Air temperature
Under conditions of thermodynamic equilibrium, in stable weather, air temperature drops by about 0.6°C every 100 m of gain in altitude. However, from a purely theoretical point of view this calculation is of little use, since the atmosphere is not in a state of static equilibrium. From the hiker's point of view, on the other hand, this figure is a good enough guide and becomes fairly meaningful when it is combined with observations of the environment you are in.
Air, contrary to what you might imagine, is not heated directly by the sun.
Most of the sun's rays pass through the air and land directly on the earth, lighting up ground, rocks and woods. Depending on the reflectivity (albedo) of the sunlit ground, the earth holds on to a quantity of light, converting it into heat and giving part of it back to the atmosphere in the form of infrared rays. It is these that heat the air.
In snow-covered areas, with ice, dazzling white snow or particularly pale rocks, reflectivity is very high, that is, the ground does not absorb heat but reflects it completely in the form of light. This is also why high-mountain air is colder than the air down in the valley. Pale surfaces, in fact, are unable to hold on to the heat by converting it into infrared rays, and as a result the air above them cannot warm up.
Air humidity
Air is able to hold a certain amount of water in suspension in the form of vapour. This amount of water depends on temperature. For the same absolute humidity, as temperature rises the air's capacity to hold water vapour in suspension increases, with a consequent drop in the relative humidity of the air. Conversely, if the air temperature falls, its water vapour content falls too. When this decrease reaches the saturation limit typical of that temperature, the excess water is expelled from the air in the form of fog, rain, snow, hail or dew (on the ground). A mass of moist air that rises undergoes a drop in temperature and so can no longer hold the same amount of moisture. At altitude, the excess water tends to condense, being expelled from suspension and giving rise to clouds.
Conclusions
As far as weather observation is concerned, if temperature anomalies appear locally, that is, variations not directly linked to the daily action of the Sun, you can assume you are facing a change in the weather.
A concrete example: it's midday, the sun is high but in spite of everything the temperature starts to drop, there are scattered signs of condensation in the air, small scattered cloud formations: we are facing a deterioration in the weather.
Sources:
- Polvere di mare, freeride a Roccaraso - Rotili, Verdecchia
Frequently asked questions
How much does the temperature drop as you climb in the mountains?
In stable weather the air temperature drops by about 0.6 °C every 100 metres of altitude. It is a rough guide, but for hikers it becomes useful when combined with observation of the environment around them.
How can you tell from the temperature that the weather is getting worse?
One sign is a drop in temperature that is not linked to the sun's daily cycle: for example at midday, with the sun high, the temperature falls and condensation and small scattered clouds appear in the air. It is a clue that the weather is deteriorating.