"TudorHgh" wrote in message
...


but
I have also got some patches of cirrus which are coming
from a NW direction. Now if this was the first signs of the
front, then shouldn't they simply appear to be coming from the SW?



Not necessarily, Col. The advection of warm air from the SW means
there
must be some form of upper ridge. Ahead of this, i.e. before the surface
warm
front, the upper winds will be W'ly or even NW'ly. I have seen Ci moving
thus
many times ahead of a Low. It also ties in well will the rule-of-thumb
that
with advection of warm air the wind veers with height, and vice versa,
though
this latter case is difficult to observe from the ground.


A book I read many years ago explained this using a concept called the
"thermal wind". Basically, this notes that pressure falls more rapidly with
height in cold air than in warm due to the density difference so at high
levels the wind tends to blow parallel to the boundary between warm and
cold, with the warm air on the right and the cold air on the left (think
upper westerlies!).

From this it follows that (in the Northern hemisphere) if you face the upper
wind (as seen in the high level clouds), low cloud moving in from the left
indicates warm advection between the cloud layers and vice versa. Perhaps
the purists may not find it as satisfactory as the blurb about the troughs
advancing, etc, but qualatatively it seems to me easier for the beginner /
man in the street to get their heads around.

The advection of cold air behind a cold front is easily seen provided
cumulus development starts close enough to the upper cloud overhang. It may
also be visible at the end of the rain if low cloud forms in the cold air as
it first arrives.

This observation is actually a very useful rule for "local forecasting".
The rate of advection is greatest the nearer to right-angles the wind
directions are, so you get a qualatative estimate of how active the system
is and how rapidly the rain or whatever may be advancing. If a front
"stalls", the lower wind may "back" until it is blowing in the opposite
direction to the high clouds. You can also detect the approach of "waves"
on a trailing cold front by careful observation, or approaching thundery
conditions by cold advection over warm where several levels of cloud are
visible.

One has to bear in mind the following:

1) Very low cloud level or surface winds will be "backed" to the upper
wind by surface friction, giving a false indication of warm advection. A
little allowance may need to be made.

2) In hilly or mountainous areas, the topography can disrupt airflow to a
level somewhat above the summits and this may render this sort of simple
forecasting tool worthless.
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