Condensation, Precipitation, Types of Rainfall Part 2

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Condensation

Condensation is the process by which atmospheric water vapour changes into water or ice crystals. When the temperature of saturated air falls below dew point, the air cannot hold the amount of humidity which it was holding earlier at a higher temperature. This extra amount of humidity changes into water droplets or crystals of ice depending upon the temperature at which condensation takes place.

Process of Condensation

The temperature of the air falls in two ways. Firstly, cooling occurs around very small particles of freely floating air when it comes in contact with some colder object. Secondly, loss in air temperature takes place on a massive scale due to rising of air to higher altitudes. The condensation takes place around the smoke, salt and dust particles which attract water vapour to condense around them. They are called hygroscopic nuclei. When the relative humidity of an air is high, a slight cooling is required to bring the temperature down below dew point. But when the relative humidity is low and the temperature of the air is high, a lot of cooling of the air will be necessary to bring the temperature down below dew point. Thus, condensation is directly related to the relative humidity and the rate of cooling.

Forms of Condensation

Condensation takes place in two situations. Firstly, when dew point is below freezing point or below 0°C. Secondly, when it is above freezing point. In this way, the forms of condensation may be classified into two groups:

  • Frost, snow, and some clouds are formed when dew point is below freezing point.

  • Dew, mist, fog, smog, and some clouds are formed when dew point is above freezing point.

The forms of condensation may also be classified on the basis of place where it is occurring. For example, on the ground or natural objects such as grass blades and leaves of the plants or trees, in the air close to the earth’s surface or at some height in the troposphere?

Dew: When the atmospheric moisture is condensed and deposited in the form of water droplets on cooler surface of solid objects such as grass blades, leaves of plants, and trees and stones, it is termed as dew. It occurs when there is clear sky, little or no wind, high relative humidity, and cold long nights. These conditions lead to greater terrestrial radiation and the solid objects become cold enough to bring the temperature of air down below dew point. In this process the extra moisture of the air gets deposited on these objects.

Frost: When the dew point is below freezing point, under above mentioned conditions, the condensation of extra moisture takes place in the form of very minute particles of ice crystals. It is called frost. In this process, the air moisture condenses directly in the form of tiny crystal of ice. This form of condensation is disastrous for standing crops such as potato, peas, pulses, grams, etc. It also creates problems for road transport system.

Mist and Fog: When condensation takes place in the air near the earth’s surface in the form of tiny droplets of water hanging and floating in the air, it is called mist. In mist the visibility is more than 1 km and less than 2 km. But when the visibility is reduced to less than 1 km, it is called fog. Ideal conditions for the formation of mist and fog are clear sky, calm, and cold winter nights.

Smog: Smog is a fog that has been polluted and discoloured by smoke, dust, carbon monoxide, sulphur dioxide, and other fumes. Smog frequently occurs in large cities and industrial centres. It causes respiratory illness.

Cloud: Clouds are visible aggregates of water droplets, ice particles, or a mixture of both along with varying amounts of dust particles. A typical cloud contains billions of droplets having diameters on the order 060.01 to 0.02 mm. Liquid or solid water accounts for less than 10 parts per million of the cloud volumes. Clouds are generally classified on the basis of their general form or appearance and altitude. Combining both these characteristics, clouds may be grouped as:

Low Clouds: The base level of low clouds varies from very near the ground to about 2000m. The basic type of this family is the status, a low, uniform layer resembling fog but not resting on the ground. Stratocumulus clouds form a low, grey layer composed of globular masses or rolls which are usually arranged in groups, lines, or waves.

Clouds with vertical development fall into two principal categories: cumulus and cumulonimbus. Cumulus clouds are dense, dome-shaped and have flat bases. They may grow to become cumulonimbus, the extent of vertical development depending upon the force of vertical currents below the clouds as well as upon the amount of latent heat of condensation liberated in the clouds as they form. To an observer directly beneath, a cumulonimbus cloud may cover the whole sky and have the appearance of nimbostratus. It applies to a cloud from which rain is falling.

Medium Clouds: These clouds are formed at altitudes between 2000 to 6000 metres. This group of clouds include altocumulus and altostratus.

High Clouds: These clouds are formed above the altitude of 6000 metres and include cirrus, cirrostratus, and cirrocumulus.

Cloud Types (After Budyoko and Bradshaw)

Cloud Types (After Budyoko and Bradshaw)

Cloud Types (After Budyoko and Bradshaw)

Precipitation

Precipitation is defined as water in liquid or solid forms falling to the earth. It happens due to the continuous condensation in the body of air that helps the water droplets or ice crystals to grow in size and weight that the air cannot hold them. As a result, these starts falling on the ground under the force of gravity.

Forms of Precipitation

The form that precipitation takes is largely dependent upon the method of formation and temperature during the formation. The forms of precipitation are as follows:

Drizzle and Rainfall: Drizzle is a fairly uniform precipitation composed exclusively of fine drops of water with diameter less than 0.5 mm. Only when droplets of this size are widely spaced are called rain.

Snowfall: When condensation takes place below freezing point i.e., -0° C, the water vapour changes into tiny ice crystals. These tiny ice crystals grow in size and form ice flakes which become big and heavy and start falling on the ground. This form of precipitation is called snowfall.

Sleet: Sleet is frozen rain, formed when rain before falling on the earth, passes through a cold layer of air and freezes. The result is the creation of solid particles of clear ice.

Hail: Hail is precipitation of small balls or pieces of ice with diameters ranging from 5 to 50 mm, falling either separately or agglomerated into irregular lumps. Hailstones are comprised of a series of alternating layers of transparent and translucent ice.

Types of Rainfall

When a mass of moist air ascends to high altitudes it cools down to lower temperatures. In doing so it attains dew point which leads to condensation and precipitation. Thus, the cooling of air occurs mainly when it rises. There are three important ways in which a mass of air can be forced to rise and each of these ways produces its own characteristic precipitation or rainfall.

Convectional Rainfall: Excessive heating of the earth’s surface in tropical region results in the vertical air currents. These currents lift the warm moist air to higher strata of atmosphere. When the temperature of such a humid air starts falling below dew point continuously, clouds are formed. These clouds cause heavy rainfall which is associated with lightning and thunder. This type of rainfall is termed conventional rainfall. It is very common in equatorial region where it is a daily phenomenon in the afternoon.

Orographic or Relief Rainfall: Orographic rainfalls are formed where air rises and cools because of a topographic barrier. When their temperature falls below dew point, clouds are formed. These clouds cause widespread rain on the windward slopes of the mountain range. This type of rain is termed orographic rainfall. When these winds cross over the mountain range and descend along the leeward slopes, they get warm and cause little rain. Region lying on the leeward side of the mountain receiving little rain is called rain shadow area. A famous example of orographic rainfall is Cherrapunji on the southern margin of the Khasi Hills in Meghalaya India.

Convergence or Cyclonic Rainfall: Convergence rainfall, produced where air currents converge and rise. In tropical regions, where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by convention. Convectional activity frequently occurs along fronts where the temperature of the air masses concerned is quite different. Mixing of air along the front also probably contributes to condensation and therefore to the frontal rainfall. When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one. The rising warm air mass condenses to form clouds which cause extensive down pour. This rainfall is associated with thunder and lightning. This type of rainfall is also called frontal rainfall. This type of rainfall is associated with both warm and cold fronts. It is generally steady and may persist for a whole day or even longer.

Different Types of Rainfall

Different Types of Rainfall

Different Types of Rainfall

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