Rainfall Types, Convectional, Orographic, Frontal, Map, Distribution

Rainfall

Rainfall is defined as precipitation in liquid form. Precipitation occurs when the local air becomes saturated with vapour and begins to pour because it can no longer keep the water vapour in the gaseous state. Precipitation comes in three forms: liquid, freezing and frozen. Precipitation falls on the earth in various forms. The form depends on the method of formation and the temperature during formation. The article below discusses the various Types of Rainfall based on their origin which is beneficial for UPSC aspirants.

Read More: Evaporation and Condensation

Rainfall Necessary Conditions

There are some necessary conditions required for Rainfall formation such as:

• There should be a sufficient amount of evaporation from the water bodies.
• There should be a wind to carry the water vapour from one place to another
• There should be some way of decreasing the temperature of the moist air.
• Rainfall occurs when the cloud droplets change to raindrops.

Rainfall Types Diagram

Various Types of Rainfall based on their origin are discussed below.

Read More: List of Major Local Winds

Types of Rainfall

Rainfall is classified as Convectional, Orographic, and Convergence rainfall based on the mode of occurrence:

1. Convectional Rainfall

Because different surface areas are heated differently, a parcel of air near the ground may be warmed by conduction more than the air around it. As the warm air is less dense, it rises to the upper strata of the atmosphere. When the temperature of the warm, humid air decreases below the dew point, condensation takes place, and clouds are formed. These clouds cause thunder and heavy rainfall.

This type of rainfall is called convectional rainfall. It is common in equatorial regions and in interior parts of continents mainly in the northern hemisphere.

Read More: Structure of the Atmosphere

2. Orographic Rainfall

It occurs when warm air rises and cools because of a topographic barrier. As the warm air rises, it cools and the temperature falls below the dew point, forming clouds. These clouds cause widespread rain on the mountain range’s windward slopes. This type of rain is called orographic rainfall. When the air cross over the mountain range, it descends along the leeward slopes. After descending, they get warm and cause little rain on the leeward side.

The region on the leeward side that receives less rain is called the rainshadow area. Orographic rainfall occurs in Cherrapunji on the southern margin of the Khasi Hills in Meghalaya, India.

3. Frontal Rainfall or Cyclonic Rainfall

This occurs as a result of the convergence of large air masses. A front is formed when two opposing air masses, such as a cold polar air mass and a warm westerly air mass, converge along a line. The warm wind is lifted upward along this front, where the colder air settles.

These cyclonic fronts form in temperate regions where cold polar winds and warm westerlies meet. Warm air lying on top of cold air cools and becomes saturated, causing condensation to form around hygroscopic nuclei.

Warm air is lifted along the cyclonic front in an inclined plane rather than vertically like convective currents. The front is a zone of high-pressure intensification, cooling condensation, cloud formation, and rainfall—the entire process is known as frontogenesis.

Frontal rainfall is most common in the mid-latitudes, where warm westerlies meet cold polar easterlies. Frontogenesis is also the process by which temperate cyclones form. Temperate cyclones also cause frontal rainfall in India during the winter season, which is referred to as a western disturbance.

Read More: Types of Winds

Rainfall Global Distribution

Rainfall is not uniformly distributed on earth. Different regions of the world receive different amounts of rainfall depending upon their location and seasons. The world can be divided into the following areas based on the rainfall received.

1. Areas of Heavy Rainfall

These regions receive more than 200 cm of annual precipitation. The main areas include the equatorial belt, the mountain slopes of the western coasts in the cool temperate zone and the coastal areas of monsoon lands.

2. Areas of Moderate Rainfall

The regions receive 100 cm to 200 cm of annual precipitation. The main areas lie near the regions of heavy rainfall. The coastal areas in the warm temperate zone also receive moderate precipitation.

3. Areas of Low Rainfall

These regions receive 50 cm to 100 cm of annual precipitation. The main areas lie in the central part of the tropical lands, in the eastern and the interior parts of the temperate lands.

4. Areas of Scanty Rainfall

These regions receive less than 50 cm of annual precipitation. The main areas are the leeward slopes of the mountain ranges, the continental interiors, the western margins of the continents in the tropical areas and the arid deserts.

Read More: Heat Waves

Rainfall Global Distribution Map

Below is the map for the distribution of rainfall globally:

Read More: Types of Clouds

Rainfall Precipitation Seasonal Distribution

The conditions that can lead to precipitation do not exist in the same combination all year. As a result, the seasonal distribution of rainfall varies. However, the majority of the world receives a significant amount of Precipitation during the summer season. The following are the main characteristics of seasonal rainfall distribution:

The equatorial region experiences heavy rainfall all year.

• A few degrees north or south of the equator have wet summers and dry winters.
• The monsoon circulation brings more seasonal contrasts resulting in wet summers as the wind blows onshore and dry winters as the wind blows offshore.
• Seasonal variation, due to the monsoons, is well-developed in the Indian Subcontinent and in Southeast Asia.
• Most of the western coastal areas in the mid-latitudes have dry summers and wet winters due to the presence of the subtropical high-Pressure Belts.
• In the temperate region, the precipitation is cyclonic in nature and cyclones are more common in the winter season. Thus heavy rainfall occurs in winter and not in summer.

Read More: Types of Rocks

Rainfall UPSC

Rainfall is the gravitational fall of atmospheric moisture in the form of water. Snow is the precipitation of white opaque crystals that occurs when the temperature falls below zero degrees Celsius. Rain does not fall unless the cloud droplets become so large as a result of coalescence that the air can no longer hold them. This article discusses the various types of rainfall based on their origin which is beneficial for UPSC aspirants.

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