
 CHAPTER TWO
AVIATION WEATHER
page. Therefore, in Figure 21, the winds are from the northwest. At the end of this stick are any
numbers of barbs, which come in three shapes, to indicate the wind speed. A long barb
represents 10 knots, short barbs are 5 knots, and pennants are 50 knots.
The numbers to the left of the station symbol indicate the temperature (top left) and dew point
(bottom left). In between the temperature and dew point, there may be a symbol from Figure 22
representing the present weather at the station. Additionally, the circle (or square) may be filled
in to represent the amount of sky covered by clouds, in eighths. An empty circle means clear
skies, while a fully darkened circle indicates a completely overcast sky (also from Figure 22).
Figure 22 Major Station Model Symbols
The righthand side of the station model describes the pressure at the station. On the top right,
there will be three digits to represent the sea level pressure (SLP) in millibars and tenths. Since
SLP will always be somewhere around 1000 millibars, the hundreds digit (and thousands, if
present) is dropped, and the decimal point is also omitted. Thus, depicted pressures beginning
with large numbers (such as a 9) really start with a hidden "9" and pressures beginning with
small numbers (such as a 1) actually have a "10" in front of them. Below the current SLP is the
pressure tendency over the last three hours, beginning with a (+) or () sign to denote an overall
rise or fall, and then the value of that total pressure change. After this notation is a set of two
24 Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability

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