dgpcolorado said:
Some rough numbers for the reduction in air density versus altitude, other conditions being equal (temperature, humidity, weather air pressure):
Sea Level (0)
1000 feet -4%
2000 feet -7%
3000 feet -10%
4000 feet -14%
5000 feet -17%*
6000 feet -20%
7000 feet -23%
8000 feet -26%
9000 feet -29%
10,000 feet -31%
The conditions are not accurate for equal temperatures and pressures. The density altitude percentages you specify are based on
standard conditions at that altitude.
A rule of thumb for standard temperature is to subtract 2C per each 1000 feet higher in altitude. "Standard" temperature at sea level is 15C (60F), so where you live, at about 8000 feet, the standard temperature would 8 * 2 = 16 degrees Celcius less than 15C, or -1C.
So, you can use
this calculator to plug in:
> 8000 feet (2438 meters)
> 30 (-1C) temperature
> 29.92 inches (1013mb)
> 0 dew point for this example
The answer you'll get is very close to 8000 feet
density altitude. That's standard conditions that will equal the approximate 26% reduction in air density. If the temperature were 15C at sea level, and then 15C at 8000 feet, the difference in density altitude would be about 10,000 feet, or about 31%.
When you're in that big airplane at 30,000 feet, and see on the info screen that it's -45C outside, that fits the rule of thumb, 15 - (30 * 2) = -45.
Also, air pressure are not the same. There is a rule of thumb of 1 inch of mercury pressure loss per thousand feet increase in altitude. So, your 8000 foot mountain home would only have a standard pressure of:
29.92 [inches at sea level] - (8 * 1) = 21.92
The pressures reported by the weatherman and aviation "METAR" reports are corrected for altitude, therefore would report 29.92 (QNH). Only one airline, that I'm aware of, in the western world used "QFE", or actual pressure at Field Elevation; American Airlines. Every time they landed, the altimeter would read "0". Some former Soviet and Chinese countries also use QFE.
A crash of a U.S. Air Force aircraft in eastern Europe was attributed to mistaking QFE for QNH.