When you see the numbers '9900' on Winds-Aloft chart, what does that mean?

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Multiple Choice

When you see the numbers '9900' on Winds-Aloft chart, what does that mean?

On Winds-Aloft charts, wind is shown in two two-digit groups: the first two digits indicate wind direction, and the second two digits indicate wind speed in knots. If the wind direction is not fixed at that level, the direction digits are 99. If the wind speed is less than 5 knots, the speed digits are 00. So the value 9900 means variable wind direction with winds lighter than 5 knots (essentially calm). Why the other ideas don’t fit: it isn’t a steady wind of 9900 knots—the numbers aren’t a single speed, and such a speed isn’t plausible. It isn’t unknown wind speed; the digits have a specific encoding. And 00 doesn’t mean no wind exactly but less than 5 knots, so it isn’t simply “no wind.”

On Winds-Aloft charts, wind is shown in two two-digit groups: the first two digits indicate wind direction, and the second two digits indicate wind speed in knots. If the wind direction is not fixed at that level, the direction digits are 99. If the wind speed is less than 5 knots, the speed digits are 00. So the value 9900 means variable wind direction with winds lighter than 5 knots (essentially calm).

Why the other ideas don’t fit: it isn’t a steady wind of 9900 knots—the numbers aren’t a single speed, and such a speed isn’t plausible. It isn’t unknown wind speed; the digits have a specific encoding. And 00 doesn’t mean no wind exactly but less than 5 knots, so it isn’t simply “no wind.”

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