In meteorology, rainfall types can include the character or phase of the precipitation which is falling to ground level. There are three distinct ways that rain can occur. These methods include orographic rainfall. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Precipitation can also fall in two phases, either liquid or solid. Liquid forms of precipitation include rain and drizzle. Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding word of freezing, becoming known as freezing rain or freezing drizzle. Frozen forms of precipitation include snow, ice needles, sleet, hail, and graupel. Intensity is determined either by rate of fall, or by visibility restriction.
Precipitation falls in various forms, or phases. They can be subdivided into:
- Freezing precipitation:
- Frozen precipitation:
Precipitation occurs when air is saturated; this occurs when air rises; which in turn usually occurs in one of three ways.
Convective precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of convection. Stratiform precipitation occurs when large masses over air rise slant-wise as larger-scale atmospheric dynamics force them to move over each other. Orographic precipitation is similar, except the upwards motion is forced when a moving airmass encounters a rising slope.
Convection occurs when the Earth's surface, mainly in the equatorial region, within a conditionally unstable, or moist atmosphere, becomes heated more than its surroundings, leading to significant evaporation. Convective rain, or showery precipitation, occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation thunderstorms. Graupel and hail indicate convection. In mid-latitudes, convective precipitation is associated with cold fronts (often behind the front), squall lines, and warm fronts in very moist air.
Stratiform rainfall is also caused by frontal systems surrounding extratropical cyclones or lows, which form when warm and often tropical air meets cooler air. Stratiform precipitation falls out of nimbostratus clouds. When masses of air with different density (moisture and temperature characteristics) meet, warmer air overrides colder air. The warmer air is forced to rise and if conditions are right becomes saturated, causing precipitation. In turn, precipitation can enhance the temperature and moisture contrast along a frontal boundary. Fronts cause sudden changes in general temperature, and in the humidity and pressure in the air. Warm fronts occur where the warm air scours out a previously lodged 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, because, after the warm air rises above the cooler air (which sinks to 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 a mass of cooler air dislodges a mass of warm air. This type of transition is sharper, since cold air is more dense than warm air. The rain duration is less, and generally more intense, than that which occurs ahead of warm fronts. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.
Orographic or relief rainfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains. The lift of the air up the side of the mountain results in adiabatic cooling, 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. Moisture is removed by orographic lift, leaving drier air (see Foehn) on the descending (generally warming), leeward side where a rain shadow is observed.
In Hawaii, Mount Waiʻaleʻale (Waiʻaleʻale), on the island of Kauai, is notable for its extreme rainfall, as it has the highest average annual rainfall on Earth, with 460 inches (12,000 mm). Storm systems affect the state with heavy rains between October and March. Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.
In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming the Great Basin desert, Mojave Desert and Sonoran Desert.
Precipitation is measured using a rain gauge. When classified according to the rate of precipitation, rain can be divided into categories. Very light rain describes rainfall with a precipitation rate of less than 0.25 millimetres (0.0098 in) per hour. Light rain describes rainfall which falls at a rate of between 0.25 millimetres (0.0098 in) and 1 millimetre (0.039 in) per hour. Moderate rain describes rainfall with a precipitation rate of between 1 millimetre (0.039 in) and 4 millimetres (0.16 in) per hour. Heavy rain describes rainfall with a precipitation rate of between 4 millimetres (0.16 in) and 16 millimetres (0.63 in) per hour. Very heavy rain terminology can be used when the precipitation rate is between 16 millimetres (0.63 in) and 50 millimetres (2.0 in) per hour. Extreme rain can describe rainfall with precipitation rates exceeding 50 millimetres (2.0 in) per hour.
Snowfall's intensity is determined by visibility. When the visibility is over 1 kilometre (0.62 mi), snow is determined to be light. Moderate snow describes snowfall with visibility restrictions between .5 kilometres (0.31 mi) and 1 kilometre (0.62 mi). Heavy snowfall describes conditions when visibility is restricted below .5 kilometres (0.31 mi).
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