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Sea Level Pressure and
1000-500mb Thickness
The colored contours
indicate sea level pressure in millibars. High pressure is red, low
pressure in green or blue. Only the last 2 digits shown -- sea level
pressure is usually around 1000 millibars, so add 1000 to values in the
range of 00-50, and add 900 to values in the range of 50-98.
o Low sea level pressure
indicates cyclones or storms near the surface of the earth. High sea
level pressure indicates calm weather.
The black contours indicate
the vertical distance, or thickness, between the 1000 millibar surface
and the 500 millibar surface, measured in tens of meters.
o Since air behaves nearly as
an ideal gas, and vertical distance is proportional to volume over a
specified surface area, the thickness between two pressure levels is
proportional to the mean temperature of the air between those levels.
Thus, low values of thickness mean relatively cold air.
o The 540 line is highlighted,
since this line is often used as a rule of thumb to indicate the division
between rain and snow for low terrain. When there is precipitation where
the thickness is below 540dam, it is generally snow. If the thickness is
above 540dam, it is usually rain (or sleet if the air next to the surface
is below freezing).
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