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Blowing hot and cold

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Peter Gibbs Peter Gibbs | 12:30 UK time, Monday, 10 October 2011

Distance travelled ~ 725'583'200 km

The first weekend of October saw UK beaches packed with day-trippers, while Londoners headed to the parks in an effort to stay cool during what turned out to be the hottest October weekend on record. Gravesend in Kent reached 29.9C, hotter than most days this summer.

Just a few days later the shorts were back in the drawer, the coats were on and happy hikers were tramping through fresh snow on the Scottish mountains. Elsewhere, late summer warmth was replaced by snow in the Alps and in California's Sierra Nevada, just 3 months after the last of the previous winter's snow had melted, 2 feet of fresh stuff arrived.

Such wild swings in the weather are not unusual at this time of year as the seasons change. The exceptional UK heatwave was caused by persistent southerly winds bringing very warm air from North Africa and the Mediterranean, while the early Scottish, Alpine and Californian snowfalls were brought in by a rapid switch to northerly Arctic airstreams.

Having passed the equinox, the longer nights and shortening days in the northern hemisphere lead to a net loss of warmth during each 24-hour cycle. The Arctic ocean, surrounded by rapidly cooling land, begins to freeze over. Further south, the cooling is slower. The warm waters of the Atlantic and Mediterranean act as huge stores of summer heat. As executive producer Jonathan Renouf said recently in this blog (In a constant search for Equilibrium), weather is all about gradients. In this case it's the north/south temperature gradient that's tightening, which means a switch of wind direction can bring a big change in temperature.

So, wondering what to wear tomorrow? The answer, my friend, is blowing in the wind.


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