Monday, 23 May 2011

Bouncy bouncy... a small lesson on met.

Bouncy Bouncy
Oh what a good time!
Bouncy bouncy....  Hmm, enough with the Boosh references, but you should have got the idea, it's been rough out there today! If you watch the UK weather forecast you will have gathered that there's been a big old low pressure going over us today, and this one was particularly vile. For those of you who've done met at college or just know this stuff, please feel free to skip this one, I don't want to teach grandmother to suck eggs, but for those of you who've not yet had that pleasure:

Here's the surface pressure analysis for midnight this morning:

And then the forecast for 1200 midday:

 The thin black lines are called isobars, they are basically contours, like you get on an OS map, only these show pressure. A low pressure system moves in an anti-clockwise direction and as it moves, so the wind goes too. Wind generally moves along the isobars, and on a low pressure system the wind circles inwards. The closer the lines of equal pressure, the faster the wind will be. (Think of it as a funnel, the closer the isobars, the steeper the slope of the funnel and the faster a ball will roll down the inside). The red lines with semi-circles on are warm fronts, and the blue lines with triangles on are cold fronts. Our part of the world is a mixing bowl of sorts; to the north we have cold air around the pole, and to the south we have warm air around the equator. If the earth didn't spin and butterflies didn't flap their wings (bit of chaos theory there for you) then these two masses of air would sit side by side quite happily, but if anything stirs things up a bit then things get interesting. When warm air and cold air meet and mix, they don't want to mix, so one body of air will actually slide under the other. This makes air rise, and when air rises it cools, and the water vapour in it condenses, condensed water vapour is generally know as clouds, and you know what they bring! So that cold front is a big ass wedge of cold air, pushing up warm wet air (warm air can hold more water than cold air) from the south into the atmosphere, which is why a cold front brings with it rain in buckets. 

So there you go, that's not the whole met syllabus, there are further complications and possible permutations of the scenario, but those are the basics. High pressure systems of course go clockwise (think of the two types of system as cogs) and are gentler beasts. Then when you head to the other side of the equator things flip and low pressure systems rotate clockwise and high pressure systems rotate anti-clockwise. The direction in which they move depends on what latitude you are at, this is is due to something called the Coriolis Effect, caused by the spin of the earth. Google it, if you're bored!


  1. Winds blow anti-clockwise around a low, IN THE NORTHERN HEMISPHERE. They Blow Clockwise around a low in the Southern Hemisphere.

    That doesn't really matter to those in the Irish Sea - but it makes the information more accurate. Some guy might be joining a ferry in New Zealand - and reading your blog - he'd get the wrong message in that case.

    But you blog is going to be more attractive now - it's what aspiring seafarers will want to read - so be as carefull and accurate as you can be.

    You are now doing fine though - no one remembers everything all of the time.

    Albatross :)

  2. Albatross- I refer you to the last paragraph of the post :)

    It's not meant to be a full blown lecture on met, just a taster.