full text of the classic FAA guide
Pressure patterns can be a clue to weather causes and movement of weather systems, but they give only a part of the total weather picture. Pressure decreases with increasing altitude. The altimeter is an aneroid barometer graduated in increments of altitude in the standard atmosphere instead of units of pressure. Temperature greatly affects the rate of pressure decrease with height; therefore, it influences altimeter readings. Temperature also determines the density of air at a given pressure (density altitude). Density altitude is an index to aircraft performance. Always be alert for departures of pressure and temperature from normals and compensate for these abnormalities.
Following are a few operational reminders:
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.
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. Then check your aircraft manual to ensure that you can become airborne from the available runway. Check further to determine that your rate of climb permits clearance of obstacles beyond the end of the runway. This procedure is advisable for any airport regardless of altitude.
When planning takeoff or landing at a high altitude airport regardless of load, determine density altitude. The procedure is especially critical when temperature is abnormally warm or pressure abnormally low. Make certain you have sufficient runway for takeoff or landing roll. Make sure you can clear obstacles beyond the end of the runway after takeoff or in event of a go-around.
Sometimes the altimeter setting is taken from an instrument of questionable reliability. However, if the instrument can cause an error in altitude reading of more than 20 feet, it is removed from service. When altimeter setting is estimated, be prepared for a possible 10- to 20-foot difference between field elevation and your altimeter reading at touchdown.