Even before humans learned to fly in balloons or in aircraft, we ventured up into the atmosphere by climbing mountains. But how high were we? Lacking a long tape measure, another procedure was needed! Pascal and Perier had noted back in the 17th Century that barometer readings decrease as one climbs a mountain. This relation between pressure decrease and altitude increase can be used to find your altitude using an instrument, called an altimeter, which displays the altitude. A pressure altimeter consisting of an aneroid barometer mechanism is used in aircraft and by hikers. Even some digital wristwatches have altimeters. The scale of the altimeter is calibrated in terms of the altitude at which one would expect to find a particular atmospheric pressure under average conditions as described in the Standard Atmosphere.
Of course, atmospheric conditions are constantly changing and are seldom at average values. Movements of large scale weather systems cause pressure changes. One can compensate for these pressure variations by setting the beginning of the altimeter scale using the "altimeter setting", provided in hourly weather reports available from the National Weather Service. Another deviation in altitude readings from the average occurs when temperatures vary from those of the Standard Atmosphere. A column of air warmer than average will give altimeter altitude readings that are lower than the actual altitude, while a cold air column will give readings higher than the actual altitude. Pilots have to be especially mindful of these temperature variations when flying at low levels or in reduced visibility conditions.
For more information on pressure altimeters and their readings, see Thursday's optional Supplemental Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 5 Chapter Progress Response Form, under section B. Daily Summary.