Condensation is the process whereby water vapour in the atmosphere is returned to its original liquid state. In the atmosphere, condensation may appear as clouds, fog, mist, dew or frost, depending upon the physical conditions of the atmosphere. Condensation is not a matter of one particular temperature but of a difference between two. Condensation of water vapour occurs when the temperature of air is lowered to its dew point.
All air contains water vapour of varying quantities. The lower the air temperature, the smaller the maximum possible capacity for vapour. When air is cooled, relative humidity increases, until at a particular temperature, called the dew point, the air becomes saturated. Further cooling below the dew point will induce condensation of the excess water vapour.
The temperature of the dew point will depend upon the absolute content of water vapour, that is the absolute humidity, measured in g/m3 (grams per cubic metre). The dew point of humid air will be higher than the dew point of dry air. Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled. If air in contact with the ground is cooled to its dew point, dew or frost will form, dew if the point is above 0°C, or frost if it is below 0°C. Cooling of a larger layer of air near to the ground may produce mist of fog, which freezes if the dew point is below 0°C. Air that is cooled to its dew point by rising and expansion will condense to form clouds. Above 0°C, small droplets of water are formed. Condensation may also result in ice crystals at temperature well below 0°C. When temperatures are near or a little below 0°C, supercooled water droplets can form.