On 16/07/2011 12:03, Yokel wrote:


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.


Thanks Yokel. I sometimes think that a '3D' type chart may explain this
better than just looking at a normal synoptic chart with fronts.

--
Phil
Guildford