Mechanism of precipitation

The term precipitation as used in hydrology is meant for all forms of moisture emanating from the clouds and all forms of water like rain, snow, hail and sleet derived from atmospheric vapors, falling to the ground. Precipitation is one of the most important events of hydrology. Floods and droughts are directly related to the occurrence of precipitation. Water resources management, water supply schemes, irrigation, hydrologic data for design of hydraulic structures and environmental effects of water resources development projects are related to precipitation in one way or the other. So it is important to study various aspects of precipitation.

Precipitation takes place when a body of moisture cools sufficiently for it to become saturated in water droplets and ice crystals, upon condensation. Air may be cooled in a number of ways i.e.

By meeting of air masses of different temperature

Or

By coming into contact with a cold object such as ground (mountains). The most important mechanism is uplift

of air. By coming into contact with a cold object such as ground (mountains). The most important mechanism is uplift of air, as air rises, its pressure decreases, it expands and cools down. Its ability to hold water is reduced to a certain point. The air becomes saturated and condensation occurs. The formation of cloud does not itself result in precipitation as there must be a mechanism to provide a source of inflow of water. In fact some clouds are associated with dry weather or light rain while other clouds are indication of heavy rainfall.

1. Air Masses:

If the surface is warm and moist, air in the region becomes warm and moist also. If the air then moves to different regions it takes these properties along, gradually changing in response to the new area.

Air masses are designated as cold or warm and moist or dry.

Cold air masses:

• Originate over the pole and near the polar region.
• Warm air masses originate from tropic and sub tropic.
• The air masses that originate over tropical oceans are moistest.
• Whereas mass from over cold, dry land are driest.

Continental polar mass:

Cold and dry air masses that originate over landmass are called continental polar mass.

Maritime Polar mass

Cold and moist air masses that originate over water are called maritime polar mass.

Continental tropical mass:

Warm and dry air mass originating over land mass

Maritime tropical mass:

Warm and humid air mass originating over water

2. Front:

It is a boundary between two air masses. The term has its origins analog to military operations as the armies in battle encounter each other at the front.

A. Cold Air front:

If cold air advances, the air front is called cold air front

B. Warm air front:

If cold air retreats, it is called warm air front.

C. Stationary air front:

If cold air neither advances nor retreats, it is a stationary front.

Occluded/Occlusion:

When cold front overtakes warm front or warm front overtakes cold front.

3. Cyclone:

A cyclone is more or less circular area of low atmospheric pressure in which the wind blows counterclockwise in the northern hemisphere. A typical cyclone is a large whirling mass of air ranging 500-100 miles or more in diameter and with a velocity of 300miles/hr.

At the center of the cyclone the barometric pressure is less. In northern hemisphere the wind reaches the center spirally in counter clockwise direction with a vertical component. The central portion acts as a chimney through which air rises, expands, cools and produce condensation and usually precipitation.

4. Hurricane:

It is a tropical cyclone 300 miles in diameter with a speed of 74 miles/hr or more, which forms over warm ocean water in maritime tropical air.

5. Tornado:

A rotating air accompanied by a funnel shaped downward and having several hundred yards in diameter whirling destructively at speed of 300 mph

6. Typhoon:

Every tropical ocean in different locations use different masses as typhoon (western pacific) Cyclone (Indian Ocean)

Forms of precipitation

Various forms of precipitation are as follows:

1. Drizzle:
2. Rain
3. Glaze
4. Rime
5. Snow
6. Snowflakes
7. Hail
8. Sleet

Drizzle:

These are the minute particles of water at start of rain. These consist of water drops under 0.5 mm diameter and its intensity is usually less than 1.0 mm/hr. Their speed is very slow and we cannot even feel them. Therefore they cannot flow over the surface but usually evaporate. It consists of tiny liquid water drops between 0.1 and 0.5 mm diameter. It is also called mist. Drizzle fall from low strata’s and rarely exceeds 1mm/hr. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Due to small size, drizzle evaporates before reaching the earth.

Rain:

It is form of precipitation in which the size of drops is more than 0.5 mm and less than 6.25 mm in diameter. It can produce flow over the ground and can infiltrate and percolate. Both the duration as well as rate of rainfall are important. If the rainfall per unit time is greater than the rate of infiltration, the rain water can flow over the surface of earth. Consists of liquid drops mostly > 0.5 in diameter. Rain is reported in three intensities.

• Light: for rates of fall up to 2.5mm/hr
• Moderate: From 2.8 to 7.6 mm/hr
• Heavy: Over 7.6 mm/hr

Glaze:

It is ice coating formed on exposed surface by freezing of super cooled water deposits by rain or drizzle. Its specific gravity is as high as 0.8 to 0.9

Rime:

It is white opaque deposit of ice granular mass or less separated by trapped air and formed by rapid freezing of super cooled water drops. Its specific gravity may be as low as 0.2 to 0.3.

Snow:

Snow is precipitation in the form of ice crystals resulting from sublimation i.e. change of water vapor directly to ice. It is composed of ice crystals formed by the process of sublimation. The average specific gravity of snow is 0.1. The density of freshly fallen snow varies greatly; 185 to 500mm of snow is generally required to equal 25mm of liquid water.

Snowflakes:

A snowflake is made up of a number of ice crystals fused together. Snowflakes are bigger particles of snow formed by combination of snow crystals in atmosphere and fall as heavy masses. They may reach several centimeters in dia.

Hail:

Hail is the type of precipitation in the form of balls or lumps of ice, formed by alternate freezing and melting as they are carried up and down by highly turbulent air currents. The impact of these is also more. A single hailstone weighing over a pound has been observed. Precipitation in the form of ball of ice produced in convective clouds.

Hailstones may be spherical, conical or irregular in shape and from 5 to 125 mm in dia. They are composed of alternative layers of glaze and Rime. Their specific gravity may reach 0.8. The largest hailstone in US fell of size 44cm circumference and 776g weight.

Sleet:

Sleet is frozen rain drops cooled to the ice stage while falling through air at subfreezing temperatures. Consists of transparent globular solid grain of ice formed by freezing of raindrops or freezing of largely melted ice crystals fully through a layer of sub-freezed air, near the earth surface. These are simply frozen raindrops.

As it falls toward the ground, it goes through a layer of warmer air somewhere between the cloud and the ground.  If that layer is thick & warm enough, the snowflake melts and becomes a rain drop.

That liquid rain drop falls back into colder air (temperature below 0 C or 32 F), and refreezes into a small, hard ice pellet.

Sleet can accumulate on the ground and look like snow, but you cannot make a sleet-man like a snowman! If the cold layer of air near the ground is not thick enough, the rain does not freeze again until it gets to the ground. That is when you have freezing rain!