Several centuries ago it was recognised that storms were associated with low barometric pressure. But there was a paucity of reliable data, and it took many years for anyone to figure out exactly what was happening.
The key was found when the atmospheric pressure was measured simultaneously at many different points on the Earth's surface. When these pressure values were plotted on a map, the most convenient way of making sense of the complex jumble of figures was to draw lines on the chart through points where the atmospheric pressure had the same value. The regions of low pressure appeared, as they do today, as a series of concentric and almost circular lines which came to be called isobars.
Soon it was noted that the lower the pressure at the centre of these "depressions", the more vigorous and stormy the weather in the vicinity. Moreover, it was seen that the wind blew around the depressions in an anti-clockwise direction, and its strength was discovered to be inversely proportional to the distance apart of the isobars: the closer together the isobars the stronger the wind.
A depression, as we now know, is a vast swirl of air, not too unlike the whirlpool to be seen as the water leaves a bath. Many of the depressions which affect Ireland begin as a small disturbance in the vicinity of the Gulf of Mexico, and can be easily followed on successive weather charts as they move northeastwards at 30 or 40 m.p.h. across the Atlantic, becoming deeper and more vigorous as they do so. Their most common path lies close to the northwest coast of Ireland, from where they continue northeastwards towards Norway, before gradually "filling up" and dying away.
It is a cliche that everyone remembers exactly where they were at the time they first heard that President John F. Kennedy had been shot. For older meteorologists, a similar chronological beacon shining from the dim and distant past is the first sighting of a picture taken by a weather satellite; it is remembered as a dramatic visual revelation of a concept, vaguely realised in theory, but now graphically obvious, quite literally, in black and white.
In my case, I recall two things that struck me about that first image that I saw at Shannon Airport in the 1960s: amazement that the geographers should have got the shape of Europe so exactly right, and surprise that a depression, viewed from space, appears not as the series of concentric circles so familiar from the weather map, but as a line of cloud spiralling around and around inexorably towards the centre of the low.