Precipitation occurs when evapotranspiration takes place and local air becomes saturated with water vapor, and so can no longer maintain the level of water vapor in gaseous form, which creates clouds. This occurs when less dense moist air cools, usually when an air mass rises through the atmosphere to higher and cooler altitudes. However, an air mass can also cool without a change in altitude (e.g. through
radiative cooling, or ground contact with cold terrain).
Convective precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of
convection.
Stratiform precipitation occurs when large air masses rise diagonally as larger-scale winds and atmospheric dynamics force them to move over each other.
Orographic precipitation is similar, except the upwards motion is forced when a moving air mass encounters the rising slope of a landform such as a mountain ridge or slope.
Convectional s. Convection occurs when the Earth's surface, especially within a conditionally unstable or moist
atmosphere, becomes heated more than its surroundings and in turn leading to significant evapotranspiration. Convective rain and light precipitation are the result of large convective clouds, for example
cumulonimbus or
cumulus congestus clouds. In the initial stages of this precipitation, it generally falls as showers with a smaller area and a rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited vertical and horizontal extent and do not conserve much water. Most precipitation in the
tropics appears to be convective; however, it has been suggested that stratiform and convective precipitation often both occur within the same complex of convection-generated cumulonimbus.
Graupel and
hail indicate convection when either or both are present at the surface. They are indicative that some form of precipitation forms and exists at the freezing level, a varying point in the atmosphere in which the temperature is 0°C. In mid-latitude regions, convective precipitation is often associated with
cold fronts where it is often found behind the front, occasionally initiating a
squall line.
Cyclonic Frontal precipitation is the result of frontal systems surrounding
extratropical cyclones or lows, which form when warm and tropical air meets cooler, subpolar air. Frontal precipitation typically falls out from
nimbostratus clouds. When masses of air with different densities (moisture and temperature characteristics) meet, the less dense warmer air overrides the more dense colder air. The warmer air is forced to rise and, if conditions are right, creates an effect of saturation and condensation, causing precipitation. In turn, precipitation can enhance the temperature and dewpoint contrast along a frontal boundary, creating more precipitation while the front lasts. Passing weather fronts often result in sudden changes in environmental temperature, and in turn the
humidity and
pressure in the air at ground level as different air masses switch the local weather.
Warm fronts occur where advancing warm air pushes out a previously extant cold air mass. The warm air overrides the cooler air and moves upward. Warm fronts are followed by extended periods of light rain and drizzle due to the fact that, after the warm air rises above the cooler air (which remains on the ground), it gradually cools due to the air's expansion while being lifted, which forms clouds and leads to precipitation.
Cold fronts occur when an advancing mass of cooler air dislodges and plows through a mass of warm air. This type of transition is sharper and faster than warm fronts, since cold air is more dense than warm air and sinks through in gravity's favor. Precipitation duration is often shorter and generally more intense than that which occurs ahead of warm fronts. A wide variety of weather can be found along an
occluded front, usually found near
anticyclonic activity, but usually their passage is associated with a drying of the air mass.
Orographic Orographic or relief rainfall is caused when masses of air are forced up the side of elevated land formations, such as large
mountains or
plateaus (often referred to as an upslope effect). The lift of the air up the side of the mountain results in
adiabatic cooling with altitude, and ultimately condensation and precipitation. In mountainous parts of the world subjected to relatively consistent winds (for example, the
trade winds), a more moist
climate usually prevails on the
windward side of a mountain than on the
leeward (downwind) side, as wind carries moist air masses and orographic precipitation. Moisture is precipitated and removed by orographic lift, leaving drier air (see
Foehn) on the descending (generally warming), leeward side where a
rain shadow is observed. isolated towering vertical thunderhead shower in
Mojave Desert, western USA In
Hawaii,
Mount Waiʻaleʻale (
Waialeale), on the island of Kauai, is notable for its extreme rainfall. It currently has the highest average annual rainfall on Earth, with approximately per year. Storm systems affect the region with heavy rains during winter, between October and March. Local climates vary considerably on each island due to their topography, divisible into windward (
Koolau) and leeward (
Kona) regions based upon location relative to the higher surrounding mountains. Windward sides face the east-to-northeast
trade winds and receive much more clouds and rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. On the island of Oahu, high amounts of clouds and often rain can usually be observed around the windward mountain peaks, while the southern parts of the island (including most of Honolulu and Waikiki) receive dramatically less rainfall throughout the year. In South America, the
Andes mountain range blocks
Pacific Ocean winds and moisture that arrives on the continent, resulting in a desert-like climate just downwind across western Argentina. The
Sierra Nevada range creates the same drying effect in North America, causing the
Great Basin Desert,
Mojave Desert, and
Sonoran Desert. == Intensity ==