value then the wind will be strong and the waves will be big. On a weather chart a low can easily recognised quickly by a thick mass of closely packed isobars. The closer these isobars are packed the lower the surface pressure inside the depression, and so the quicker the air tries to rush in from outside and therefore the stronger the surface wind. Because the air rushing in is deflected by almost 90 degrees by the Coriolis force, the wind is more or less along the isobars
As the depression propagates from west to east direction, the warm and cold fronts eventually catch up with each other and they form an occluded front. This is when the low starts to weaken and ultimately soon loses its identity altogether

The longer a low stays in a mature state, the longer the two fronts stay apart and the longer the winds blow the bigger the waves will be. A very deep low that stalls in its path is rare but when it does occur it produces very large clean waves. Normally if a low like this exists it moves quite rapidly bringing in the next low which can cause a criss cross of different swell conditions being formed.

The Jet stream
Lows that move north east tend to deepen very quickly. There is a reason for this and its also to do with why the low moves the way it does. About 5000 m the behaviour of the upper air affects the pressure on the surface through three dimensional convergence and divergence combined with the Coriolis force effects.

Airflows at 5,000m are much stronger than those on surface. The Jet stream is a constant westerly wind that blows around the globe at mid latitudes but it has meanders which make the flow waver north and south. The path that a low follows closely tracks the track of the jet stream. The amount of energy pumped into the low by the upper air stream depends on how strong the jet is and its direction. So in effect the deepest lows tend to be where the jet stream is strongest. Lows form somewhere off eastern seaboard off the USA and then deepen rapidly while tracking toward Scotland and Ireland.

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