Reading the winds aloft forecast

The winds and temperatures aloft forecast, still called FB or FD by most people, is a wall of four and six digit numbers. Each group is a wind direction, a wind speed and usually a temperature, compressed into as few characters as possible.

Decoding a group

Take 2820+05. The first two digits are the wind direction in tens of degrees true, so 28 means 280. The next two are the speed in knots, so 20 knots. The last part is the temperature in Celsius, plus 5. So: wind from 280 true at 20 knots, plus 5 degrees.

A four digit group like 2310 is the same thing without a temperature. Temperatures are omitted for the 3,000 foot level and for any level within 2,500 feet of the station elevation.

The special cases that catch people

Light and variable. 9900 means the wind is forecast to be less than 5 knots, direction not specified. It is not 990 degrees at nothing.

Winds over 100 knots. If the direction digits are greater than 36, subtract 50 from the direction and add 100 to the speed. So 731960 is 73 minus 50 equals 23, meaning 230 degrees, and 19 plus 100 equals 119 knots, at minus 60 degrees. Speeds of 200 knots or more are encoded as 199.

Temperatures above 24,000 feet. Assume negative. The sign is dropped because it is always minus up there.

Direction is true, not magnetic. The tower gives you magnetic wind, the forecast gives you true. Mixing them up puts your correction angle out by the local variation, which is around 7 degrees in Florida and much more elsewhere.

Interpolating to your altitude

Levels are published at 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000 and 39,000 feet. Fly at 4,500 and you are between the first two.

The tempting shortcut is to average the directions and average the speeds. That is wrong when the direction changes much between levels, because a wind veering from 230 to 280 does not blow from 255 at half strength. The correct way is to break each level into its north-south and east-west components, interpolate those two numbers linearly, then convert back into a direction and speed.

For 230 at 10 and 280 at 20, halfway up, the vector method gives roughly 264 at 14. Averaging directions gives 255 at 15, close in this case but increasingly wrong as the turn between levels increases.

Temperature matters more than students expect

The same forecast that gives you wind gives you temperature aloft, which tells you the freezing level. If you are IFR and the forecast at your altitude is at or below zero in cloud, that is an icing conversation, not a footnote. It also feeds density altitude, which is what actually determines your true airspeed and your climb performance.

Picking an altitude for the wind

Once you can read the forecast, the obvious next question is which altitude to file. A tailwind twenty knots better two thousand feet up may not be worth the climb on a short leg, because the climb costs time and fuel before you ever see the benefit. EasyNavLog compares every legal cruising altitude for your route, with the climb already accounted for, and flags the freezing level from the same forecast temperatures.

Which altitudes are legal in the first place is a separate question, covered in VFR and IFR cruising altitudes.

EasyNavLog is a planning and training aid, not a substitute for an official briefing. Verify every number against your POH or AFM, current charts and an approved weather source.