Fog is not precipitation, but on Titan it rains methane
Precipitation means anything that condenses in a cloud and then falls because gravity wins. That rules out fog and mist, which simply hang in the air. Every year about 505,000 cubic kilometres of water comes down on Earth, and on Saturn's moon Titan, a slow drizzle of liquid methane fills puddles at the equator and poles.
Air becomes saturated in two ways, by cooling or by gaining extra water vapour, and often both at once. Inside a cloud, tiny droplets merge by colliding with one another or with ice crystals until they are too heavy for rising air currents to hold up. The result can be drizzle, rain, sleet, snow, ice pellets, graupel or hail, sorted into three families: liquid, liquid that freezes on contact, and ice. Different kinds can fall together.
Meteorologists distinguish three main mechanisms. Convective rain comes from powerful vertical motion that can overturn the atmosphere locally within an hour and unleash downpours; most precipitation in the tropics forms this way. Stratiform rain comes from gentler uplift and falls less intensely. Orographic rain happens when air is pushed up mountainsides, which is why windward slopes can be soaked while the lee side, warmed by sinking air, may be desert. Freezing rain forms when moist air rides over a layer of sub-freezing air near the ground, often near a stationary front. Warm lakes under cold air can generate intense lake-effect snow and even thundersnow.
Averaged across the globe, the yearly total works out to about 990 millimetres, but land receives less, roughly 715 millimetres, since about 398,000 of those cubic kilometres fall on the oceans and only 107,000 on land. Precipitation delivers most of the planet's fresh water, and climate systems such as the Koppen classification use average rainfall to tell regions apart. Global warming is already shifting patterns, bringing more rain to some places, less to others, and more extreme weather.
Rain is measured with a gauge and reported in millimetres of depth. The unit is neat: a litre spread over a square metre makes a layer one millimetre deep, so litres per square metre and millimetres are the same thing. Before metrication, Australians counted rainfall in points, each a hundredth of an inch.
Source: Precipitation
