As air rises it expands, owing to the decrease in pressure with height, and as it expands, in theory it cools at an average rate of 1°C for every 100 m of altitude. As the air cools, it becomes saturated with water vapour which condenses around small particles in the air. These particles may occur naturally, such as soil particles or salt particles residual to evaporation of sea spray, or they may be produced artificially during combustion. A measure of the necessary cooling to produce condensation is the dewpoint, which is the temperature at which air of a given absolute humidity and at a given pressure begins to give up its water as drops of dew. When moist air is chilled to a temperature below its dewpoint, in the presence of suitable minute particles, a cloud or mist will form.