What is precipitation?
A hydrologic cycle, also simply known as the water cycle, is a constant and continuous circulation of the total water (excluding deep groundwater) from the earth to the atmosphere and again back to the earth.
It involves the circulation of various physical processes like evaporation, condensation, precipitation, infiltration, surface runoff, and subsurface flows.
Liquid water present on land or ocean or other water resources is turned into vapors due to heat from the sun which is known as evaporation.
These vapors rise up to the atmosphere, collect and form clouds. Further accumulation of the clouds and lowering of temperature due to rise in altitude causes the clouds or vapors to condense forming heavy droplets or solid forms of water.
These heavy particles then fall back to the earth and ocean in various forms. This process is known as precipitation.
It is further followed by processes like ground infiltration, surface and subsurface flow ultimately distributing water back to the oceans or other water resources. These physical processes are continuous and cyclic. This cycle is responsible for the production of freshwater on earth.
Thus precipitation can be defined as the phenomenon by which the water from the atmosphere falls back to the earth and oceans in various forms.
Thus, precipitation is an important physical process that completes the water cycle and helps to maintain the water balance on the earth.
Before different types of precipitation let us read about various forms of precipitation.
Various forms of precipitation
Precipitation can occur in various forms depending upon the climatic condition.
Precipitation in the form of water droplets whose size are larger than 0.5mm is called rain. The sizes of such water droplets during rainfall can be measured up to 6mm.
These water droplets are formed due to condensation of water vapor around some cloud nuclei such as small dust particles which become heavier on further condensation due to lowering of atmospheric temperature and ultimately fall back to the earth in the form of water droplets or rain.
Rainfall can be classified on the basis of rainfall intensity.
- Light rain: Intensity <= 2.5mm/hr
- Moderate rain: Intensity>2.5mm/hr and <= 7.5mm/hr
- Heavy rain >7.5mm/hr
Rainfall is the most dominant form of precipitation.
It is a form of precipitation in which the water droplets are of size smaller than 0.5mm. These droplets can be seen to float in air freely. Intensity of drizzle are also less than 1mm/hr.
Glaze are thin sheets or coating of ice which are formed when drizzle or rain falls on the cold ground surface whose temperature is around freezing point of water ie. 0°C and freezes.
Sleet are forms of precipitation which are formed when rain drops passes through the atmosphere with subfreezing temperature (temperature below freezing point) and gets frozen. Initially sleets are liquid raindrops which freezes only with it falls through the atmosphere with sub-freezing temperature.
Snow consists of unique ice crystal which are individually formed in clouds. But they fall in the form of cluster during precipitation. Snow have an average density of 0.1g/cc which makes them seem to float in air.
Snow usually occurs in presence of cold weather and humidity in the winter season, whereas in the Himalayas above 5000m altitude snowfall occurs the whole year.
Hailstones are an irregular lump of ice which are more than 8mm in diameter. They are formed by repeated freezing and melting of ice. They usually occur during cold storms.
Types of precipitation
Precipitation can be majorly classified into 3 types on the basis of the lifting mechanism of the water vapor up to the atmosphere which leads to the condensation of vapor clouds into water drops due to the lower temperature at higher altitudes.
1) Convectional precipitation
Localized heating of a certain area causes the air at that area to warm up much faster causing local evaporation at the same time. Since the warmer moist air is lighter than cold ones, these rise up to the atmosphere forming a vacuum.
These vacant areas are then filled by surrounding cold air resulting in the wind (movement of air). As the warm air continues to rise, they undergo cooling due to colder temperatures at high altitudes. Under saturated conditions and adequate lower temperature, this moist air condenses into water drops and falls on the earth.
This type of lifting and precipitation mechanism is called convective precipitation. It is similar to that of the convective heat transfer phenomenon. Convective precipitation usually occurs in the tropical convective equatorial regions like Amazon, with high intensity localized rainfall for a short duration and within a smaller area extending of about 10km in diameter.
They are also found to occur in Pakistan in summer times. Convective precipitation may also occur in the form of hails depending on the climatic condition.
2) Orographic precipitation
The forward-moving moist air mass or clouds gets lifted up to a higher altitude when they confront tall natural topographic barrier like hills and mountains on their path.
These causes consequent adiabatic cooling and condensation of the moist air mass as a result of low temperature at higher altitudes, thus resulting in immediate precipitation. Such a type of lifting and precipitation mechanism is termed orographic precipitation.
This type of mechanism is responsible for most of the heavy rainfall. The hill slope where the moist air ascends receives more rainfall and is termed as windward side whereas, the slope where the air descends receives less or no rainfall and is termed as Leeward side or rain shadow.
Rain shadows have a harsh and dry climate. They are usually found to occur in places with high mountain ranges like in the Himalayan region, Andes mountain range regions, etc. Nepal is an example among those places which receive orographic precipitation.
The easterlies/ trade wind flowing east to west carries the moist air from the Bay of Bengal, which then enters into Nepal can elevate along the hills and south slope of the Himalayas and results in orographic precipitation. They usually result in the heavy steady rain.
3) Cyclonic precipitation
Cyclones are the low-pressure zones in the atmosphere having circular wind motion. Cyclonic precipitation occurs when the warm moist air moves from the high-pressure zone to these low-pressure zones-cyclones. These usually occur in plain regions. They are further classified into two types:
1) Non Frontal Cyclonic precipitation
Nonfrontal cyclonic precipitation occurs when moist air from the surrounding high-pressure zone converges laterally into the central low-pressure zone causing the air to rise up and undergo cooling which then results in precipitation.
2) Frontal cyclonic precipitation
Fronts are the boundary between the warm and cold air masses. Frontal cyclonic precipitation occurs when one air mass passes horizontally over the other air mass having different temperatures, densities, vapor content, and pressure.
Such motion of air into the low-pressure zone and rotational motion of the earth causes rotational motion of air current forming cyclone. When a warmer air mass passes over the cold air mass then it causes the warm front precipitation. Warm front precipitation results in continuous rainfall or drizzle.
Whereas when the advancing cold air forces the warmer air to rise up and cool then cold front precipitation occurs. Cold front precipitation usually results in short duration intense rainfall.
Cyclonic precipitation can be violent resulting in hail storms.
Necessary condition for the occurrence of precipitation
- Moisture is the most important requirement for precipitation to occur. It is generated through evaporation.
- The temperature must be lower enough for condensation to occur along with other climatic condition to satisfy the condition for condensation
- Condensation nuclei such as dust particles, oxides of nitrogen, salt crystals, carbon dioxide must be present for vapor to condense around them
- Precipitation must reach the earth’s surface in any form. Such precipitation which is evaporated in the middle of falling is called virga.
I hope this article on “Different types of precipitation” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Zenish Shakya