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Atmospheric Dynamics
Summary of Salient Results
Lecture 5 – Meteorological charts
1. Surface charts
These are similar to the ones printed in
newspapers etc. Observations are shown as
station circles with a cluster of symbols and
numbers around them, the most important of
which are shown in the diagram. Pressure is
in tenths of a mb with the leading digits
omitted (i.e. 1010.7) and temperature/ dew
point in tenths of a degree (5.7° and 5.6°).
Wind barbs show the direction the wind is
coming from and the speed in knots (2 kt = 1
ms-1).
From these charts comes the concept of synoptic scale – major features have horizontal scale ~
1000 km and pressure variation ~ 30 mb. (Remember 1° latitude = 110 km). They also illustrate the
concept of geostrophy – wind tends to blow along isobars, especially over the sea. Fronts are areas
of cloud and rain, as illustrated by satellite images.
2. Upper air charts
Derived from radiosonde observations, so the only measurements are p, T, TD and wind. These
charts plot variations in height on a constant pressure surface, which is equivalent to the variations
in pressure at a constant height in the atmosphere. As we are primarily interested in gradients in
pressure, not its absolute value, we can transform the pressure gradient ∂p/∂x|z using the
mathematical identity:
p z x
p
z p
z
.
.
 1
so

.
 g
by the hydrostati c equation
x z p x z p
x z
x p z x
x p
This means that the horizontal gradient in p at a constant height is just ρg times the horizontal
gradient in z at constant pressure – i.e the height of a pressure surface.
These charts are less detailed than surface charts and illustrate both the synoptic scale and
geostrophy more clearly – winds are more parallel to the isobars now.
As we look at a succession of charts in the vertical two things become evident:
a) the pattern doesn’t change – the depth scale of synoptic features is comparable to the depth
of the troposphere, or 10 km.
b) the winds get stronger with altitude.
3. Geostrophy
Purely empirically, we observe that the winds tend to flow along isobars and the wind strength
increases when the isobars are close together – i.e. speed is proportional to the horizontal gradient in
height or pressure.
When the flow is curved this relation changes.
Anticlockwise curvature (in the N. hemisphere) is cyclonic. Winds are slower than geostrophic.
Clockwise curvature (in the N. hemisphere) is anticyclonic. Winds are faster than geostrophic.
My email: Geraint.vaughan@manchester.ac.uk
My web page: http://tinyurl.com/mh5jg