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## Altitude 

As altitude increases, air density decreases. At altitude, the atmospheric pressure that acts on a given volume of air is less, allowing the air molecules to space themselves further apart. The result is that a given volume of air at high altitude contains fewer air molecules than the same volume of air at lower altitude. As altitude increases, density altitude increases, and glider takeoff and climb performance is reduced. 

## Temperature 

Temperature changes have a large effect on density altitude. When air is heated, it expands—the molecules move farther apart, making the air less dense. Takeoff and climb performance is reduced, while the length of runway required for landing is increased. 

Consider the following METAR for two airports with same altimeter setting, temperature, and dewpoint. Love Field (KDAL) airport elevation of 487 feet versus Denver International (KDEN) at 5,431 feet. 

KDAL 240453Z 21007KT 10SM CLR 25/15 A3010 RMK AO2… KDEN 240453Z 24006KT 10SM FEW120 SCT200 25/15 A3010 RMK AO2… 

The computed density altitude for Love Field is 1,774 feet; for Denver, 7,837 feet—almost twice the density altitude increase compared to the increase for Love Field. The effects of attitude and temperature are significant, a fact pilots must consider when computing aircraft performance. 

## Wind 

Wind affects glider performance in many ways. Headwind during launch results in shorter ground roll, while tailwind causes longer ground roll before takeoff. [Figure 5-2] Crosswinds during launch require proper crosswind procedures or control input to track along the runway. 

During cruising flight, headwinds reduce the groundspeed of the glider. A glider flying at 60 knots true airspeed into a headwind of 25 knots has a groundspeed of only 35 knots. Tailwinds, on the other hand, increase the groundspeed of the glider. A glider flying at 60 knots true airspeed with a tailwind of 25 knots has a groundspeed of 85 knots. 

Crosswinds during cruising flight cause glider heading (the direction in which the glider nose is pointed) and glider track (the path of the glider over the ground) to diverge. In a glider, it must be remembered that crosswinds have some head or tailwind component that results in a lower or higher true groundspeed. When planning the landing, the wind effects must be factored into the landing pattern sight picture 

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and allowances must be made for the winds, indicated or expected. It is a lot easier to lose altitude than it is to make up altitude when the glider is down low. When gliding toward an object on the ground in the presence of crosswind, such as on final glide at the end of a cross-country flight, the glider pilot should keep the nose of the glider pointed somewhat upwind of the target on the ground. For instance, if the crosswind is from the right during final glide, the nose of the glider is pointed a bit to the right of the target on the ground. The glider’s heading is upwind (to the right, in this case) of the target, but if the angle of crab is correct, the glider’s track is straight toward the target on the ground. [Figure 5-3] 

Headwind during landing results in a shortened ground roll, and tailwind results in a longer ground roll. Crosswind landings require the pilot to compensate for drift with the proper flight control input, such as a sideslip or a crab. Glider pilots must be aware of the apparent angle versus the rate of descent. The glider descends at a constant rate, but the different groundspeeds will result in different approach angles. These different approach angles require specific techniques to ensure safe touchdowns in the landing zone. For example, if landing in a strong headwind, the glider pilot should plan for a closer base leg to allow for the apparent steeper approach due to the slower ground speed. Another technique would be the delayed extension of spoilers or drag brakes and accepting the faster airspeed to counter the headwind component. In the case of a tailwind and an apparent lower angle of approach, the glider pilot can use more spoiler or drag brake extension or slipping or a combination as allowed by the GFM to touchdown in the landing zone. In any condition, the glider pilot must make allowances for the current conditions of wind and density altitude to ensure a safe landing in the landing area. A glider pilot must respond to the current conditions and amend the traffic pattern and/or modify their procedures to compensate 

