I have realized that the Altimeter of my bike computer (it uses Air Pressure to determine altitude and needs to be calibrated every start) is not consistent at different temperatures.
At the Start the altimeter automatically calibrates to my home altitude 842m. When I reach the top of the mountain altitude varies depending on the temperature. At 30 degrees Celsius I get readings as low as 1050m, at -10 degrees Celsius I get readings as high as 1080m. This is NOT because the temperature decreases as I go up the mountain. The temperature difference between the valley and the top of the mountain is almost insignificant (1-2 degrees). I'm well aware pressure increases as temperature drops.
The conclusion I take is that when temperatures are low, air pressure does not drop as much with increasing altitude as when temperatures are high. We have gravity and temperature as relevant sources of pressure changes but it seems they do not behave as linearly as we would like them to. Assuming the altimeter was designed to operate optimally at 10 degrees Celsius and 900m (Trusting Google Earth, the true altitude is 1069m which is what my altimeter displays at around 10 degrees celsius) how do I conclude by how much the altimeter is off at different temperatures & altitudes?