Reading winds and temperatures aloft

A winds and temperatures aloft forecast tells you what the wind and air temperature will be at a range of levels, and reading one lets you pick a cruising level and work out your real ground speed before you ever start the engine.

This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.

What the forecast is

The winds and temperatures aloft forecast, known in the United States as the FB or FD product, gives the forecast wind direction, wind speed, and air temperature at a set of standard levels above a station. In the US it is produced by the NOAA/NWS Aviation Weather Center, and the coding and decoding rules are set out in the FAA Aviation Weather Handbook (FAA-H-8083-28), the modern home of the material that older pilots will know from Advisory Circular AC 00-45. Other authorities publish the same information graphically as wind and temperature charts, often called spot wind or significant weather charts.

The standard levels in the US forecast are 3000, 6000, 9000, 12000, 18000, 24000, 30000, 34000, and 39000 feet. There are two sensible omissions: no wind is forecast for a level within 1500 feet of the station elevation, and no temperature is forecast for the 3000 foot level or for any level within 2500 feet of the station elevation.

How a group is built

Each level is given as a short coded group. A full group has six characters: two for direction, two for speed, and two for temperature.

• Direction is the first two digits, in tens of degrees true. So 24 is 240 degrees. This is the same true reference used by the written wind in a METAR, not the magnetic wind you hear on the radio.
• Speed is the next two digits, in knots.
• Temperature is the last group, in Celsius. From 6000 feet up to and including 24000 feet the sign is shown. Above 24000 feet the temperature is always negative, so the sign is left off.

Two special cases are worth committing to memory:

• 9900 means the wind is light and variable, less than 5 knots.
• When the forecast speed is 100 knots or more, the coders add 50 to the direction and subtract 100 from the speed so the numbers still fit two digits each. A direction figure between 51 and 86 is your clue. To decode, subtract 50 from the direction and add 100 back to the speed. A speed of 200 knots or more is always coded as 199, which appears as a speed of 99 after the addition.

A worked example

Here is a forecast for one station, level by level:

3000 2113 6000 2125+05 9000 2235-01 12000 2348-07 24000 2576-30 39000 731950

Reading it across:

• 2113 at 3000 ft: wind from 210 degrees true at 13 knots. No temperature is given at 3000 ft.
• 2125+05 at 6000 ft: wind 210 at 25 knots, temperature plus 5 Celsius.
• 2235-01 at 9000 ft: wind 220 at 35 knots, temperature minus 1 Celsius. This is close to the freezing level, which matters for icing.
• 2348-07 at 12000 ft: wind 230 at 48 knots, temperature minus 7.
• 2576-30 at 24000 ft: wind 250 at 76 knots, temperature minus 30.
• 731950 at 39000 ft: the direction code 73 is above 36, so subtract 50 to get 23, meaning 230 degrees true; add 100 to the speed of 19 to get 119 knots; the temperature is minus 50, with the sign assumed because the level is above 24000 ft. This is the kind of fast, cold flow you find near a jet stream.

What you do with it

A winds aloft forecast earns its keep in planning:

• Choosing a level. Compare the wind at adjacent levels and pick the one that gives you the best tailwind or the smallest headwind for your route. A few thousand feet can be worth ten or twenty knots of ground speed.
• Ground speed, time, and fuel. The forecast wind is what turns your true airspeed into a ground speed, and ground speed is what sets your time en route and your fuel burn.
• Icing and the freezing level. Watch for levels where the temperature is near zero and the air is moist; that is where airframe icing lives. The level where the forecast crosses zero is a useful estimate of the freezing level.
• Turbulence and shear. A large change of wind speed or direction between adjacent levels is a hint of wind shear and possible turbulence, and very strong high-level winds point to the jet stream.

Turning the wind into a ground speed

The reason a winds aloft forecast is worth decoding is that it lets you turn your true airspeed into a ground speed before you fly, which is what sets your time en route and your fuel. The tool for this is the wind triangle, described in the navigation chapter of the Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25).

Take a worked example. You plan to cruise at 9000 feet on a track of 090 degrees true, at a true airspeed of 120 knots. The decoded forecast for 9000 feet is 2235-01, a wind from 220 degrees true at 35 knots. The wind is blowing from your right and slightly behind you, so part of it pushes you along and part of it pushes you sideways.

• The angle between the wind direction (220) and your track (090) is 130 degrees. Because that is more than 90 degrees, the wind has a tailwind component: it is helping.
• A rough mental estimate splits the wind into along-track and across-track parts. With the wind well behind the beam, a useful chunk of those 35 knots adds to your ground speed, so a 120-knot true airspeed might give a ground speed nearer 140 knots, while the crosswind part is what you cancel with a small drift correction to hold the track.

Now switch the track to 270 degrees, flying the reverse route, and the same wind comes from ahead of the beam. The along-track part now subtracts, so the ground speed drops below the true airspeed, perhaps to around 100 knots, and the leg takes noticeably longer on the same fuel flow. The wind did not change; your direction relative to it did.

Two conventions are worth pinning down while you are at it. The forecast levels are referenced to mean sea level, and above 18000 feet in the United States they are expressed as flight levels (FL180, FL240) flown on the standard altimeter setting, not as feet on the local QNH. And the freezing level is wherever the forecast temperature passes through zero, which is the first place to look for airframe icing if there is moisture about.

A forecast, not an observation

It is worth remembering what kind of data this is. A winds and temperatures aloft forecast is exactly that, a forecast, generated from a numerical weather model for fixed valid periods and use-windows. The actual wind you find at a level can differ from the forecast, sometimes by a useful margin, especially around fronts, jet streams, and terrain where models struggle. The NOAA/NWS Aviation Weather Center issues the product in cycles, each valid for a stated window, so the version you use should be the current one for your planned departure.

Two refinements help you read it sensibly. Pilot reports of the actual wind and temperature at a level are a direct check on the forecast, and a string of reports showing the wind running stronger than forecast is worth more than the printed figure. And the winds aloft forecast is usually read alongside the significant weather chart and any freezing-level chart, which together show the fronts, the jet stream, and the icing layers that explain why the wind and temperature change between levels.

The practical habit, then, is to treat the forecast as your planning baseline, the best estimate of the wind that turns your true airspeed into a ground speed, while staying ready for the real wind to differ and updating your time and fuel against the ground speed you actually achieve once airborne.

Common pitfalls

• Reading the direction as magnetic. Forecast winds are true. Only spoken winds from ATC or the ATIS are magnetic.
• Missing the strong-wind code. A direction over 36 is not a real heading; it means 50 was added and the wind is 100 knots or more.
• Forgetting the temperature sign. Above 24000 feet the temperature is always negative even though no minus sign is printed.
• Expecting a temperature near the surface. There is none at 3000 feet or within 2500 feet of the station.
• Treating the levels as height above ground. They are referenced to mean sea level, like all forecast levels.

In Pilot EFB

Pilot EFB fetches and displays the winds and temperatures aloft for the airports you have selected, at the standard levels, showing wind direction, speed, and temperature so you can compare levels at a glance when you plan a cruise. It presents the forecast; it does not choose your level or compute your fuel for you, so apply your own judgement and your aircraft's data. Forecasts you have already pulled stay readable offline, because your device is the source of truth for what you have saved, while fetching a fresh forecast needs a connection. Pilot EFB is offline-first and is not a certified Electronic Flight Bag.