Weather: Weather Watch

Ever since the weekend, the forecasters have promised us blustery weather and rain today. That's what happens when a cold front runs into a warm one.

Yesterday's weather map has the typical formation of a cold front approaching Britain from the south east, preceded by two warm fronts. It's a picture that promises wind and rain. Since cold air circulating round a depression is heavier than warm air, it undercuts the warm air ahead of it. But cold air is also slowed less by friction than warm air, so near the surface of the Earth, cold-air winds tend to be stronger.

That's what brings us the blustery feature of the forecast. The rain comes as the air of the warm front rises and is cooled below its condensation point. It's the behaviour of the higher-level winds that makes precise prediction of where and when the rain will fall so difficult. Sometimes, high upper-level winds will take the warm, moist air ahead of the cold front. Then you can get the curious feature of rain preceding the sort of conditions that are usually seen as causing it.

Yesterday, prediction was made more difficult by the presence of easterly winds blowing in from the Continent, while a low pressure area was drifting in from the south west. Listen to today's forecast, and applaud if they get it right.