The winds and temperatures aloft forecast and the numbers that tell you what is waiting at your cruise altitude before you leave the ground

The winds and temperatures aloft forecast (FB winds) tells you exactly what the atmosphere is doing at your planned cruise altitude before you leave the ground. Learning to decode this forecast lets you choose smarter altitudes, calculate more accurate fuel burns, and identify potential hazards like icing and turbulence. It is one of the most overlooked tools in a student pilot’s weather briefing, and one of the most valuable.

What Is the Winds and Temperatures Aloft Forecast?

The winds and temperatures aloft forecast, officially designated FB (sometimes labeled FD in older references), is issued by the National Weather Service twice daily based on 00Z and 12Z observations. It provides forecast winds and temperatures at specific altitudes for reporting stations across the country.

You can find it on Leidos Flight Service, ForeFlight, Aviation Weather (aviationweather.gov), or anywhere you pull a standard weather briefing. The forecast is published for multiple time periods: 6-hour, 12-hour, and 24-hour. Always use the one closest to your planned departure time.

How Do You Decode the Winds Aloft Format?

A typical winds aloft entry looks like 2145+07 — a string of digits that makes perfect sense once you know the structure.

The first four digits represent the wind:

• The first two digits are the wind direction in tens of degrees true. So 21 means 210 degrees true (south-southwest).
• The next two digits are the wind speed in knots. 45 means 45 knots.

So 2145 means the wind at that altitude is from 210 degrees true at 45 knots.

The remaining digits are the temperature:

• +07 means positive 7 degrees Celsius. A minus sign indicates below zero.

One critical detail: winds aloft are reported in degrees true, not magnetic. If you are planning on a sectional chart, your true course aligns directly with the reported wind direction. But if you are thinking in terms of runway headings or your heading indicator, you must apply local magnetic variation.

What Do the Special Codes Mean?

Two encoding quirks trip up new pilots:

9900 does not mean a wind from 9,900 degrees. It is the code for light and variable, meaning the wind at that altitude is less than 5 knots.

Wind speeds over 100 knots use a special encoding found at higher altitudes. The Weather Service adds 50 to the direction code and subtracts 100 from the speed. For example, 7845 decodes as: 78 minus 50 = 280 degrees true, and 45 plus 100 = 145 knots. It looks strange the first time, but the rule is consistent.

Which Altitudes Are Reported?

The forecast provides data at 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet MSL.

An important detail that catches people off guard: no wind is reported at an altitude within 1,500 feet of the station elevation, and no temperature is reported at the 3,000-foot level. For a station like Denver (elevation approximately 5,300 feet), you will not see data at 3,000 or 6,000 feet because 6,000 feet is only about 700 feet above the station. Data would begin at 9,000 feet.

For flights at lower altitudes near high-elevation airports, you may need to interpolate or reference nearby lower-elevation stations.

How Do Winds Aloft Help You Pick the Best Altitude?

Consider a 200-nautical-mile cross-country from central Texas to Oklahoma City with a course of roughly 360 degrees true.

• At 6,000 feet, the wind is from 180 degrees at 25 knots — a direct 25-knot headwind.
• At 9,000 feet, the wind shifts to 240 degrees at 30 knots. The wind is stronger, but the angle is far more favorable. The headwind component drops significantly, and you may achieve a better groundspeed despite the higher wind speed.

This is exactly the kind of analysis the Airman Certification Standards (ACS) expect on a checkride. The examiner wants to see that you considered the winds aloft and chose an altitude that makes sense for your direction of flight — not just one that follows the hemispheric rule.

Speaking of which, FAR 91.159 requires that above 3,000 feet AGL, you fly odd thousands plus 500 on eastbound courses and even thousands plus 500 on westbound courses. You cannot always pick the perfect altitude, but the winds aloft forecast tells you which of your legal altitudes is the smartest one.

Why Do the Temperatures Aloft Matter?

The temperature data serves three important purposes:

Performance. True airspeed changes with temperature. On a warmer-than-standard day, your true airspeed is higher for the same indicated airspeed. On a colder day, it is lower. This directly affects groundspeed calculations, fuel burn, and time en route.

Icing risk. If the temperature at your cruise altitude is between 0 and -20 degrees Celsius and visible moisture is present, you are in the icing zone. Even VFR pilots can encounter conditions where clouds are building. Knowing the freezing level helps you assess whether ice is a realistic threat.

Turbulence clues. A large temperature difference between altitude levels can indicate wind shear. If it is +15°C at 6,000 feet and -2°C at 9,000 feet, that steep drop suggests a front or shear zone. The transition layer between those altitudes is likely rough.

How Should You Use Winds Aloft in Flight Planning?

Step 1: Pull the winds aloft for stations along your entire route — not just departure and destination. Winds can change dramatically along the way.

Step 2: Identify your legal altitude options under the hemispheric rule. Compare the headwind or tailwind component at each altitude. A 10-knot groundspeed difference between 4,500 and 6,500 feet adds up significantly over a two-hour flight.

Step 3: Check temperatures. Are you near freezing? Is there a large temperature change between levels suggesting turbulence? Is the temperature significantly different from standard, affecting your performance calculations?

Step 4: Factor everything into your navigation log — wind correction angle, groundspeed, time between checkpoints, and fuel burn. Showing up to a checkride with a completed nav log and a clear explanation of your altitude choice demonstrates thorough preparation.

How Accurate Is the Forecast?

The winds aloft forecast is a prediction, not a measurement. Once airborne, verify actual winds by comparing your GPS groundspeed to your true airspeed. If there is a significant discrepancy between the forecast and what you are experiencing, the weather may be developing differently than predicted. Treat that as a signal to reassess the rest of your briefing.

Key Takeaways

• The first four digits are wind direction (true) and speed in knots; the remaining digits are temperature in Celsius. Code 9900 means light and variable (under 5 knots).
• Winds aloft are in degrees true, not magnetic — apply magnetic variation when comparing to compass headings.
• No data is reported within 1,500 feet of station elevation, so high-altitude airports may lack low-level wind data.
• Compare headwind/tailwind components at each legal altitude before defaulting to your usual cruise level. A slightly higher altitude with a better wind angle can save meaningful fuel and time.
• Temperature aloft affects performance, icing risk, and turbulence — never skip it in your briefing.

References: FAA Advisory Circular on Aviation Weather Services; Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B).