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The air resistance acting on a rider is directly related to the density of the air, the greater the density, the greater the force. 

Under standard conditions, the air's density at altitude is less than at sea level. The following table shows typical values:

Elevation Air Density Units
Sea Level 1.226 kg/m3
1500 m 1.056 kg/m3
3000 m 0.905 kg/m3

Although the above values are typical, on any given day the air's density may be much different from values given in the above table. 

Was it a bad day, or was it Air Density?   As an example, Denver, Colorado, at an elevation of 1500 meters with temperature of 24°C and barometric pressure of 29.01 inches of Hg could have an air density of 0.960 kg/m3 (lower than standard pressure).  At the same time Wilmington, DE, at sea level with a temperature of 12°C and a barometric pressure of 30.29 inches of Hg could have an air density of 1.253 kg/m3 (higher than standard).  Due to differences in Air Density it would require 28% more power to ride the same pursuit time in Wilmington as in Denver, even though 15% is more or less the nominal difference. 

Air density depends on temperature, barometric pressure and altitude and to some extent on water vapor (humidity). Air density is calculated here as a function of temperature, barometric pressure, and altitude, neglecting the effect of water vapor which is small. 

Current and historical daily temperature and barometric pressure for most places in the world can be found at 
Weather Underground, Inc.

An explanation of the relationship between Air Density and barometric pressure can be found at the USA Today Weather Page. 

© 1997 Tom Compton