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
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.