10.2.6. Types of Rainfall

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

Rainfall can be categorized into three distinct types based on how it is formed. Understanding these types is essential for recognizing patterns in weather and climate across different regions. The three types are convectional rain, orographic (or relief) rain, and cyclonic (or frontal) rain. Each type has unique characteristics and processes that determine when and where it occurs.

Imagine rainfall as a recipe; depending on the ingredients (types of air masses, temperature, and geography), you can create different types of dishes (rainfall). Just like how a chef uses different techniques to prepare various meals, nature utilizes distinct processes to generate different types of rain.

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 cumulus clouds are formed. With thunder and lightning, heavy rainfall takes place but this does not last long. Such rain is common in the summer or in the hotter part of the day. It is very common in the equatorial regions and interior parts of the continents, particularly in the northern hemisphere.

Convectional rain occurs when the sun heats the earth's surface, causing the air above it to warm up. This warm air is lighter and rises. As it ascends, it cools down, leading to condensation where water vapor forms droplets, resulting in cumulus clouds. Eventually, these clouds can produce heavy rain, often accompanied by thunderstorms. This type of rain is most common during warm summer days, especially in equatorial regions where it can occur frequently.

Think of a pot of water on the stove. As the water heats up, steam rises. Similarly, hot air rises from the earth's surface, creating convection currents. Just like how the steam eventually condenses back into droplets, the rising warm air cools and forms clouds, ultimately leading to rain.

When the saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; the temperature falls, and the moisture is condensed. 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. It is also known as the relief rain.

Orographic rain occurs when moist air encounters mountains. As the air rises over the mountain, it expands and cools, resulting in condensation and precipitation on the windward side of the mountain. This area gets significant rainfall. Conversely, as the air descends on the leeward side, it warms up and can hold more moisture, leading to a drier climate known as a rain-shadow area. This phenomenon explains why some regions are lush and green while others, just a few miles away, are arid.

Imagine blowing up a balloon. As you blow air into it, the balloon expands. When air hits a wall, it rises, and the behavior changes. Similarly, moist air rises over mountains, leading to rainfall on one side but leaving the other side dry, just like how a wall alters the flow of air.

You have already read about extra tropical cyclones and cyclonic rain in Chapter 9. Please consult Chapter 9 to understand cyclonic rainfall.

Cyclonic rain is associated with weather systems known as cyclones, which are large-scale air mass systems characterized by low atmospheric pressure. When warm, moist air meets cold air, they create a cyclone, leading to significant rainfall as the air rises and cools rapidly. Detailed understanding of cyclonic rainfall can be found in Chapter 9, where the processes of cyclone formation and their role in bringing rain to various regions are discussed.

Think of a spinning top. When it spins, the air around it swirls. Similarly, cyclones drag warm air upwards, forming a spinning mass of clouds and rain. Just like how a spinning top can create a whirlwind, cyclonic systems can generate large storms that bring heavy rainfall.

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