by the National Weather Service

“ When temperature is colder than standard, you are at an altitude lower than your altimeter indicates. When temperature is warmer than standard, you are higher than your altimeter indicates. When flying cross country, keep your altimeter setting current. This procedure assures more positive altitude separation from other aircraft. When flying over high terrain in cold weather, compute your true altitude to ensure terrain clearance. When your aircraft is heavily loaded, the temperature is abnormally warm, and/or the pressure is abnormally low, compute density altitude. ”

“ Altitude If it were possible for a pilot always to determine mean temperature of the column of air between the aircraft and the surface, flight computers would be designed to use this mean temperature in computing true altitude. However, the only guide a pilot has to temperature below him is free air temperature at his altitude. Therefore, the flight computer uses outside air temperature to correct indicated altitude to approximate true altitude. ”

“ Beware of the low pressure-bad weather, high pressure-good weather rule of thumb. It frequently fails. Always get the complete weather picture. When flying from high pressure to low pressure at constant indicated altitude and without adjusting the altimeter, you are losing true altitude. ”

“ The millibar is rapidly becoming a universal pressure unit.
Station Pressure Obviously, we can measure pressure only at the point of measurement. The pressure measured at a station or airport is "station pressure" or the actual pressure at field elevation. We know that pressure at high altitude is less than at sea level or low altitude. For instance, station pressure at Denver is less than at New Orleans. Let's look more closely at some factors influencing pressure. ”