Special Circumstances

CHAPTER TWO

METEOROLOGY FLIGHT PLANNING

Wind information on the Winds-Aloft Forecast is given with a series of four digits. The first two

represent the true wind direction to the nearest ten degrees true, and the last two digits represent

the speed in knots. For example, 2435 indicates a wind from 240 T at 35 knots.

For most altitudes, the temperature follows the wind information in a set of two digits that may

or may not include a sign for positive or negative. For example, 2435 + 07 indicates the wind

will be 240T at 35 knots with a temperature of +7 Celsius. Notice that all temperatures are

negative above 24,000 feet as indicated in the heading information by the phrase "TEMPS NEG

ABV 24000." At these altitudes, all the digits are run together, eliminating the redundant minus

sign between the wind and the temperature. For example, 274650 forecasts a wind from 270 T

at 46 KIAS with a temperature of 50Celsius.

Special Circumstances

The above procedures are used for all "normal" wind information; however, there are exceptions

for unusual wind conditions. A direction of "99" indicates a variable wind direction. When

forecast wind speeds are less than 5 knots, direction is difficult to determine, and the winds are

called "light and variable," and the code "9900" will be listed.

When a wind speed of 100 knots or greater is forecast, the simple four-digit wind code no longer

works satisfactorily and an additional set of rules is used. For example, if the winds are forecast

to be 230 at 145 knots, the normal code would require five digits, requiring a change to the

format of the entire Winds-Aloft Forecast. Therefore, if you see a direction that would translate

to be greater than 360T, it was not a mistake; it is this extra rule. The wind was encoded by

adding 500 to the direction and subtracting 100 knots from the speed, thus requiring a total of

only four digits again. To decode such winds, then, one must subtract 50 from "unrealistic"

direction codes and add 100 to the indicated speed. For example, a code of 7345 would forecast

winds of 230T at 145 knots. If winds are forecasted to be 200 knots or greater, the wind group

is coded as 199 knots. For example, 8299 would be decoded as 320T at 199 knots or greater.

Additionally, it is sometimes impractical to forecast the temperature and wind. This is

particularly true for conditions near the surface, where the temperature is more likely to deviate

from the standard lapse rate, and where the winds are more likely to be gusty and variable due to

thermal or mechanical turbulence. So, for the following conditions, wind and temperature are

omitted from the Winds-Aloft Forecast.

Wind information is never forecast for altitudes within 1500 feet of the surface.

Temperature information is never forecast within 2500 feet of the surface.

Temperature information is never forecast for the 3000-foot level.

2-24 DATA DISPLAYED ON WEATHER IMAGERY PRODUCTS