Air Pressure

Air Pressure Altimetry is the measurement of altitude using air pressure. A pressure altimeter is an instrument that measures altitude based on a strict relationship between air pressure and altitude. Air pressure is measured within the altimeter by an aneroid barometer capsule, with the instrument graduated in increments of altitude. A pilot must constantly adjust the altimeter, or “zero” the altimeter, during flight as atmospheric densities change with air temperature. Another type of altimeter—a radar altimeter—sends and receives radio wavelengths between the plane and the ground to determine altitude. Altitude (km) Pressure (mb) Altitude (km) Pressure (mb) 0.00 1013.25 10 264.99 0.50 954.61 12 193.99 1.00 898.76 14 141.70 1.50 845.59 16 103.52 2.00 795.01 18 75.65 2.50 746.91 20 55.65 3.00 701.21 25 25.49 4.00 616.60 30 11.97 5.00 540.48 35 5.75 6.00 472.17 40 2.87 7.00 411.05 50 0.79 8.00 356.51 60 0.23 9.00 308.00 70 0.06 The information in the table allows a determination of the rate of pressure decrease with altitude which is not at a constant rate. Remember that half of the weight of the total atmosphere occurs below 5,500m (18,000 ft); at that altitude only about half of the total atoms and molecules of atmospheric gases remain to form the mass of the atmosphere. Determine the decrease in pressure between the following altitudes. Express the difference in millibars. 1 km interval difference in pressure: 10 km interval difference in pressure: 0 and 1 km [114.49] mb 0 and 10 km [748.26] mb 2 and 3 km 93.8 mb 10 and 20 km 209.34 mb 5 and 6 km 68.31 mb 20 and 30 km 43.68 mb