The ront September-OOctober, 2002 National Weather Service Central Region Volume 1 Number 5 Atmospheric Pressure and Altimeters In this issue: Pressure altimeters are simple, silent sentries that follow air pressure surfaces, the topography of the atmosphere. Pilots Pressure distribution in the need to know how these surfaces change and affect altitude. atmosphere that your altime- Craig Sanders, Lead Forecaster informed about the horizontal pressure ter senses. NWS, Duluth, MN changes at ground level. Pressure also varies vertically. A Pressure altimeters have been a cru- given pressure that the altimeter uses is cial part of every cockpit since the early not at the same altitude everywhere. Pressure surfaces years of aviation. Paul Kollsman’s efforts Knowing a few more details about this - The invisible topography to make this an accurate instrument ush- aspect of the atmosphere should give you of the air used by ered in the art of instrument flying. new appreciation about the importance of The altimeter is very accurate in per- keeping your altimeter properly adjusted. altimeters forming its designed mission, vertically Basic Structure separating you from other aircraft, ob- structions, and the terrain. Altimeter set- If the earth were of uniform tempera- International Standard tings frequently provided by air traffic ture, if the sun provided no heat, and if the controllers enable an altimeter to do its earth didn’t rotate, then pressure, and air Atmosphere. silent task. density would be fairly uniform. The - A condition that rarely Air pressure is constantly changing. earth would have an uncomplicated pres- exists, but from which ASOSs and surface maps keep us sure distribution as shown in Figure 1. your altimeter is calibrated. Pressure altitude, sea level pressure, altimeter setting - What’s that all about? Isobars on a surface chart - What they really signify METAR reports - Hourly pressure data from ASOS you can trust. Mission Statement To enhance aviation safety by Figure 1. Air pressure decreases from a maximum near the earth’s surface to nothing at increasing the pilots’ knowledge the edge of space. If the world were of uniform temperature, then specific pressure read- of weather systems and processes ings would exist at known altitudes around the world. The earth would be encircled with and National Weather Service concentric, invisible pressure surfaces. Specific pressure surfaces, namely 850, 700, 500 products and services. and 300 mb are used by computers to forecast upper level weather patterns. 1 The pressure would decrease smoothly Reducing the Pressure and predictably from about 1013.2 mil- To make plots of air pressure mean- libars (29.92 inches of mercury) at the ingful, barometer readings are mathemat- surface to nothing at the limit of the atmo- ically adjusted to read as if they were all shpere. All the parcels of air feeling a taken at the same elevation. Mean Sea pressure of 850 mb, 700 mb, or 500 mb, if Level (MSL) is very nearly the same visible, would appear as a set of concen- throughout the day despite the tides and is trically nested spheres. Figure 1 shows used as the plane of reference. this arrangement with half the pressure This is done by taking a barometer surfaces removed for easy viewing. reading and mathematically lowering its At about 5,000 feet above ground elevation to the standard sea level. This is level (AGL), the pressure would be about what is meant by “reducing” a reading to 850 mb. At 10,000 feet AGL, a pressure sea level. Figure 3 depicts the process. Air of 700 mb would be measured, and at pressure increases as you reduce its actu- 18,000 feet, a barometer would show a al elevation to where sea level would be pressure of 500 mb. directly beneath the barometer if the The real world is not that simple. ground weren’t there. The surface map in Figure 4 has a pressure pattern where all That simple structure, however, is not ASOS barometer readings have been reality. Planetary heat exchange due to the reduced to sea level pressure. sun’s energy, coupled with the rotation of Calibration is accomplished by using the earth, causes tremendous exchanges Figure 2. If the sun didn’t heat the earth an idealized atmospheric distribution of of air having different temperatures and and the earth didn’t rotate, the atmo- temperature, pressure and air density densities around planet Earth. Regions of shere would have a uniform distribution known as the International Standard high and low pressure develop, and the of attributes (top). However, due to heat- Atmosphere (ISA). The familiar pressure simple vertical distribution of pressure, ing and rotation, areas of high (H) and levels of 850 mb, 700 mb, 500 mb would temperature and density is made complex. low (L) pressure exist, and the vertical exist at their respective altitudes of 5,000 The bottom portion of Figure 2 shows location of specific pressure levels vary. feet, 10,000 feet and 18,000 feet every- what happens to the pressure spheres when mixing occurs. Pressure altimeters follow these wavy pressure surfaces. If barometers were used to report air pressure at ground level throughout the country, the plotted values would have lit- tle use. Barometers at lower terrain eleva- tions would report high pressure because of the weight of the air above pressing A n Sea Le down. Mountain locations would always Mea vel report lower air pressure. B Isobars, or lines of equal pressure, Mean connecting stations with identical pres- Sea sure readings would locate regions of Level high and low pressure, but the maps would be very similar to height lines on a topographic map. Tightly packed lines would indicate steep terrain. Widely space lines would imply gently sloping terrain. To correctly analyze the current weather pattern and to track any recent changes, you would have to carefully compare charts from day to day and even from hour to hour. You would have to Figure 3 ASOS and AWOS barometers are calibrated to read pressure that would be visually determine where pressure found if the altimeter were at sea level instead of the higher elevation where it current- changes were occurring by comparing ly is. The actual atmospheric pressure (barometer A) is mathematically ‘reduced” to sea positions of these pressure lines and not- level (barometer B). Mean sea level is the reference plane because the ocean’s level is ing the very small changes over time. The fairly stable despite tidal fluctuations. The International Standard Atmosphere is used job would be painstaking and tedious. as the standard to calibrate altimeters. The hole in the mountains of Washington in this image shows where measn sea level would be. All altimeters at or above FL180 use mean sea level pressure of 29.92 inches of Hg (1013.2 mb) as their reference setting. 2 atmospheric motion actually displaces those pressure surfaces upward over high pressure regions and downward over low pressure systems. Figure 6 depicts this more realistic situation. Pressure planes or surfaces exist as domes over high pressure and as bowls under low pressure. They intersect the surface map at the isobars. A section of these pressure surfaces in Figure 6 has been removed and every other pressure surface has also been removed for clarity of explanation. The height of the pressure surfaces is also greatly exaggerated for better viewing. ASOS altimeter settings tell which pressure surface is currently intersecting the earth at that airport. That pressure sur- face is at the same height above mean sea level as the runway is. Departing the air- port, your aircraft is climbing above that pressure surface. Pressure surfaces are changing alti- Figure 4. Areas of high and low pressure result from the atmosphere’s attempt to re- tude all the time in response to the air’s distribute excesses and deficiencies of heat and mass. Strong storms develop and pro- natural mixing, rising, and falling. But the mote rising air. Next to these storms, areas of high pressure with descending or subsid- altimeter continues to reference that pres- ing air complete the process. Depicted here are conditions for 1200 UTC on February sure plane. The altitude shown does not 9, 2002. The lines are isobars or lines of equal sea level pressure. The units are mil- tell you whether the pressure plane is libars, an international unit for pressure measurement. moving up or down or whether it’s close to the ground. To avoid problems, you need to keep your little cockpit pal adjust- ed to the nearest altimeter setting or pres- sure plane. Let’s see how this works on a flight from Seattle, WA (SEA) to Rapid City, SD (RAP). Figure 7 is that route using a close up view from Figure 6. Before departing Seattle, you set the altimeter to read the field elevation of 429 feet. The pressure setting of 30.59 inches (1036 mb) shows in the Kollsman window, and is the same pressure given to you on the ATIS broadcast. Enroute the altimeter reports alti- tudes relative to that 1036 mb (30.59 inch) pressure surface unless or until you change the setting in the Kollsman win- dow. An indicated altitude of 9,500 ft Figure 5. The International Standard Atmosphere (ISA) is an average, uniform distrib- MSL means that you are 9,500 ft above ution of pressure, temperature, and density. Actual conditions on any given day as sea level or 9, 071 ft above that pressure measured by balloon soundings are often different. When differences are large, altime- surface. As far as the altimeter knows, the ter errors increase because pressure surfaces move to different levels vertically.