What does "9900" indicate in a winds aloft forecast?

In a winds aloft forecast, "9900" indicates light and variable winds. This coding is a standardized way to report conditions where wind speeds are generally calm, typically less than 5 knots. In practical terms for pilots, this means that the winds are not strong enough to have a significant effect on flight, making it important for flight planning, particularly in terms of fuel usage and flight stability.

The other options represent different meteorological conditions that are specified differently. For example, winds of 50 knots would be specifically coded, and mechanisms like tornado warnings are addressed through different alerts unrelated to the wind strength denoted by "9900." Additionally, "wind speed not measurable" would involve a different numerical representation usually indicating that instruments couldn't effectively capture any wind speeds at that altitude.