Weather Musing...April Showers

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Weather Musing...April Showers

Postby weatherman » Fri Apr 11, 2008 9:34 am

Hi all,

Just thought you might be interested in the classic example today of an unstable day developing, with large Cu and Cb clouds forming. I might be teaching granny to suck eggs, but hopefully not everyone!

Weatheronline produce forecast skew-t diagrams in 6-hour steps to 36-hours ahead. By using these you can see the potential for showers to develop (as well as skew-t's unlocking the door to some other goodies!).

The red line is the dry bulb temperature, the blue line is dewpoint and the grey line is the predicted path of an air parcel at saturation (more on this in a bit). Down the left hand side are heights in mb, remember roughly near the surface 1mb=30ft.

Where the red and blue lines touch the air is saturated, i.e. cloud. The further apart, the drier the air.

Anyway, just wanted to show you how things develop through today. Heres the forecast skew-t for Birmingham for 0600Z:


Now, the red and blue lines are fairly close together, espcially at lower levels in the atmosphere. But the grey line is to the left. This means the air parcel will not rise.

Next have a look at the 12z skew-t forecast:

Now the grey line is way to the right of the red line. This says that is a parcel of air were to rise it would do so right the way up to 425mb! This is large Cu and Cb cloud. The base of the cloud would be around 920mb.

By the 18z forecast:

Things are starting to settle down again, but still the grey is to the right of the red and so could still rise, but it won't be long before it is to the left.

On days like those we will see over the weekend this is a good way of predicting the onset of showers. Bear in mind it only works in convective weather, not in frontal situations; then we have to look for different information from the skew-t.

Hope that is useful. Mega busy today so can't promise to answer lots of questions, but you can ask!

Best wishes,
Simon Keeling, PhD MSc, FRMetS
My new book The Pocket Weather Forecaster is out now. Read sample pages here.

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Postby Silkie » Sun Apr 13, 2008 10:15 pm

Jeez Simon!

Should I know this stuff or can I just rely on you to tell me when a convective situation will be showery or thundery? :)

Seriously, it is amazing to see the kind of tools available to those who know what to do with them and the insights that can be gained into the infinitely complex system that is the weather.
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Postby sahona » Mon Apr 14, 2008 10:56 am

Never seen these graphs before, and like Silkie, not really sure I want to add them to the in-tray of pre-sail tasks. I think I'll accept the clouds, try to be aware of what they mean, and leave the technicalities to the technically competent....
Having said that, we are in the middle of the varnishing season at the moment, so it's very useful information.
Thanks Simon, that's the lid off another jar of worms! 20' - 30' Berths available, Clyde.
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