Atmospheric
Circulation
Analyzing air pressure patterns
Meteorologists have a variety of ways to visualize weather data, a map being the
most common. To analyze pressure patterns, a constant height map is often used.
A constant height map (also known as a "constant height chart") shows the
distribution of pressure at sea level. Isobars, lines connecting points of equal
air pressure are used to show pressure patterns on constant height maps. Most of
the maps of air pressure in this book are constant height maps.
Figure 6.4 Constant height map with isobars
Another map that is used to analyze pressure patterns is called a constant
pressure map (also known as a "constant pressure chart"). A constant pressure
map shows the change in elevation of an isobaric surface which is a surface upon
which the pressure is the same at all locations. By examining the height of an
isobaric surface relative to it's normal elevation, one can discern where areas
of high and low pressure are.
Figure 6.5 500 mb constant pressure (isobaric) surface
The 500 mb surface is commonly used to by meteorologists in weather forecasting.
The normal height of 500 mb surface is 5600 meters. Recall, that at the surface
we consider high pressure to be greater than normal sea level pressure and
low pressure to be less than normal sea level pressure. In a similar way
we can use the normal height of the 500 mb surface to identify where high and
low pressure areas are located.
Figure 6.6 Constant pressure map with contour lines
Let's consider a simple situation to understand how the height of an isobaric
surface relates to air pressure. Recall that air pressure is related to force
exerted by the weight of a column of air above a given point. If air temperature
varies through any part of an air column, the density and pressure will also
vary. Figure 6.7 shows a column with warm air to the left and cool air to the
right. The 500 mb surface is shown as the white surface dipping from left to
right. Let's assume that the surface pressure remains constant. When air is
heated it becomes bouyant causing it to rise and when cooled it sinks. As the
warm air rises more air molecules will be found above 5600 meters than
normal and thus the 500 mb surface is found at a higher
elevation.
To the right where cold air has sunk to the surface fewer air molecules than
normal are found above 5600 meter and thus the 500 mb surface is found at a
lower elevation. If the re are more molecules above 5600 meter in the warm
region, then high pressure is located at 5600 meters. If fewer air
molecules are above 5600 meters in the cold region, then low pressure is located at 5600 meters.
Figure 6.7 Air pressure - isobaric surface height
relationship
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