flight level. Density altitude can also be determined by referring  to the table and chart in  Figures 11-3 and 11-4  respectively.  Since air is a gas, it can be compressed or expanded. When  air is compressed, a greater amount of air can occupy a  given volume. Conversely, when pressure on a given volume  of air is decreased, the air expands and occupies a greater  space. That is, the original column of air at a lower pressure  as air density decreases (higher density altitude), aircraft  performance decreases. A decrease in air density means a  high density altitude; an increase in air density means a lower  density altitude. Density altitude is used in calculating aircraft  performance. Under standard atmospheric condition, air at  each level in the atmosphere has a specific density; under  standard conditions, pressure altitude and density altitude  identify the same level. Density altitude, then, is the vertical  distance above sea level in the standard atmosphere at which  a given density is to be found.  Density altitude is computed using pressure altitude and  temperature. Since aircraft performance data at any level is  based upon air density under standard day conditions, such  performance data apply to air density levels that may not be  identical to altimeter indications. Under conditions higher  or lower than standard, these levels cannot be determined  directly from the altimeter.  Density altitude is determined by first finding pressure  altitude and then correcting this altitude for nonstandard  temperature variations. Since density varies directly with  pressure, and inversely with temperature, a given pressure  altitude may exist for a wide range of temperature by allowing  the density to vary. However, a known density occurs for  any one temperature and pressure altitude. The density of the  air, of course, has a pronounced effect on aircraft and engine  performance. Regardless of the actual altitude at which  the aircraft is operating, it will perform as though it were  operating at an altitude equal to the existing density altitude.  For example, when set at 29.92 "Hg, the altimeter may  indicate a pressure altitude of 5,000 feet. According to the  AFM/POH, the ground run on takeoff may require a distance  of 790 feet under standard temperature conditions. However,  if the temperature is 20 °C above standard, the expansion of  air raises the density level. Using temperature correction data  from tables or graphs, or by deriving the density altitude with  a computer, it may be found that the density level is above  7,000 feet, and the ground run may be closer to 1,000 feet.  Air density is affected by changes in altitude, temperature,  and humidity. High density altitude refers to thin air while  low density altitude refers to dense air. The conditions that  result in a high density altitude are high elevations, low  atmospheric pressures, high temperatures, high humidity, or  some combination of these factors. Lower elevations, high  atmospheric pressure, low temperatures, and low humidity  are more indicative of low density altitude.  Using a flight computer, density altitude can be computed by  inputting the pressure altitude and outside air temperature at  11-4  Figure 11-4.  Density altitude chart.  Outside air temperature (OAT)  Density altitude (feet)  15,000  14,000  13,000  12,000  11,000  10,000  9,000  8,000  7,000  6,000  5,000  4,000  3,000  2,000  1,000  Sea level  14,000 13,000  12,000 11,000  10,000 9,000 Pressure altitude (feet) 8,000 7,000 6,000  5,000  4,000 3,000  2,000  1,000 -1,000 Sea level -2,000 -20° -10° 0° 10° 20° 30° 40° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°  100°  Standard temperature C  F  Effects of Pressure on Density 