Factors Affecting Performance High and Low Density Altitude Conditions Glider performance during launch depends on the power  output of the launch mechanism and on the aerodynamic  efficiency of the glider itself. The four major factors that affect  performance are density altitude, weight, design, and wind. Every pilot must understand the terms “high density altitude”  and “low density altitude.” In general, high density altitude  refers to thin air, while low density altitude refers to dense  air. Those conditions that result in a high density altitude  (thin air) are high elevations, low atmospheric pressure,  high temperatures, high humidity, or some combination  thereof. Lower elevations, high atmospheric pressure, low  temperatures, and low humidity are more indicative of low  density altitude (dense air). However, high density altitudes  may be present at lower elevations on hot days, so it is  important to calculate the density altitude and determine  performance before a flight. One way to determine density altitude is to use charts  designed for that purpose.  [Figure 5-1]  For example, you  plan to depart an airport where the field elevation is 1,600 feet  MSL. If the altimeter setting is 29.80, and the temperature  is 85 °F, what is the density altitude? First, correct for  nonstandard pressure (29.8 "Hg) by referring to the right side  of the chart and adding 112 feet to the field elevation. The  result is a pressure altitude of 1,712 feet. Then, enter the chart  at the bottom, just above the temperature of 85 °F (29.4 °C).  Proceed up the chart vertically until intercepting the diagonal  1,712-foot pressure altitude line, then move horizontally to  the left and read the density altitude of approximately 3,500  feet. This means a self-launching glider or towplane will  perform as if it were at 3,500 feet MSL on a standard day. Most performance charts do not require a pilot to compute  density altitude. Instead, the computation is built into the  performance chart itself. A pilot needs only the correct  pressure altitude and the temperature. Some charts, however,  may require computing density altitude before entering them.  Density altitude may be computed using a density altitude  chart or by using a flight computer. Due to changing weather conditions, atmospheric pressure  at a given location changes from day to day. The following  is the METAR report for Love Field observed on the 23rd  at 21:53Z (GMT) which indicates a local pressure of A2953,  or altimeter setting of 29.53 "Hg. When barometric pressure  drops, air density decreases. The reduced density of the air  results in an increase in density altitude and decreased glider  performance. This reduces takeoff and climb performance  and increases the length of runway needed for landing. KDAL 232153Z 21006KT 7SM -RA BKN025 BKN060  OVC110 12/11 A2953 RMK AO2 PRESFR SLP995 P0005  T01220106 The 12/11 notation of this report indicates the reported  temperature and dewpoint for Love Field. When barometric pressure rises, air density increases. The  greater density of the air results in lower density altitude.  Thus, takeoff and climb performance improves, and the  length of runway needed for landing decreases. Atmospheric Pressure 5-2 Figure 5-1.  Density altitude chart. Density Altitude Chart  Outside air temperature  Approximate density altitude (thousand feet)  15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 S.L. 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 -18° -12° -7° -1° 4° 10° 16° 21° 27° 32° 38° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°  100° Standard temperature F  C  28.0 1,824 28.1 1,727 28.2 1,630 28.3 1,533 28.4 1,436 28.5 1,340 28.6 1,244 28.7 1,148 28.8 1,053 28.9 957 29.0 863 29.1 768 29.2 673 29.3 579 29.4 485 29.5 392 29.6 298 29.7 205 29.8 112 29.9 20 29.92 0 30.0  −73  30.1  −165  30.2  −257  30.3  −348  30.4  −440  30.5  −531  30.6  −622  30.7  −712  30.8  −803  Altimeter setting ("Hg)  Pressure altitude  conversion factor 