Density-altitude is a term to describe how dense the atmosphere is in comparison to the International Standard Atmosphere. The datum point for ISA is 59 degrees Fahrenheit (15°C), with a pressure of 29.92 inches of mercury (1013mb) at sea level. The following chart shows the ISA temperature values for some different altitudes above mean sea level.
MSL F C 8000 31°F -1°C 7000 34°F 1°C 6000 38°F 3°C 5000 41°F 5°C 4000 45°F 7°C 3000 48°F 9°C 2000 52°F 11°C 1000 55°F 13°C Sea Level 59°F 15°C
Air density decreases as altitude increases. A rough rule of thumb is that with about every 18,000 to 20,000 ft of altitude gained, there is about half as much air density. So at about 40,000 ft, there is about one quarter the amount of molecules per cubic centimeter of air as found at sea level.
Air density varies with temperature. For every ±15°F or ±8.5°C variation from the standard temperature at your pressure altitude, the density-altitude is increased or decreased by 1000 ft.
To create the same amount of lift, for each 1000 ft increase in density-altitude the true airspeed will increase by 2 percent.
Since kinetic energy increases at the square of the velocity, for each 1000 ft increase in density-altitude, the landing rollout (or the energy required to be absorbed by your legs) will increase by 4 percent.
Going from a low altitude DZ like Monterey, California in the winter to a high DZ like Mile-Hi Colorado in summer, be aware that your canopy will land quite differently (a lot faster)!
These effects can even be seen at a lower altitude DZ like Perris, California when it starts to get, uh, "warm". Landing at Perris (1410 MSL) in the summer with a temperature of 105°F can be the equivalent of landing at a DZ located over 5,000 feet above sea level on a "normal" day.
Humidity is not much of a factor in determining the landing effects of density-altitude. Humidity is a factor for pilots during takeoff as it affects engine performance.