Mechanism 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
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:
Cold and dry air masses that originate over landmass are called continental polar mass.
Cold and moist air masses that originate over water are called maritime polar mass.
Warm and dry air mass originating over land mass
Warm and humid air mass originating over water
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.
If cold air advances, the air front is called cold air front
If cold air retreats, it is called warm air front.
If cold air neither advances nor retreats, it is a stationary front.
When cold front overtakes warm front or warm front overtakes cold front.
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.
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.
A rotating air accompanied by a funnel shaped downward and having several hundred yards in diameter whirling destructively at speed of 300 mph
Every tropical ocean in different locations use different masses as typhoon (western pacific) Cyclone (Indian Ocean)
Various forms of precipitation are as follows:
It consists of tiny liquid water drops between 0.1 and 0.5mm 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.
Consists of liquid drops mostly > 0.5 in diameter. Rain is reported in three intensities.
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
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.
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.
Bigger particles of snow formed by combination of snow crystals in atmosphere and fall as heavy masses. They may reach several centimeters in dia.
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 125mm 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.
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!