Meteorology Applied to Navigation: Upper Winds, Temperatures and Altimetry
Upper winds and temperatures
Read winds aloft from forecasts and charts: direction is from which the wind blows, and speeds increase with altitude in the jet stream. Understand how to interpolate wind between levels and how temperature lapse affects TAS and true altitude.
Altimetry
The altimeter is set to QNH (airfield elevation reference), QFE (height above the datum) or standard 1013.25 hPa (flight levels). Traps: flying from high to low pressure without resetting the subscale causes the altimeter to over-read (you are lower than indicated); high to low temperature likewise. The "high to low, look out below" rule is essential. Know the transition altitude/level and the transition layer.
Temperature effects on performance and navigation
Cold temperature reduces true altitude for a given indicated altitude and increases TAS for a given IAS (TAS increases about 1% per 1°C above ISA at constant IAS, roughly). Know how to compute density altitude.
Rule of thumb: True altitude error ≈ 4 ft per 1°C ISA deviation per 1000 ft. Example: at 10 000 ft with ISA−10°C, the aircraft is about 400 ft lower than indicated.
Also understand how wind and temperature forecasts are used to refine fuel and ETA.