Humidity is the amount of water vapour in the air; condensation is the change of that vapour back into liquid (or ice) when air is cooled to its dew point, producing dew, fog, mist, clouds, and precipitation.
- Absolute humidity is the actual mass of water vapour per unit volume of air; relative humidity is the ratio of the vapour present to the maximum the air can hold at that temperature, expressed as a percentage.
- Warm air can hold more water vapour than cold air, so relative humidity rises as air cools even without adding moisture; the temperature at which the air becomes saturated (100 per cent) is the dew point.
- Condensation needs cooling to the dew point and the presence of hygroscopic nuclei (such as dust and salt) around which droplets form.
- Forms of condensation: dew (on cold surfaces), white frost (below freezing), fog and mist (near the ground, reducing visibility), and clouds (in the free air).
- Clouds are classified by height and form: cirrus (high, wispy), cumulus (heaped, fair-weather or storm), stratus (layered, overcast), and the rain-bearing cumulonimbus and nimbostratus.
The absolute versus relative humidity distinction, the dew point, the role of condensation nuclei, and the cloud types (cirrus, cumulus, stratus, cumulonimbus) are recurring climatology facts; fog has an operational bearing on border visibility.
Absolute humidity (actual amount) versus relative humidity (percentage of capacity); relative humidity rises when air cools even if no moisture is added; fog (suspended near the ground) versus cloud (in the free air); cirrus (high) versus cumulus (heaped).
Humidity is vapour in the air; cooling to the dew point gives condensation as dew, fog, and clouds (cirrus, cumulus, stratus, cumulonimbus).