The Winds Aloft Forecast, the Six-Digit Code That Changes Your Altitude Decision Before You Leave the Ramp, and the Temperature Column That Doubles as Your First Icing Check

The winds aloft forecast is one of the most information-dense weather products available to pilots, and one of the most commonly skimmed. A six-digit code at each altitude level contains your wind direction, wind speed, and temperature - enough to choose a cruise altitude, calculate groundspeed, and flag potential icing risk before you leave the ramp.

What Is the Winds Aloft Forecast and Where Do You Find It?

The official name from the National Weather Service is the Forecast Winds and Temperatures Aloft, sometimes abbreviated FB winds in older references and briefing tools. It is available directly from the Aviation Weather Center at aviationweather.gov, and it populates automatically in standard briefing tools like ForeFlight and 1-800-WX-BRIEF.

The product is issued four times a day and covers three valid time windows: 6 hours, 12 hours, and 24 hours out. When planning the evening before a flight, confirm you are viewing the window that covers your actual departure time. Checking the wrong valid period means planning against the wrong winds.

How Do You Read the Six-Digit Code?

The forecast is organized by three-letter reporting station identifier, then broken out by altitude. Standard levels are 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet MSL.

Each altitude entry is a six-character code: four digits for wind, two digits with a sign for temperature. The code 271818-04 breaks down like this:

• First two digits (27): Wind direction in tens of degrees. Multiply by 10. 27 × 10 = 270° - a direct west wind.
• Second two digits (18): Wind speed in knots, read directly. 18 knots.
• Last two digits with sign (-04): Temperature in Celsius. -4°C.

That single code tells you everything: wind from 270° at 18 knots, temperature -4°C.

What Does 9900 Mean in the Winds Aloft Forecast?

When winds at a given level are less than 5 knots with no meaningful direction, the forecast publishes 9900. This indicates light and variable winds - essentially calm. No wind correction angle calculation is needed for that altitude. Note it and continue planning.

How Do You Decode High-Speed Wind Encoding?

The forecast format uses only two digits for wind speed, which tops out at 99 knots. Winds in the jet stream routinely exceed that, so a separate encoding handles speeds between 100 and 199 knots.

When wind speed falls in that range, the meteorologist adds 50 to the direction digits and subtracts 100 from the speed digits before writing the code. To decode, check whether the direction digits exceed 36 - a number above 36 represents an impossible compass direction and signals the high-speed encoding is active.

Example: You see the code 7799.

• Direction digits: 77. Greater than 36, so high-speed encoding applies.
• Subtract 50: 77 - 50 = 27. Direction is 270°.
• Speed digits: 99. Add 100: 99 + 100 = 199 knots.

A pilot flying a Cessna 172 below 12,000 feet will rarely encounter this. Pilots flying instrument cross-countries and checking high-altitude winds will. Know the decode before it appears on an oral exam.

Are There Special Rules for Temperature in the Winds Aloft Forecast?

Two temperature conventions are worth knowing.

Above 24,000 feet, temperatures are always negative - cold enough that the forecast drops the minus sign entirely. A temperature of -40°C at 30,000 feet is written simply as 40. No minus sign. The convention assumes the reader knows temperatures at that altitude are negative.

At 3,000 feet MSL, the forecast does not include a temperature column. The surface thermal environment dominates at that altitude, making the temperature at altitude less meaningful to isolate. A 3,000-foot entry will show direction and speed only. That is normal.

How Do You Use the Winds Aloft Forecast to Choose a Cruise Altitude?

Match your planned cruise altitude to the nearest forecast levels and interpolate. If cruising at 5,500 feet, your reference levels are 3,000 and 6,000 feet. If 3,000 shows winds from 210° at 10 knots and 6,000 shows winds from 220° at 20 knots, a reasonable estimate for 5,500 feet is roughly 215° at 15–16 knots. No calculator needed - proportional reasoning is enough.

Then compare levels relative to your heading. Flying north (roughly 360°), a tailwind comes from the south (roughly 180°). If 6,000 feet shows a southwest wind and 9,000 feet shows a west wind, the 6,000-foot level delivers a better tailwind component on a northbound track. You pick 6,000 feet.

The reverse logic applies when minimizing headwinds. If 6,000 feet has winds screaming out of the west at 30 knots and 9,000 feet has dropped to 12, climbing to 9,000 costs time on the way up but recovers groundspeed over the leg. That trade-off is calculable on the ground before engine start.

How Do Winds Aloft Affect Fuel Planning?

Groundspeed depends directly on the wind component along your track. A 20-knot headwind on a Cessna 172 true-airspeed of 110 knots yields a 90-knot groundspeed. That changes fuel burn per nautical mile and can flip a fuel stop from optional to required. Work through this before you brief the flight, not during it.

How Can the Winds Aloft Forecast Help You Identify Icing Risk?

The winds aloft forecast is not a replacement for AIRMET Sierra, which covers icing explicitly. But the temperature column tells you the thermal environment at each altitude level, and thermal environment is half the icing equation.

When the area forecast shows clouds or precipitation and the temperature at your cruise altitude is between 0°C and -10°C, that is the zone where supercooled liquid water survives long enough to accumulate on airframe surfaces. Zero to -10°C is where ice accumulates fastest on a piston trainer without de-ice equipment.

As you work through altitude options, cross-reference two things: is there reported or forecast moisture at this level, and what does the temperature column show? The winds aloft do not confirm ice is present - they tell you whether the temperature is favorable for ice if moisture is also present. That distinction matters.

What Does a Full Altitude Decision Look Like in Practice?

Consider a morning flight from Nashville to Knoxville, Tennessee - roughly 120 nautical miles, heading east-northeast. The winds aloft show:

• 3,000 feet: winds from 240° at 8 knots, no temperature
• 6,000 feet: winds from 250° at 14 knots, temperature -2°C
• 9,000 feet: winds from 260° at 22 knots, temperature -8°C

The area forecast shows scattered clouds at 4,000 feet with tops to 6,000 feet. Freezing level is reported around 5,500 feet MSL.

• 3,000 feet: Legal, but below the cloud bases with a moderate headwind component on the eastbound track.
• 6,000 feet: Inside the clouds at -2°C - right at the freezing level. A Cessna 172 with no de-ice equipment has no business in that environment.
• 9,000 feet: Above the cloud tops. The winds from 260° give a modest headwind component on the eastbound track, but the flight is clear of moisture and at -8°C, supercooled water droplets are less likely to survive long enough to accumulate.

The answer is 9,000 feet. You accept a modest groundspeed penalty in exchange for a significant safety margin. Calculate groundspeed with the wind component, figure fuel burn, and brief the flight.

Why Do Wind Directions in the Forecast Reference True North, Not Magnetic?

Wind directions in the winds aloft forecast are referenced to true north. Your sectional, your VOR radials, and your compass are all magnetic. This is not a problem when the math is done correctly.

When calculating a wind correction angle to convert a true course into a true heading, you are working in consistent units throughout - true course and winds aloft direction are both true. Magnetic variation enters the calculation only at the end, when converting true heading to magnetic heading to compass heading. Keep those conversions in order and the math flows cleanly.

Key Takeaways

• The winds aloft forecast is a forward-looking product, not a current observation. Confirm you are using the valid time window that covers your departure.
• Each six-digit code contains wind direction (first two digits × 10), wind speed in knots (second two digits), and temperature in Celsius (last two digits with sign).
• 9900 means light and variable winds. If the direction digits exceed 36, the high-speed encoding is active: subtract 50 from direction digits and add 100 to speed digits.
• The temperature column is your first icing screen at each altitude. The danger zone for piston trainers without de-ice equipment is 0°C to -10°C in the presence of moisture.
• Wind directions in the forecast reference true north. Apply magnetic variation only at the final step when converting to magnetic heading.