What is the difference between a four-digit and six-digit winds aloft entry?

Four-digit entries appear only in the 3,000-foot column and give direction and speed only - no temperature, because reliable surface-level temperature forecasts are impractical at that altitude. Six-digit entries, used at 6,000 feet and above, add a temperature value in degrees Celsius.

What does 9900 mean in the winds aloft forecast?

An entry of 9900 indicates winds are forecast to be less than 5 knots at that altitude and station. There is no meaningful wind direction to report, and this altitude has essentially no wind factor for groundspeed planning.

How do I decode a winds aloft entry with direction digits above 50?

Direction digits above 50 signal the high-wind encoding used when speed exceeds 99 knots. Subtract 50 from the direction digits to get the true direction (in tens of degrees), and add 100 to the speed digits to get the true speed. For example, 7760 decodes to 270° at 160 knots.

How do I pick a cruising altitude using the winds aloft table when the listed altitudes don't match my legal options?

The hemispherical rule (FAR 91.159) gives you altitudes like 8,500 and 10,500 feet, which fall between the table's listed columns. Interpolate between the adjacent columns - for 10,500 feet, look at both the 9,000-foot and 12,000-foot entries and estimate wind conditions in between.

How does the winds aloft temperature data help me assess icing risk?

The structural icing risk band for most general aviation aircraft is 0°C to -20°C. Check the temperature column at your intended cruising altitude, then cross-reference with any active AIRMET for icing. The AIRMET confirms moisture is present; the winds aloft temperature confirms you're in the temperature range where ice forms.

The Winds Aloft Forecast, the Coded Four-Digit Groups That Confuse Every Student on Their First Cross-Country, and the Altitude Selection Math That Could Change Your Flight Time by Half an Hour

The Winds and Temperatures Aloft Forecast is one of the most-accessed and least-understood weather products in general aviation. Once you know how to read it, it transforms your preflight planning from guesswork into a calculated decision about groundspeed, fuel, and route efficiency. This guide covers everything from basic decoding to the altitude selection math that can change your flight time by 25 minutes or more on a 200-mile leg.

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

The official product name is the Winds and Temperatures Aloft Forecast. Its current format uses the identifier FB. Older pilots may refer to it as the FD winds, which was the label on the legacy text product - same concept, updated encoding.

You’ll find it on the Aviation Weather Center (aviationweather.gov), ForeFlight, Garmin Pilot, and through any Flight Service standard briefing.

The National Weather Service issues this forecast twice daily, built from radiosonde data - weather balloons launched from surface stations, climbing through the atmosphere and measuring winds, temperature, and humidity as they ascend. The forecast model projects those observations forward into three products: valid at 6 hours, 12 hours, and 24 hours from the issue time.

For a flight this afternoon or evening, use the 6-hour or 12-hour valid time. For a flight planned for tomorrow, use the 24-hour product. Most weather apps select the appropriate window automatically once you enter your estimated departure time.

How Is the Winds Aloft Table Structured?

The table is not a continuous grid blanketing the country. It’s a set of point forecasts tied to specific stations, identified by three-letter codes usually aligned with a VOR or major airport weather reporting site.

For a short cross-country, one station near your route midpoint is typically sufficient. For longer trips, check stations near your departure, midpoint, and destination. Winds aloft change across distance - a 30-knot tailwind at the western end of a 500-mile route can become a 20-knot headwind at the eastern end depending on the pressure pattern.

The standard forecast covers these altitudes: 3,000 | 6,000 | 9,000 | 12,000 | 18,000 | 24,000 | 30,000 | 34,000 | 39,000 feet. For piston general aviation, 3,000 through 12,000 feet covers the vast majority of student and new private pilot flying.

How Do You Decode a Four-Digit or Six-Digit Group?

Entries come in two forms depending on altitude.

Four-digit groups appear only in the 3,000-foot column. They give direction and speed only - no temperature. At 3,000 feet, close enough to the surface that a reliable temperature forecast is impractical, the METAR at your departure field gives a more accurate picture anyway. You also won’t see temperature data at any altitude within 2,500 feet of the station’s field elevation.

Six-digit groups start at 6,000 feet for most low-elevation stations and give you direction, speed, and temperature in one entry.

How to decode a six-digit entry - example: 2318-04

First two digits: 23 → wind direction in tens of degrees → 230° (from the southwest)
Next two digits: 18 → wind speed in 18 knots
Remaining digits with sign: -04 → temperature -4°C

That one entry tells you: winds from 230° at 18 knots, temperature -4°C.

What Does 9900 Mean?

When winds at a given altitude and station are forecast to be less than 5 knots, the direction isn’t meaningful. The entry reads 9900.

When you see 9900, that altitude has essentially no wind factor for planning. No tailwind to chase, no headwind to fight.

How Do You Decode the High-Wind Code?

When wind speed exceeds 99 knots, the two-digit speed field can’t hold a three-digit number. The forecast uses a specific encoding convention: add 50 to the direction digits, subtract 100 from the speed digits.

Your flag: direction digits above 50 in a normal entry would be impossible (valid directions only go 01–36). Direction digits of 51 or higher mean the high-wind code is active.

Example: 7760

First two digits are 77. Greater than 50 - high-wind code in effect.
Subtract 50: 77 − 50 = 27 → direction 270° (due west)
Last two digits are 60. Add 100: 60 + 100 = 160 knots

Result: winds from the west at 160 knots - a jet stream core. You’re not flying a trainer into that, but knowing which side of that river of wind you’re on matters for altitude selection on longer cross-countries.

How Is Temperature Reported - and Why Does It Matter for Icing?

Below 18,000 feet, the temperature entry shows an explicit sign: +14 or -14.

Above 18,000 feet, all temperatures are negative by definition, so the minus sign is dropped. The table shows magnitude only - you supply the negative sign mentally.

This matters most for icing analysis. The structural icing risk band for most general aviation aircraft is 0°C to -20°C. If the temperature column at 9,000 feet shows -8°C, and there’s an AIRMET for icing along your route, those two products together define the hazard. The winds aloft forecast gives you the temperature data; the AIRMET confirms the moisture. Together they tell you where the ice lives.

How Do You Use Winds Aloft to Select the Best Cruising Altitude?

The altitude you cruise at changes your groundspeed. Groundspeed changes flight time. Flight time changes fuel requirement. On a 200-mile leg, the difference between a poor altitude and an optimal one can be 25 minutes - and 25 minutes is the difference between comfortable reserves and uncomfortable math.

Westbound scenario, magnetic course ~270°:

Altitude	Wind	Assessment
6,000 ft	210° at 12 kts	Partial headwind component
9,000 ft	240° at 22 kts	Better angle, partial tailwind
12,000 ft	270° at 30 kts	Pure tailwind - 115 KTAS becomes 145 kt groundspeed

But you can’t simply pick 12,000 feet because you want the tailwind. VFR cross-country flight above 3,000 feet AGL requires compliance with the hemispherical rule (FAR 91.159).

What Is the Hemispherical Rule and How Does It Apply?

Magnetic course 000°–179°: fly at odd thousands + 500 feet (3,500 | 5,500 | 7,500 | 9,500 | 11,500)
Magnetic course 180°–359°: fly at even thousands + 500 feet (4,500 | 6,500 | 8,500 | 10,500 | 12,500)

A westbound 270° course falls in the 180°–359° range: even thousands plus 500. Your legal altitude choices are 4,500 | 6,500 | 8,500 | 10,500 | 12,500 feet.

Now the nuance: the winds aloft table lists 6,000, 9,000, and 12,000 feet - not 8,500 or 10,500. You interpolate between adjacent columns.

8,500 feet (between the 6,000 and 9,000 columns): expect roughly 15–17 knots with an improving tailwind component
10,500 feet (between the 9,000 and 12,000 columns): moving toward the pure tailwind end of the range - best available groundspeed under the hemispherical rule on this heading

10,500 feet is your optimal choice: best winds aloft, legal altitude, no guessing.

How Do You Spot Wind Shear and Turbulence Potential in the Table?

Read the winds aloft table horizontally across altitude bands, not just at your intended cruising altitude.

A large speed jump between adjacent columns at the same station is a shear signature. If 6,000 feet shows 10 knots and 9,000 feet shows 45 knots, that 35-knot increase across 3,000 feet marks a layer with turbulence potential.

Direction reversals tell the same story: wind from the south at 6,000 feet and wind from the north at 9,000 feet at the same station is textbook wind shear. That layer is where the chop lives.

The winds aloft table can show you this before it appears in the AIRMETs, especially early in a developing weather system. If two cruising altitude options are otherwise equal and one sits in the middle of a shear band, take the other. Smooth air and a few fewer knots of tailwind is always the better trade.

What Does a Complete Preflight Weather Workflow Look Like?

A systematic preflight weather analysis follows this sequence:

METAR at departure and destination for current conditions
TAF at destination and alternate for trend while en route
AIRMETs and SIGMETs for broad hazard zones along the route
Winds aloft table: find closest stations, decode relevant altitude columns, note temperatures
Apply the hemispherical rule to identify legal altitude choices
Select the optimal altitude based on winds, groundspeed, and performance
Check for shear between altitude bands
Cross-reference temperature entries with any icing AIRMETs

After decoding a few dozen entries, 9900 becomes immediately obvious. Direction digits above 50 automatically trigger the high-wind decode. The table becomes transparent with repetition.

What Will the Examiner Ask on the Private Pilot Checkride?

The Airman Certification Standards (ACS) for the private pilot certificate require demonstrated weather analysis for cross-country planning, and winds aloft is a central part of that evaluation.

The examiner wants you to point to the table, decode an entry, explain what it means for your specific route, and show how you combined it with the hemispherical rule to select your cruising altitude.

A complete answer sounds like: “At 10,500 feet, which is close to the 12,000-foot column, I’m looking at winds from the southwest at approximately 28 knots. My calculated groundspeed is roughly 140 knots. That’s what I used for my fuel and time estimate.”

That answer shows you used the product, understood it, and made a decision from it. That’s the conversation the examiner is looking for.

Key Takeaways

The Winds and Temperatures Aloft Forecast (FB) is issued twice daily and valid at 6, 12, and 24 hours; use the time window closest to your departure
A six-digit entry gives direction (tens of degrees), speed (knots), and temperature (°C) - decode 2318-04 as 230° at 18 knots, -4°C
9900 means winds under 5 knots; direction digits above 50 signal the high-wind encoding (subtract 50 from direction, add 100 to speed)
Combine the winds aloft table with FAR 91.159 (the hemispherical rule) to identify both legal and optimal cruising altitudes - interpolate between listed altitudes for the actual choices available to you
Read across altitude bands to spot wind shear layers; large speed jumps or direction reversals between adjacent columns indicate turbulence potential, sometimes before AIRMETs catch up