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2(d). Atmosphere

Temperature affects an aircraft's performance and is perhaps most crucial when making altimetry readings. The altimeter is basically an aneroid barometer that is graduated to use increments of height. The standard used for graduating the altimeter is that of standard atmosphere. Standard atmosphere was developed by engineers and meteorologists who needed a fixed standard with which they could reference for aircraft performance and weather, respectively. They arrived at standard atmosphere by averaging conditions throughout the atmosphere for all latitudes, seasons and altitudes. This gave them specific sea-level temperature, pressure and rates of change of temperature and pressure with height. In this hypothetical atmosphere, pressure falls at a fixed rate upward through the atmosphere unlike the actual atmosphere. Think of the standard atmosphere as a measuring stick to which pilots compare their altitude based upon other factors (density, pressure and temperature). So, why is a standard atmosphere necessary? The altimeter measures altitude (that is vertical distance above a level plane, such as sea level), so at any time a pilot should know the aircraft's distance from sea level, right? The airplane's altimeter measures the atmospheric pressure at flight level. In other words it is measuring the weight of the air pressing down on it from above. What the altimeter cannot measure is the air density and the air temperature, both of which affect the flight characteristics of the aircraft. An airplane performs differently in different temperature and air density situations. By establishing a standard atmosphere, pilots can compare their airplanes performance under certain conditions to this standard. Pilots can then make adjustments accordingly to their instruments and flight plan.

Look at the bar graph below for an example of how temperature affects the reading of true altitude. According to standard atmosphere an airplane flies at 10,000 feet given a standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars. Let's take 3 aircraft, each flying atop a measurable column of air. The air pressure at the top and bottom of each column of air is equal. Now let's change the temperature of the columns making one warmer than the standard and the other column colder than the standard. When the air is warmer than the standard, the altimeter reads lower than the airplane's true altitude. When the air is colder than the standard, the altimeter reads higher than the airplane's true altitude.