When it is not possible to forecast a prevailing surface wind direction due to its expected variability, for example, during light wind conditions (less than 3 kt) or thunderstorms, the forecast wind direction should be indicated as variable using "VRB". When the wind is forecast to be less than 1 kt the forecast wind speed should be indicated as calm. When a wind speed of 20oKm/h (100kt) or more is forecast, it should be indicated as 200Km/h (or 100kt) [ICAO ANNEX 3, recommendation 6.2.12]