Precipitation

Precipitation

Condensation alone does not put moisture back on the ground. In free air, continuous condensation makes the tiny water particles grow larger and heavier. As long as the air can hold them up against gravity, they stay suspended. But once they grow big enough that the resistance of the air can no longer support them, they fall to the earth's surface. This release of moisture after condensation is called precipitation, and it can take either liquid or solid form.

When precipitation falls as water, it is called rainfall. When the temperature is below 0°C, moisture is released as fine flakes of snow — this is snowfall. Snow forms when moisture is released in the shape of hexagonal crystals, and these crystals join together to make snowflakes.

Besides rain and snow, there are two other forms of precipitation: sleet and hail. These are limited in occurrence and are sporadic in both time and space — they do not happen regularly or everywhere.

Sleet is essentially frozen raindrops and refrozen melted snow-water. It forms when a layer of air with temperature above freezing point sits over a subfreezing layer near the ground. Raindrops leave the warmer air above, enter the colder air below, solidify, and reach the ground as small pellets of ice. These pellets are not bigger than the raindrops from which they were formed.

Hailstones are different. Sometimes raindrops, after being released from clouds, solidify into small rounded pieces of ice before reaching the earth's surface. This happens when rainwater passes through colder layers of air. A distinctive feature of hailstones is that they have several concentric layers of ice, one over the other — like the rings of an onion.

Types of Rainfall

On the basis of origin, rainfall is classified into three main types: convectional, orographic (or relief), and cyclonic (or frontal).

Convectional Rain

When air is heated, it becomes light and rises in convection currents. As it rises, it expands and loses heat. This cooling leads to condensation, and cumulus clouds are formed. The rainfall that follows comes with thunder and lightning and is heavy, but it does not last long. This type of rain is common in summer or during the hotter part of the day. It is very common in the equatorial regions and in the interior parts of continents, particularly in the northern hemisphere.

Orographic Rain

When a saturated air mass comes across a mountain, it is forced to ascend. As it rises, it expands, the temperature falls, and moisture condenses. The chief characteristic of this rain is that the windward slopes — the side facing the wind — receive greater rainfall. After giving rain on the windward side, the winds cross over to the other slope and descend. As they descend, their temperature rises, and their capacity to hold moisture increases. So these leeward slopes remain rainless and dry. The area on the leeward side that gets less rainfall is called the rain-shadow area. This type is also known as relief rain.

Cyclonic Rain

This type of rainfall is linked to extra-tropical cyclones and has already been covered in the chapter on atmospheric circulation and weather systems. Cyclonic rain occurs when warm and cold air masses meet, and the warm air is lifted over the cold air, leading to condensation and precipitation along the front.

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. Some broad patterns stand out.

As we proceed from the equator towards the poles, rainfall goes on decreasing steadily. Coastal areas receive greater rainfall than the interiors of continents. Rainfall is also more over the oceans than over landmasses, because oceans are great sources of water.

Between latitudes 35° and 40° N and S of the equator, rain is heavier on the eastern coasts and decreases towards the west. But between 45° and 65° N and S, due to the westerlies, rainfall is first received on the western margins of the continents and decreases towards the east. Wherever mountains run parallel to the coast, rain is greater on the coastal plain and on the windward side, and it decreases towards the leeward side. …