IRELAND is sometimes described as lying "in the mid latitude westerlies of the northern hemisphere". Now if you assess the wind regime some thousands of feet above the surface, you will indeed find that the westerly direction overwhelmingly predominates, but here at ground level, our winds are directionally volatile. The pattern is complicated by the presence of depressions.
When atmosphere pressure values for thousands of places on the Earth's surface are plotted on a chart, the most convenient way of making sense of the complex jumble of figures is to draw lines through points where the values coincide. These lines are called isobars, and nowadays, as you might expect, are usually put in place by a computer. In any event, the pattern that appears resembles the height contours on a map showing the physical features of an area of land.
The regions of low pressure, which appear as a series of concentric and almost circular isobars, are referred to as depressions, or simply "lows". As long ago as the early part of the 19th century it was recognised that storms were associated with these low pressure systems, but it took many years for meteorologists to figure out exactly what was happening that the wind blows along the isobars in an anti-clockwise direction around a depression, and that its strength is inversely proportional to the distance apart of the lines the closer together the isobars, the stronger the wind.
In real life, as distinct from its representation on the weather map, a depression is a vast swirl of air, not too unlike the whirlpool seen as the water is being emptied from a bath. This characteristic is very apparent from satellite pictures, where the bands of cloud associated with the weather fronts, or areas of rain, can be seen spiralling in an anti-clockwise direction towards the centre of low pressure.
Depressions are highly mobile. Many of those which affect Ireland make their first appearance over the ocean on the western side of the Atlantic, and can be easily followed on successive weather charts as they move north-eastwards at 30 or 40 m.p.h. In summertime their most common path takes them well to the north of Ireland, so frequently they do not interfere with our relatively settled weather. At this time of year, however, they tend to pass close to our northwest coast, where they are often at the "deepest" phase of their life cycle. This is why at this time of year we often experience strong winds and periods of blustery rain at regular intervals as has been happening recently.