What is Precipitation?
- Precipitation may be defined as the fall of moisture from the atmosphere to the earth’s surface in any form.
- Water evaporates from water surfaces like streams, rivers, oceans, lakes, ponds, etc. And also from land and plants in the form of water vapor.
- These water vapors get collected in the atmosphere and behave like a gas.
- Under a normal range of temperature and pressure, the water vapor obey the various gas laws (like Boyle’s law, Charles law,etc.)
- As the evaporation continues, the amount of atmospheric vapor goes on increasing.
- But, since space can hold only a certain fixed amount of water vapor in the presence of a solid or a liquid surface, a stage is reached when any further addition of vapor will get condensed on the surfaces.
- The vapor may get condensed in different forms, such as mist, rain, snow, hail, sleet, etc.
- The evaporated water thus returns to the Earth’s surface in any of these forms.
- The water which comes back to the surface of the earth in its various forms like rain, hail, snow, sleet, etc. is known as precipitation.
- In India, a major portion of precipitation is due to rainfall and a small portion of snowfall in the Himalayan region.
- For precipitation to form, it therefore necessary to have the following favorable conditions in the environment :
- The atmosphere must have moisture
- There must be sufficient nuclei (particles) present to aid condensation over them.
- Weather conditions must be favorable to condensation of water vapor.
- The products of condensation must reach to the earth.
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- Types of Precipitation
Types of Precipitation
- Cyclonic precipitation is caused by the lifting of an air mass because of the pressure difference.
- If low pressure occurs at an area, air will flow horizontally from the surrounding area, causing the air at the low-pressure area to lift.
- The cyclonic precipitation may be divided into two-part
- Frontal Precipitation
- Non-Frontal Precipitation
- When two air masses, because of contrasting densities and temperatures, clash with each other, precipitation and condensation, occur in the surface of contact.
- The surface of contact is called a ‘Front‘ or ‘Frontal Surface,’ and the precipitation is called frontal precipitation.
- When a cold air mass drives out a warm air mass, it is called a ‘Cold Front,’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘Warm Front.’
- On the other hand, if both air masses are drawn simultaneously towards a low-pressure area, the front developed is stationary and is called a ‘Stationary Front.’
- Cold Front causes intense precipitation in comparatively small areas, while the precipitation due to the warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts.
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- In the case of non-frontal precipitation, the moist warm air mass is stationary, and the moving cold air mass meets it.
- Thus, due to the lightness of the warm air mass, there is a passive ascent of warm air over cold air owing to this active undercutting.
- When this lifted warm air cools down at higher altitude, precipitation occurs.
- Convective Precipitation is caused by the natural rising of warmer lighter air at colder, denser surroundings.
- The difference in temperature can result from unequal heating in the surface, unequal cooling in the top of the air layer.
- Generally, kind of precipitation occurs in tropics, where, on a hot day, the ground surface gets heated unequally, causing this warmer air to lift up, as the colder air comes to take its place.
- The vertical air currents develop tremendous velocities and are hazardous to aircraft.
- Convective precipitation is spotty, and its intensity vary from light showers to cloud bursts.
- Orographic precipitation is caused by moist air masses. Which strikes some natural topographic barriers like mountains, causing, rise up, condensation, and precipitation.
- This greatest amount of precipitation falls on the windward side, and the leeward side often has very little precipitation.
- The rainfall is composed of showers and steady rainfall. The winds, heavily laiden with moisture from the Bay of Bengal, strike the southern slope of Himalayas, causing intense rains so much so that a place known as Mawsynram near Cherrapunji in Meghalaya, gets the maximum average annual rainfall of the order of 1270 cm. Similarly, Agumbe In the Western Ghats of South India gets a very heavy orographic rainfall of 900 cm.
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Precipitation due to turbulent ascent
- 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.
- Winter rainfall in Madras state is mainly due to this process.
Forms of Precipitation
- Rain is the principal form of precipitation in India. When the size of the water drop is larger than 0.5 mm, it is called rainfall. The maximum size of a raindrop is about 6 mm. Any drop larger than this size tends to break down into drops of smaller sizes during its fall from the clouds. On the basis of its intensity, rainfall may be classified as light, moderate, and heavy.
- Snow is the next important form of precipitation and consists of ice crystals, usually combining to form flakes. New fresh snow has an initial density of 0.06 to 0.15 g/cu.m. and the average value.’ It is assumed to be 0.10 g/cu.m. Snowfall in India is usually confined only to the Himalayan region. But, in countries like Canada, the USA. Russia. China, etc. where snow occurs in abundance and, hence, forms the major part of their precipitation.
- It is showery precipitation in the form of irregular pellets of ice of size more than 8 mm. Hails occur at violent thunderstorms in which vertical currents are very strong
- A fine sprinkle of numerous water droplets of size, not more than 0.5 mm and Intensity less than 1 nim/h is known as a drizzle. At this, the drops are so small that they appear to float in the air
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- it is frozen raindrops of transparent grains which form if rain falls through the air at subfreezing temperature.
- When rain or drizzle comes at contact with the cold ground at around 0°C. This water drops freeze into form an ice coating called glaze or freezing rain.
- It is the white opaque deposit of ice granules more or less separated by trapped air and formed by the rapid freezing of supercooled water drops impinging on exposed objects: Specific gravity can be as low as 0.2 to 0.3.
- Dew forms directly by condensation on the ground mainly during the night when the surface has been cooled by outgoing radiation.
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