Yokel wrote:
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.

Yes, I noticed exactly the same thing here when I was in the main
rain band. Generally moderate rain but every so often it would
suddenly intensify to heavy rain for just a few minutes.

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.

No, that's a good exlanation, it's right at my level
--
Col

Bolton, Lancashire
160m asl