Friday, 13 June 2008

What is Wind?

It’s Father’s Day on Sunday (too late for a card now!), but the weekend is also the European Wind Energy weekend. I was asked one of those simple yet hard to explain questions like why is the sky blue (for a later week’s discussions) this week - why are there winds? Well, the answer I gave was very simply the air moves from points of high pressure to points of low pressure, which creates our wind – of course it’s a lot more complicated than simply describing what we call ‘the pressure gradient force’, but that’s basically why we have wind.

The air doesn’t always manage to move directly from High to Low pressure because it can be affected by the wider scale flow of our atmosphere (on the big scale) and local temperature differences (on a small scale), so like lots of things in meteorology we have effects going on at many levels or as we call it spatial scales.

Speaking of scales, and we like our measuring scales in meteorology, we have lots of ways of measuring winds. The most famous of course is the Beaufort Scale (named after its inventor Admiral Sir Francis Beaufort) which measures wind in a range 0 (clam) to 12 (hurricane force). For stronger winds, specifically for tornadoes, we have the Fujita scale after Ted Fujita (or more correctly now the Enhanced Fujita scale), which goes from EF1 to EF5 and relates to the scale of damage, and we have the Saffir-Simpson scale for Hurricanes, which goes from Category 1 to the most severe at Category 5.

One strong wind that has become more commonly talked about in a few recent TV documentaries is the Jet Stream (or more specifically the Polar Jet Stream). This is a very strong jet of air that travels at great speed high up in the atmosphere, in fact about 10km up (or for those of you not yet metric, that’s just over 6 miles up). This jet stream can often bring bad weather across the Atlantic to the UK, and last summer in particular when the jet was over the UK (instead of its more usual northerly summer position) we saw some pretty unseasonally wet and windy weather.

I once read that if we could harness just 1% of the power of the jet stream then we would have enough energy to power the world’s current energy needs – but 10km is a heck of a long power cable!

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