Upper winds (FB) interpolation

FB upper-wind forecast gives wind only at fixed levels (levels listed in FB upper-wind decode lesson, which also covers reading a raw FB group, ddff±tt). This lesson: interpolating wind to a cruise altitude between two forecast levels.

Method: wind changes smoothly with height, so a straight line between levels above and below is close enough.

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

• Fraction = height above the lower level / layer thickness.
• Fraction scales the change in wind, added to the lower value.

Route average:

• Direct route between two stations: average both stations' direction and speed at each level, then interpolate to cruise altitude (sample exam Q68, key 68-(2)).
• One station alone is wrong.

Unit trap:

• Cruise altitude in metres: convert (×3.28) to feet first.
• Answer stays in knots (not km/h).

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

Practice