Reading Winds Aloft: Upper Wind Forecasts for Pilots

Winds aloft are the forecast wind direction, speed, and temperature at set altitudes above the ground, from 3,000 feet up through the high flight levels. In the United States, NOAA's Aviation Weather Center issues this forecast as the FB product (Winds and Temperatures Aloft Forecast) four times a day, and the FAA makes it available through every standard flight briefing. Airlines, dispatchers, and general aviation pilots all use it to pick a cruise altitude, work out wind correction angle, and estimate groundspeed for each leg of a flight.

A winds aloft forecast is not tied to one airport. It describes the wind at a network of upper-air stations spread across the country, so a pilot flying between two of them interpolates the wind for a point in between. That sets it apart from a surface report like a METAR, which only covers the ground-level wind at a single field.

Each entry in a winds aloft forecast is a short code tied to one station and one altitude. The basic four-digit form is DDff: the first two digits are wind direction in tens of degrees true, and the last two are wind speed in knots. The code 2735 means wind from 270 degrees at 35 knots. At altitudes of 6,000 feet and above, forecasters add a two-digit temperature in Celsius after the wind group, so 273512 means wind 270 at 35 knots with a temperature of 12 degrees. Above 24,000 feet the temperature is always negative and the minus sign is dropped, since a positive temperature never occurs that high.

Two special cases cause most of the confusion. The code 9900 means the wind is calm or light and variable, and it only shows up at low altitudes. And when a forecast wind reaches 100 knots or more, the plain DDff format runs out of room, because ff can only hold two digits. Forecasters solve this by adding 50 to the direction digits and subtracting 100 from the speed digits. So a code of 7350 does not mean 730 degrees: reverse the trick to get direction 73 minus 50, or 230 degrees, and speed 50 plus 100, or 150 knots. This convention only applies when the direction digits fall between 51 and 86, since a real direction never exceeds 36 (360 degrees), so any code in that range is a flag that the high-speed rule is in play.

The third row shows the high-speed convention in action: 7350 decodes to 230 degrees at 150 knots, not 730 degrees at 50 knots, because the direction digits exceed 36.

For each leg of a cross-country flight, pull the forecast wind at your planned cruise altitude and compare it to the true course you plan to fly. The angle between the wind and your course sets the wind correction angle, the number of degrees you offset your heading from your track to fly a straight line over the ground. The same wind triangle that gives you the wind correction angle also gives you groundspeed, the number you multiply by time to find your position along the route. Run the numbers with the wind correction angle calculator, or work the full triangle by hand with the digital E6B.

A stronger wind at altitude is not always worse. A quartering tailwind at 9,000 feet can raise groundspeed enough to beat the true airspeed you would get from a lower, smoother altitude, which is why pilots check winds aloft at more than one flight level before picking a cruise altitude. Near the destination, switch from winds aloft to current surface data for the descent and approach, since a winds aloft forecast only covers the en-route portion of a flight, not the last few thousand feet.

A METAR reports the wind actually measured at one airport, updated hourly or when conditions change. Winds aloft is a forecast, not a measurement, for the air above a wide area, valid for a set window and reissued every six hours. Use METAR, and the longer-range TAF for the next 24 to 30 hours, to judge the crosswind you will land in. Use winds aloft for everything above the traffic pattern: cruise navigation, climb and descent profiles, and cruise altitude selection.

The two also differ in reference frame. Under ICAO convention, printed METAR wind is true, though in the US the spoken ATIS and tower broadcast are magnetic. Winds aloft forecasts are always true, with no exception. Feed a METAR into the METAR decoder to pull direction and speed automatically, then compare it against the forecast at altitude to see how much the wind changes between the surface and your cruise level.

Wind near the ground is slowed and twisted by friction from trees, buildings, and terrain. As altitude increases and that friction fades, wind speed goes up and direction typically veers, turning clockwise with height in the northern hemisphere as the wind lines up more closely with the pressure pattern above the friction layer. A wind that is light out of the south at the surface can become a brisk southwesterly of 40 knots or more by 9,000 feet.

Because wind changes this much with height, a single winds aloft forecast is not one number. Each station reports separate wind and temperature values for a list of standard levels, commonly 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet, though stations near high terrain skip the lowest levels that fall below the surrounding ground. A pilot planning a flight does not read every level on the list. Pick the forecast altitude closest to your planned cruise and use that one wind for your wind correction angle and groundspeed math. If your planned altitude sits between two published levels, interpolate between them, or use whichever is more conservative for fuel planning. Reading the wrong level, say using the 3,000-foot wind for a flight planned at 9,000 feet, can throw off a groundspeed estimate by 20 knots or more and leave a pilot short on fuel reserves.

The jet stream is a narrow band of very fast wind, often 100 to 200 knots, flowing generally west to east near the tropopause, typically between 30,000 and 40,000 feet. It forms along the boundary between cold and warm air masses and shifts position and strength with the seasons, running farther south and blowing harder in winter. Winds aloft forecasts capture the jet stream directly: at high-altitude stations, a sudden jump in the FB speed group, coded as something like 7530 (250 degrees at 130 knots using the high-speed convention), signals a pilot is near or inside the core.

Airlines flying eastbound near the tropopause plan routes to catch the jet stream tailwind and cut flight time and fuel burn, while westbound flights plan around it. Turbulence tends to concentrate at the edges of the jet stream, where wind shear between the fast core and the slower air around it is sharpest, which is why pilots and dispatchers watch the winds aloft forecast for the jet stream's position on any high-altitude flight.

Pilots do not need to decode raw FB text by hand every time. A standard weather briefing from flight service includes winds aloft for the requested route and altitude, read in plain language. Aviation weather sites such as NOAA's Aviation Weather Center publish the same forecast in text and graphical form, filterable by station and altitude. Flight planning apps, including ForeFlight and Garmin Pilot, pull from that same government data and overlay winds aloft on the route automatically, updating as new forecast cycles are issued every six hours. Whichever source a pilot uses, check the forecast's issue and valid time before flying, since a stale forecast can be worse than none at all.