Weather School Musing...Christmas Stratocumulus

[weatherman]

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Next Sailung Weather School (Part 1) to be held Saturday 7th February 2009. 3 places remaining. See http://www.weatherschool.co.uk for more details.
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Hi All,

As we dust away the Christmas cobwebs many of you will have been out and about of the last few days.

It’s been a classic one for anticyclonic stratocumulus cloud to form too. Thought you might be interested in a post looking at the weather we have right now and how you can relate charts and skew-t diagrams to the current conditions.

1. We’ll start with a look at the analysis chart for midnight (0001 GMT) on Saturday 27th December. The large high pressure area extends from Scandinavia to the British Isles; 1044mb (or hPa as we should officially call it) is fairly high pressure for this time of year. You can see that the easterly flow is well established over the south of the British Isles and it is here that I want you to focus your attention during this Musing.

There’s a cold front marked on the chart extending from central Scotland to western Ireland. I think this front has been drawn by the forecaster to delineate the zone of drier air which broke up the real anticyclonic gloom that pervaded southern England and Wales on Christmas Eve.

[image]http://www.wetterzentrale.de/archive/20 ... 081227.gif[/image]


2. This high resolution MODIS satellite picture from University of Dundee is for 1200 UTC on 24/12/08). This picture shows the cloud breaking off the North Sea but much of the southern UK still under the cloud (the south coast can be seen under the area of white around ¾ of the way down the picture)
Note that you may need to register with the University of Dundee to view this picture but it is free and easy to do.

[image]http://www.sat.dundee.ac.uk/abin/piccyj ... 0/ch38.jpg[/image]


3 Now, what I really wanted to show you was the classic signature of SC cloud from the skew-t diagram for Nottingham. This is the skew-t as produced by the University of Wyoming.

It has all the hallmarks of the stable, anticyclonic stratocumulus one expects from such a winter high pressure system.

Firstly, we can see the cloud height quite clearly at around 890hPa. It is at the point where the dry bulb temperature (that is the normal temperature of the air, shown by the right hand black line) and the dewpoint (shown by the left hand black line) are close together.

With a surface pressure of about 1020hPa and 1hPa equalling around 30 feet we can assume a cloud height of about 3900ft. The cloud is only very thin and it is hard to determine a top, probably no more than about 50 feet.

Above the cloud the temperature rises quickly, from +2C at 890 hPa to +8C at 850 hPa. Again, this is the signature of descending air in an anticyclone, because as the air descends, it warms, hence the formation of the inversion and the familiar ‘Roman Nose’ appearance of the skew-t. And the air was very dry above the inversion, see how far apart the dry bulb and dewpoint lines are right the way to 400 hPa( about 25,000ft).

[image]http://weather.uwyo.edu/cgi-bin/soundin ... STNM=03354[/image]



4. Gradually, as the easterly winds set in visibility below the inversion deteriorates as smoke and pollution from the continent drift into the airmass. This photgraph (taken by Vince Chadwick on 27th December over Blackpool and kindly given permission for me to use) shows the dark, mucky layer below the inversion, but the clearer more blue skies above.

[image]http://i18.photobucket.com/albums/b132/ ... 011res.jpg[/image]


There you go, thanks for indulging me once again. Hopefully this has been interesting for you?

Have fun,
Simon