What is Precipitation?
Precipitation is one of the processes of a hydrological cycle that involves the falling of water to the ground surface in various forms such as rain, snow, drizzle etc.. after condensation. When the clouds are formed followed by condensation, saturation takes place with a large amount of moisture resulting in precipitation under the influence of gravity.
Rain is the most common form of precipitation and consists of liquid water droplets that fall from the clouds. Snow occurs when the temperature is below freezing, and water vapor condenses directly into ice crystals, which then fall as snowflakes. Sleet is a mixture of rain and snow, where snowflakes partially melt into raindrops before refreezing into ice pellets before reaching the ground. Hail is formed in powerful thunderstorms when updrafts carry raindrops upward into extremely cold regions of the atmosphere, causing them to freeze and accumulate layers of ice until they become too heavy and fall to the ground as ice pellets.
Hydrological Cycle :
The Hydrological Cycle is the continuous movement of water on or below the earth’s surface. It basically involves the processes of evaporation/transpiration, condensation, precipitation, infiltration, and runoff.The hydrological cycle for precipitation involves the following steps :
1) Evaporation and Transpiration
Evaporation is the process where water from different sources such as lakes, rivers, ponds, oceans etc. is carried away in the form of vapors by air to form clouds whereas Transpiration is the process where water is evaporated from the leaves of plants. Thus, total evaporation involves :
(i) Surface Evaporation
(ii) Water Surface Evaporation (river, ocean etc)
(iii) Transpiration (Evaporation from leaves of plants)
(iv) Atmospheric Evaporation (Evaporation from water droplets in air in the form of moisture)
2) Precipitation
Precipitaion is the process where water in any form such as rain, snow, sleet or hail falls from the atmosphere to earth’s surface. It causes due to condensation of water vapor resulting in saturation of clouds, the water droplets within them grow larger and eventually fall to the ground surface under the influence of gravity.
3) Run-off
During the precipitation, some portion of water percolates into the ground and runs to ocean, river etc. Since, this water that flows through surface or sub-surface streams, thus cannot be evaporated. Therefore, such water flows are termed Run-offs. The run-offs can be classified as follows :
(i) Surface Run-Off
Water that flows over the land to reach streams and rivers, which ultimately discharges the water to the sea.
(ii) Interflow or Sub-surface Run-off
A portion of precipitation percolates into surface soil that flows as sub-surface run-off to reach nearby streams or rivers depending on geological conditions.
(iii) Groundwater flow
It is that portion of precipitation, which after infiltration percolates down and joins the ground water reservoir which is ultimately connected to the ocean.
Precipitation may be expressed as :
Precipitation = Evaporation + Run-Off
Types of Precipitation
Depending on various factors responsible for lifting, precipitation can be classified as follows :
1) Cyclonic Precipitation
Cyclonic precipitation is caused due to lifting of air masses converging into low-pressure area or cyclones. It can be divided into two types as follows :
(a) Frontal Precipitation: A border region between adjacent air masses having different characteristics such as temperature and humidity is called front. When the flow of warm air mass and cold air mass converge, then due to difference in densities they do not mix. Instead, the warm air mass rises over cold air mass and forcing the warm air mass to cool and condense, leading to the formation of clouds and precipitation.
Frontal precipitation can occur in two main types of fronts: warm fronts and cold fronts. In a warm front, warm air advances and replaces cooler air, causing a gradual lifting of the less dense warm air. As the warm air rises, it cools, and the moisture within it condenses, resulting in widespread and generally light to moderate precipitation over a broad area.
On the other hand, in a cold front, a denser and cooler air mass advances and undercuts the warmer air mass. The warm air is rapidly forced to rise, leading to intense and often convective precipitation along a relatively narrow zone. This type of precipitation is often associated with thunderstorms and can be more intense and short-lived compared to warm front precipitation.
(b) Non-Frontal Precipitation : In this case, the moist warm air mass is stationary and the moving cold air mass meets it. Thus, due to lightness of the warm air, there is the passive ascent of warm air over cold air owing to the active under-cutting. When the lifted warm air cools down at a higher altitude, precipitation occurs.
2) Convection Precipitation
Due to instability in atmosphere, the vertical movement of air occurs in warm and unstable air masses such as during summer thunderstorms. The difference in temperature results in rapid upward movement of warm air to pass over a denser colder air masses. Within these clouds, water vapor condenses and forms precipitation, which can be in the form of rain, hail, or even snow, depending on the temperature.
3) Orographic Precipitation
This is caused due to lifting of warm air masses due to topographical barriers such as mountains. Due to sudden lifting, there is a rapid change in temperature resulting in condensation and formation of clouds. Subsequently, precipitation occurs as the moisture in the clouds condenses into rain, snow, or other forms of precipitation on the windward side of the mountain or hill.
4) Cyclonic Precipitation
This type of precipitation occurs when an air mass is forced to rise up due to greater friction of the earth’s surface after its travel over the ocean. The air mass rises up because of increased turbulence and friction when it ultimately condenses and precipitation occurs.
Forms of Precipitation
The various forms of precipitation that fall under the influence of gravity due to rapid temperature variations are as follows :
Drizzle : Size of water droplets < 0.5 mm and intensity < 0.01 mm/hr.
Rain : Size of droplets > 0.5 mm.
Glaze : When drizzle or rain freezes as it comes in contact with cold objects is known as Glaze.
Sleet : It is a frozen rain drops cooled while falling through air at sub-freezing temperature
Snow : When water vapor directly changes into ice i.e. precipitation in the form of ice crystals.
Snow Flakes : Fused ice crystals together are known as snow flakes.
Hail : Hails are lumps or bulbs of ice over 5 mm in diameter formed by alternate freezing or melting as they are carried up and down in highly turbulent air currents.
