Weather: Weatherproof designs for underwear

The rounded and spiky symbols on weather charts should tell us all we need to know about the movements of air masses in the sky. When two air masses have a full frontal collision, however, the result can be confusion - as we have seen this week.

Perhaps the most frequent request I receive from readers is to explain the rounded and pointy bits on weather maps - the Macphersons and Madonnas, as I like to think of them. I have resisted this so far because I find them confusing too. But the challenge cannot be shirked for ever, so here goes.

Our changing weather is brought to us in air masses that originate over regions of uniform terrain, either oceans or land. In the past week, the warmer, wetter weather has been due to tropical maritime air from the south west, originating over the south Atlantic, while the biting cold has been mainly Arctic, blowing straight down from the north. The average temperatures at Kew in January of Arctic air vary between -2C and +2C, while the average for tropical maritime air is between 8C and 11C. So while the south-west wind has been blowing, we were breaking records for January warmth; then we all froze when the northerly wind took over.

With that in mind, we can now try to see how it is reflected in the weather map and those curious symbols that look so like designs for the underwear of female actors in Star Trek. Over the past few days, the warm front from the south west has been clearly visible. Remember that the heavy lines show the front of air masses (that's why they are called fronts) and the symbols are placed on the side of the lines indicating the direction of movement.

So there has been a constant flow of warm, wet air from the south west. So far, so good, but the messiness in the picture is the result of that air mass meeting the cold air coming from the north. Over the weekend, we saw pure cold fronts reaching us from the north and north west, then bending round eastwards into Continental Europe. Over the past few days, however, the two air masses have been fighting it out, resulting in occluded fronts - designated by alternating rounded and spiky symbols. When this happens, one thing to look for is whether the spikes and curves are on the same or opposite sides of the line. If, as is most often the case, they are on the same side, we have cold and warm air moving in the same direction. The cold air, being heavier, will blow beneath the warm, forcing it upwards, often leading to rain. Since cold fronts travel faster than warm, this type of occlusion is common. When cold and warm are travelling in opposite directions, however, the result may be a stationary front with little movement as they jostle for air space.

Looking at the various directions of the frontal systems, we can see great effects of the swirling created when the warm and cold masses met, exacerbated by cold Polar Continental air coming from the East and the usual low pressure areas coming from the west.

Under such unsettled conditions, it is hardly surprising that the weather has been so changeable. The warmer south-westerlies may be winning at the moment, but the most sensible forecast seems to be: wait and see.

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