Suunto X6 User Manual page 50

The outside temperature affects the air weight. Consequently the air pressure differ-
ence between two altitudes is also dependant on temperature.
The altitude calculation of Suunto X6/X6M is based on the air pressure at certain
normal temperatures. Each altitude has a definitive normal temperature. The normal
temperatures at each altitude are presented in Table 1.
The altitude measurement error caused by an abnormal temperature gradient can be
approximated as follows. If the sum of the temperature offsets from the
normal temperatures determined at two different altitudes is 1 ºC, the
altitude difference calculated by the Suunto X6/X6M is 0.2% off the real
altitude difference (When using imperial units the offset factor is 0.11% / 1 ºF).
This is because the real temperatures are not always the same as the normal tem-
peratures. A higher than normal temperature causes the calculated altitude differ-
ence to be smaller than the real altitude difference (your mountain ascent was
actually higher). Consequently, a lower than normal temperature causes the calcu-
lated altitude difference to be larger than the real altitude difference (you did not
ascend quite as high as displayed).
Table 2 shows an example in which the temperature offsets are positive. In this
example, the reference altitude is set at 1000 m. At 3000 m the altitude difference is
2000 m and the Suunto X6/X6M shows 80 m too little (20 ºC * 2000 m * 0.002/ºC = 80
m). Your actual altitude is thus 3080 m.
All Suunto products with an altimeter are temperature-calibrated. This means that at
constant pressure, the pressure sensor reading is always the same independent of
the unit's own temperature. However, the air temperature can still affect the altitude
readings as described above.
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