Precipitation: Types of Precipitation & Distribution of Rainfall – GEOGRAPHY UPSC NOTES

Precipitation is the process by which water vapour condenses in the atmosphere and falls to the ground under the influence of gravity. These phenomena result in rain and snow. Precipitation, for example, occurs when a portion of the atmosphere becomes saturated with water vapour and condenses and precipitates when the temperature is appropriate. The cooling of air molecules and the addition of water vapour are the two processes that cause the air to become saturated and it is a process associated with Adiabatic lapse rate.

What is Precipitation?

Precipitation refers to all liquid and solid water particles that fall from clouds and reach the ground. Drizzle, rain, snow, snow pellets, ice crystals, and hail are examples of these particles. The primary distinction between precipitation particles and cloud particles is one of size. A typical raindrop weighs the same as one million cloud droplets. Precipitation particles have high falling speeds due to their large size and can survive the fall from the cloud to the ground.

Precipitation Formation Conditions

There are some important conditions required for the formation of Precipitation such:

1. Moisture must be present in the atmosphere.
2. There must be enough nuclei present to facilitate condensation.
3. Condensation of water vapour requires favourable weather conditions.
4. Condensation products must reach the earth.

Types of Precipitation

Rain

Drizzle

Snow

Sleet

Hail

Rain

• Rain is precipitation that falls to the surface of the Earth as water droplets. Raindrops form around microscopic cloud condensation nuclei, such as a particle of dust or a molecule of pollution.
• Rain that falls from clouds but freezes before it reaches the ground is called sleet or ice pellets.
• Even though cartoon pictures of raindrops look like tears, real raindrops are actually spherical.

Drizzle

• Another variation from rain is drizzle. It consists of light water precipitation where liquid water droplets are smaller than those of rain. This can occur when updrafts in clouds are not strong enough to allow them to produce rain. Drizzle usually happens thanks to low-level clouds called ‘stratiform clouds.’
• Drizzle tends to occur more often over colder regions of the subtropics. In these locations, what scientists call a ‘supercooled drizzle’ or freezing drizzle, can also occur. This happens at temperatures as low as 10 degrees F or lower, depending on how shallow the cold air layer is.
• Drop size less than 0.5 mm.

Snow

• Snow consists of ice crystals in a flaky form, having an average density of 0.1g/cc. It is also an important form of precipitation that usually forms in colder climates and higher altitudes.

Sleet

• Sleet is frozen raindrops that are formed when rainfall passes through the air in the atmosphere at subfreezing temperatures.
• It is a type of precipitation in the form of a mixture of rain and snow.
• It is a frozen rain that forms when rain while falling to the earth passes through a layer of very cold air mass.
• Diameter > 5 mm

Hail

• Hail is a kind of showery precipitation in the form of pellets or lumps that have a size greater than 8mm. Hail occurs during violent thunderstorms.
• It falls in the form of small ice pellets. Hail is the most destructive form of precipitation produced in violent thunderstorms or cumulonimbus clouds.
• The hail consists of concentric layers of ice alternating with layers of snow. Its structure resembles that of onion.

Types of Rainfall

On the basis of origin, rainfall may be classified into three main types – convectional, orographic, or relief, and cyclonic or frontal.

Conventional Rainfall

• The air on being heated, becomes light and rises up in convection currents. As it rises, it expands and loses heat, and consequently, condensation takes place and cumulous clouds are formed. This process releases latent heat of condensation which further heats the air and forces the air to go further up.
• Convectional precipitation is heavy but of short duration, highly localized, and is associated with the minimum amount of cloudiness. It occurs mainly during summer and is common over equatorial doldrums in the Congo basin, the Amazon basin, and the islands of south-east Asia.

Orographic Rainfall

• When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands (because of a fall in pressure); the temperature falls, and the moisture is condensed.
• This type of precipitation occurs when warm, humid air strikes an orographic barrier (a mountain range) head-on. Because of the initial momentum, the air is forced to rise. As the moisture-laden air gains height, condensation sets in, and soon saturation is reached. The surplus moisture falls down as orographic precipitation along the windward slopes.
• The chief characteristic of this sort of rain is that the windward slopes receive greater rainfall. After giving rain on the windward side, when these winds reach the other slope, they descend, and their temperature rises. Then their capacity to take in moisture increases and hence, these leeward slopes remain rainless and dry. The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area (Some arid and semi-arid regions are a direct consequence of the rain-shadow effect. Example: Patagonian desert in Argentina, Eastern slopes of Western Ghats). It is also known as the relief rain.
• Example: Mahabaleshwar, situated on the Western Ghats, receives more than 600 cm of rainfall, whereas Pune, lying in the rain shadow area, has only about 70 cm.

The Wind Descending on the Leeward Side is heated adiabatically and is called Katabatic Wind.

Frontal Precipitation

When two air masses with different temperatures meet, turbulent conditions are produced. Along the front convection occurs and causes precipitation (we studied this in Fronts). For instance, in northwest Europe, cold continental air and warm oceanic air converge to produce heavy rainfall in adjacent areas.

Monsoonal Rainfall

This type of precipitation is characterized by seasonal reversal of winds that carry oceanic moisture (especially the southwest monsoon) with them and cause extensive rainfall in the south and southeast Asia.

World Distribution of Rainfall

• Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons. In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily.
• The coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more in the oceans than on the landmasses of the world because of are great sources of water.
• Between the latitudes 35° and 40° N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 45° and 65° N and S of the equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.
• Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.
• On the basis of the total amount of annual precipitation, major precipitation regimes of the world are identified as follows.
• The equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone, and the coastal areas of the monsoon land receive heavy rainfall of over 200 cm per annum.
• Interior continental areas receive moderate rainfall varying from 100 – 200 cm per annum. The coastal areas of the continents receive a moderate amount of rainfall.
• The central parts of the tropical land and the eastern and interior parts of the temperate lands receive rainfall varying between 50 – 100 cm per annum.
• Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive very low rainfall – less than 50 cm per annum.
• The seasonal distribution of rainfall provides an important aspect to judge its effectiveness. In some regions, rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of cool temperate regions.

Virga

In meteorology, virga is an observable streak or shaft of precipitation falling from a cloud but evaporates or sublimates before reaching the ground.