The weight of actual amount of water vapour present in a unit volume of air is called the absolute humidity. It is usually expressed as grams per cubic metre of air.
It is the weight of Water Vapour per unit weight of air or the proportion of the mass of water vapour to total mass of air is called the specific humidity. It is measured in the units of weight (gram per kilogram).
It is the ratio of the air’s actual water vapour content to its water vapour capacity at a given temperature. It is expressed in percentage. With the change of air temperature, the capacity to retain moisture increases or decreases and the relative humidity is also affected. It is greater over the oceans and least over the continents.
The temperature at which saturation occurs in a given sample of air or water vapour begins to change into water is known as dew point.
It is the process by which water is transformed from liquid to gaseous form. It takes approximately 600 calories of energy to convert one gram of water to water-vapour. The energy absorbed by the water molecules during evaporation is used solely to give them the motion needed to escape the surface of the liquid and become a gas. Because this energy is subsequently released as heat when vapour changes back into liquid, it is generally referred to as latent heat.
The rate of evaporation depends on a number of factors:
If moisture content is low, air has a potentiality of absorbing and retaining a larger amount of moisture. Air with high humidity has less potentialty, hence evaporation is slow and small in amount. Similarly increase in temperature increases the water absorption and retention capacity
of the given sample of air. Movement of air replaces the saturated layer with unsaturated layer which has a greater capacity of absorbing moisture. Hence, the greater the movement of air, the greater is the evaporation.
It is the process of change of state from gaseous to liquid or solid state. When moist air cooled, it may reach a level when its capacity to hold water vapour is exceeded by the actual amount present in it. Then the excess water vapour condenses into a liquid or solid form depending upon the temperature. It depends on:
Condensation in air itself can only take place if the temperature of the air is reduced to below the dew point. As the dew point of any mass of air is its saturation point, when its relative humidity is 100%, a little more cooling will being about the point at the level where condensation takes place, viz. water vapour changes into clouds or rain, on contrast, when relative humidity is low a large amount of cooling is required to first reach the dew point and then the condensation. If the moist air comes in contact with some colder object, the condensation may also take place when it is close to the dew point.
FORMS OF CONDENSATION
Forms of condensation can be classified on the basis of temperature at which the dew point is reached. Condensation can take place when the dew point is-
Where as while frost, snow and some clouds are produced when the temperature is less than the freezing point, dew, fog and clouds result even when temperature is higher than the freezing point
When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.
Frost forms on cold surfaces when condensation takes place below freezing point (00C), i.e. the dew point is at or below the freezing point. The excess moisture is deposited in the form of minute ice crystals instead of water droplets. The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.
Fog and Mist
When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. So, the fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero.
In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke is described as smog. The only difference between the mist and fog is that mist contains more moisture than the fog. In mist each nuclei contains a thicker layer of moisture.
Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface. Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by the dust, smoke, and the salt particles.
Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types: (i) cirrus; (ii) cumulus; (iii) stratus; (iv) Nimbus
Cirrus clouds are formed at high altitudes (8,000 - 12,000m). They are thin and detatched clouds having a feathery appearance. They are always white in colour.
Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000 - 7,000 m. They exist in patches and can be seen scattered here and there. They have a flat base.
As their name implies, these are layered clouds covering large portions of the sky. These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.
Nimbus clouds are black or dark gray. They form at middle levels or very near to the surface of the earth. These are extremely dense and opaque to the rays of the sun. Sometimes, the clouds are so low that they seem to touch the ground. Nimbus clouds are shapeless masses of thick vapour.
A combination of these four basic types can give rise to the following types of clouds:
High clouds – cirrus, cirrostratus, cirrocumulus;
Middle clouds – altostratus and altocumulus;
Low clouds – stratocumulus and nimbostratus and
Clouds with extensive vertical development – cumulus and cumulonimbus.
Condensation of water vapour in the air in form of water droplets and ice and their falling on the ground is called precipitation. The process of continuous condensation in free air helps the condensed particles to grow in size. When the resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface. So after the condensation of water vapour, the release of moisture is known as precipitation.
This may take place in liquid or solid form. The precipitation in the form of water is called rainfall, when the temperature is lower than the 00C, precipitation takes place in the form of fine flakes of snow and is called snowfall. Moisture is released in the form of hexagonal crystals. These crystals form flakes of snow. Besides rain and snow, other forms of precipitation are sleet and hail, though the latter are limited in occurrence and are sporadic in both time and space.
Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet. Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.
Sometimes, drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice and which reach the surface of the earth are called hailstones. These are formed by the rainwater passing through the colder layers. Hailstones have several concentric layers of ice one over the other.
Types of rainfall
On the basis of the origin: - convectional, orographic and cyclonic or frontal.
It is caused by the convectional ascent of warm and humid air to great heights. Due to excessive heating of the earth’s surface in summer months, vertical air currents are produced. As the surface air rises, it expands and cools and eventually gets saturated. This is followed by condensation and precipitation. Rainfall in the Doldrums is of convectional nature.
It occurs when warm and humid air strike landform barriers such as mountain ranges and is forced to rise. The subsequent sequence is similar to that of convectional rain. In orographic precipitation, the windward slope pf a mountain range gets more precipitation than the leeward slope because the air descends down the slope and gets warmed up. Hence, the leeward slope is drier and is known as the rain shadow area.
When precipitation is associated with a cyclonic circulation, it is called cyclonic. When warm and humid air masses converge with the cold air mass, they cause turbulent and stormy conditions, generally followed by precipitation particularly along their fronts.
World Distribution of Rainfall
Different places on the earth’s surface receive different amounts of rainfall in a year and that too in different seasons
In general, as we proceed from the equator towards the poles, rainfall goes on decreasing steadily. The coastal areas of the world receive greater amounts of rainfall than the interior of the continents. The rainfall is more over the oceans than on the landmasses of the world because of being great sources of water.
Between the latitudes 350 and 400 N and S of the equator, the rain is heavier on the eastern coasts and goes on decreasing towards the west. But, between 450 and 650 N and S of equator, due to the Westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east. Wherever mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.
On the basis of the total amount of annual precipitation, major precipitation regimes of the world are identified as follows.
Seasonal distribution of rainfall provides an important aspect to judge its effectiveness. In some regions rainfall is distributed evenly throughout the year such as in the equatorial belt and in the western parts of cool temperate regions.