Upper winds (FB) interpolation

The FB upper-wind forecast gives wind at set levels: 3000, 6000, 9000, 12000, 18000 ft ASL. To read a raw FB group (ddff±tt), see the FB upper-wind decode lesson. This lesson covers interpolating the wind to a cruise altitude that sits between two forecast levels.

Wind changes smoothly with height, so a straight line between the level above and below is close enough:

speed = lower + (cruise − lowerAlt) / (upperAlt − lowerAlt) × (upper − lower)

The fraction is your height above the lower level divided by the layer thickness, how far up into the layer you are, and it scales the change in wind you add to the lower value.

Unit trap: FB levels are in feet ASL and FB winds in knots. Convert a cruise altitude given in metres (×3.28) before placing it in the layer, and leave the answer in knots (do not convert to km/h).

Trap: reading the nearer level instead of interpolating, or inverting the fraction (it is height above the lower level, not below the upper).

Practice