On 16/07/2011 09:30, Phil Layton wrote:
On 16/07/2011 08:57, Col wrote:
In just a couple of hours, the rain band has developed from something
pretty light and fragmented into a far larger and intense area of rain.
What makes this happen so quickly, it was certainly well forecast.

Col,

There is an explanation on:

http://www.ukweatherworld.co.uk/

I cannot claim to understand it all!

The gist of it is that a disturbance in the upper troposphere (which is
where the 300mb level is) is interacting with the system below it. It
is interacting in such a way that the air in the lower to middle
atmosphere is being forced upwards. As this air is relatively warm and
moist, this forced ascent will produce plenty of condensation and hence
rain.

Upper troughs are normally associated with cold air aloft and so this
running over the warm "tongue" between the warm and cold fronts below
will destablise the atmosphere. This adds a convective element to the
rain, and it has been noticeable here in Ashurst that there has not been
continuous moderate or heavy rain but bursts (sometimes only a minute or
two long) of heavier rain embedded in generally light precipitation.

Someone else has mentioned the air being forced to rise as it reaches
the land, but this effect is taking place at a much lower level in the
atmosphere and is also more localised. This will produce areas of
persistent heavier rain on windward coasts and hills but will have much
less effect generally.

Someone can doubtless come along with a more technically rigorous
explanation, but I have tried to keep it as simple as I can.

--
- Yokel -

Yokel posts via a spam-trap account which is not read.