Flytec Windwatch Pro - Manual (Page 19)

Altimeter 19
Altimetry For the Ambitious User
Temperature of the air column
The standard altimetry formula was developed for aircraft in 1928. It is based upon an average
temperature of 59ºF (15°C) at sea level and an average temperature drop of 0.65°C per 100
meters. In the case where the actual air column is warmer, the air will be packed less densely. If
you are standing on top of a mountain, there will be more air particles above you than normal
and the air pressure will be somewhat higher. Therefore the altitude calculated according to the
standard formula will be too low. In the case where the actual air column is colder, the cold air
will be more dense and concentrated at ground level with fewer air particles above. Therefore
on top of a mountain the air pressure will be slightly lower at your position and the altitude
displayed will be higher than actual. On top of a 14,000 ft peak this situation can result in a 500
ft error!
Example: A temperature gradient deviation of 1°C per 1000 meters induces approximately a 4-
meter error (this empirical formula is valid for up to 4000m). At an altitude of 2000m during
summer where the air mass is 16°C warmer compared to the standard atmosphere, a standard
altimeter will display 2 x 4 x 16 = 128m lower than actual.
Note about correcting air column temperature: While the Windwatch PRO makes it possible
to increase altimeter accuracy by compensating for air column temperatures that deviate from
standard, total compensation is not always possible due to the layered structure of the
atmosphere. When setting a correction, the WWP proposes a +8ºC setting for warmer than
standard and -8ºC setting for colder than standard. This is based on the typical temperature
deviation for the summer and winter months in geographical mid-latitudes. If you suspect that an
air column correction other than the proposed ± 8ºC should be used, ambient temperature
measurements should be taken outside in the shade and compared to the table below. Note
also that the measured air temperature may be warmer than the overall air mass due to
localized thermal activity, and that should be factored out. If the air column temperature
correction is properly applied, a significant increase in accuracy can be achieved.
Table normal atmosphere
Alt. Meter Temperature ºC Pressure hPa
0 15.0 1013.2
1000 8.5 898.7
2000 2.0 794.9
3000 -4.5 701.1
4000 -11.0 616.4