Humidity is the general term which describes the invisible amount of water vapour present in the air. It is a highly variable climatic factor which forms only a small proportion (varying from zero to four per cent and averaging around 2% in the atmosphere.) Humidity is measured by an instrument called hygrometer.
Water vapour in the atmosphere comes through evaporation from the oceans, lakes, rivers, ice-fields and glaciers, through transpiration from plants and respiration from animals.
Significance of Atmospheric Moisture:
1. The water vapour present in rain-bearing clouds is responsible for all kinds of precipitation, and the amount of water vapour present in a given volume of air indicates the atmosphere’s potential capacity for precipitation.
2. Water vapour absorbs radiation—both incoming and terrestrial. It thus plays a crucial role in the earth’s heat budget.
3. The amount of water vapour present decides the quantity of latent energy stored up in the atmosphere for development of storms and cyclones.
4. The atmospheric moisture affects the human body’s rate of cooling by influencing the sensible temperature.
Measurement of Humidity:
Humidity is a general term and can be expressed quantitatively in different ways.
It is the weight of actual amount of water vapour present in a unit volume of air. It is usually expressed as grams per cubic metre of air. Absolute humidity of the atmosphere changes from place to place and from time to time. The ability of air to hold water vapour depends entirely on its temperature. Warm air can hold more moisture than cold air. For instance, at a temperature of 10°C, one cubic metre of air can hold 11.4 grams of water vapour.
The same volume of air can hold 22.2 grams of water vapour, once the temperature rises to 21°C. Thus, a rise in the temperature of air increases its capacity to retain water vapour, whereas a fall in temperature decreases it. However, it is not a very reliable index because changes in temperature and pressure cause changes in the volume of air and consequently the absolute humidity.
Relative Humidity It is a more practical measure of atmospheric moisture. It is the ratio of the air’s actual water vapour content to its water vapour capacity at a given temperature. This relationship between absolute humidity and the maximum moisture holding capacity of air at a particular temperature is always expressed in percentage.
Since, the relative humidity is based on the air’s water vapour content as well as on its capacity, it can be changed in either of the two ways:
(i) If moisture is added by evaporation, the relative humidity will increase.
(ii) A decrease in temperature (hence, decrease in moisture-holding capacity) will cause an increase in relative humidity.
The relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor. Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. At this temperature, the air cannot hold any additional amount of moisture. Thus, relative humidity of the saturated air is 100%. If it has half the amount of moisture that it can carry, the air is unsaturated and its relative humidity is only 50%.
A given sample of air becomes saturated without any actual change in its moisture content, provided its temperature falls or it cools to the required extent. The temperature at which saturation occurs in a given sample of air or water vapour begins to change into water is known as the dew point.
It is expressed as the weight of water vapour per unit weight of air, or the proportion of the mass of water vapour to the total mass of air. Since it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by changes in pressure or temperature.
Distribution of Water Vapour:
Latitudinally, the atmospheric moisture decreases from the equator towards the poles in an irregular manner with the latitudinal temperature gradient.
The marine air may be saturated to the extent of 80%, while the continental air may be only saturated up to 20%.
With altitude, the capacity of air to hold moisture decreases because the temperature also decreases.
Looking at the diurnal variation, the absolute humidity is high during the afternoon and comes down as the temperature comes down. The relative humidity is the lowest during early morning, especially after long, calm, clear nights due to low capacity of the air to hold moisture at a low temperature.
Evaporation and Condensation:
Evaporation Evaporation is the process by which matter changes from liquid to a gaseous or vapour state. The atmospheric moisture or humidity is nothing but water vapour which has escaped from oceans, rivers, lakes, ponds, plants, animals and humans into the atmosphere. Heat energy is required for evaporation to take place and in case of atmospheric moisture, the energy is provided by solar radiation. The water molecules, supplied with this energy, get the required motion to escape and conserve this energy as latent heat of vaporisation. Again, when the vapours get condensed into water drops, this energy is released in the form of latent heat of condensation.
Since the process of evaporation uses some amount of energy, the rest of the source mass is cooled in the process. Thus, evaporation is associated with a cooling effect. The human body, for instance, loses heat through sweat during warm weather to stay cool. The radiation absorbed by the earth balances the cooling effect of evaporation.
Factors Affecting Rate of Evaporation:
Several factors influence the rate of evaporation:
1. Amount of water available:
Rate of evaporation is greater over the oceans than over the continents.
A high temperature implies greater availability of energy for evaporation; thus, the rate of evaporation is directly proportional to the temperature of the evaporating surface.
3. Relative humidity:
Since the moisture-holding capacity of air at a given temperature is limited, drier, air (or air with lesser relative humidity) evaporates more water than moist air. Thus, evaporation is greater in summer and at mid-day than in winter and at night.
4. Wind speed:
A high wind speed removes the saturated air from the evaporating surface and replaces it with dry air which favours more evaporation. Whenever there is a combination of high temperature, very low relative humidity and strong winds, the rate of evaporation is exceptionally high. This leads to dehydration of soil to a depth of several inches.
5. Area of evaporating surface:
A larger surface area exposed to heat implies enhanced evaporation.
6. Air Pressure:
Evaporation is also affected by the atmospheric pressure exerted on the evaporating surface. Lower pressure over open surface of the liquid results in a higher rate of evaporation.
7. Composition of water:
Evaporation is inversely proportional to salinity of water. Rate of evaporation is always greater over fresh water than over salt water. Under similar conditions, ocean water evaporates about 5% more slowly than fresh water.
8. More evaporation by plants:
Water from plants generally evaporates at a faster rate than from land.
It refers to idealised conditions in which there would be enough rainfall to provide sufficient moisture for all possible evapotranspiration in an area. In order to determine the potential evapotranspiration for any place or area, several factors like temperature, latitude, vegetation, permeability and water retention capacity of soil are considered. Those places which have a surplus of precipitation over evapotranspiration are marked by surplus of water for underground storage. But, in areas where evapotranspiration is in excess of precipitation, no water is available for storage.
Thornthwaite used the concept of potential evapotranspiration in his classification of world climates.
Distribution of Evaporation:
There are two characteristic features of actual mean annual evaporation:
1. Generally, actual evaporation is greater over oceans than over continents. This is simply because of the unlimited supply of water at the ocean surface compared to the limited supply over land. However, equatorial regions are an exception to this general rule. Land areas in very low latitudes between 10°N and S of the equator lose more moisture through evapotranspiration than do oceans and other water bodies.
2. North-south distribution of actual evaporation is largely controlled by air temperature, since temperature decreases from equator towards the poles. According to Trewartha, about 60% of the earth’s evaporation occurs in the latitudinal belt extending from 20 N to 20°S and 80% occurs in the zone extending from 35°N to 35°S latitude.
Condensation is the process of change of state from gaseous to liquid or solid state. When moist air is 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. In free air, condensation results from cooling around very small particles termed ‘condensation nuclei’. Particles of dust, smoke and salt from the oceans are particularly good nuclei because they absorb water. These particles are termed “hygroscopic (water-seeking) nuclei’.
Condensation in air itself can only take place if the air temperature 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 bring the point to the level where condensation takes place, i.e. when water vapour changes into clouds or rain. In contrast, when the relative humidity is low, a large amount of cooling is required to first reach the dew point and then the condensation. Condensation, therefore, depends upon—(i) the amount of cooling and (ii) relative humidity of the air.
Condensation occurs under varying conditions which, in some way or the other, are associated with change in any of these variables—air volume, temperature, pressure and humidity.
Thus condensation takes place:
(i) When the temperature of the air is reduced but its volume remains constant and the air is cooled below the dew point; (ii) if the volume of the air is increased without addition of heat;
(iii) When the joint change of temperature and volume reduces the moisture holding capacity of the air below its existing moisture content; –
(iv) By evaporation adding moisture to the air. The most common circumstances favourable for condensation are those producing a drop in air temperature. .
Processes of Cooling for Producing Condensation:
These processes can be “studied under the headings, adiabatic and non-adiabatic.
Adiabatic Temperature Changes:
When the air rises, it expands. Thus, heat; available per unit volume is reduced and, therefore, the temperature is also reduced. Such a temperature change which does not involve any subtraction of heat, and cooling of air takes place only by ascent and expansion, is termed ‘adiabatic change’.
The vertical displacement of the air is the major cause of adiabatic and katabatic (cold, dense air flowing down a slope) temperature changes. Near the earth’s surface, most processes of change are non- adiabatic because horizontal movements often produce mixing of air and modify its characteristics.
The rate at which temperature decreases in rising air depends upon the moisture content of the air. In unsaturated air, the decrease of temperature with height is twice that in saturated air.
This is mainly due to the release of latent heat of condensation after saturation occurs. The rate at which temperature decreases in rising unsaturated air is known as dry adiabatic rate and that in the saturated air is called wet adiabatic rate.
Non-Adiabatic Temperature Changes Non- adiabatic processes include cooling by radiation, conduction or mixing with colder air. The air may be cooled due to loss of heat by radiation.
In case there is direct radiation from moist air, the cooling produces fog or clouds, subject to presence of hygroscopic nuclei in the air. Cooling may also be produced by conduction or advection of warm air across a cold surface. Cooling by contact with a cold surface produces dew, frost or fog depending on other atmospheric conditions. Sometimes, the air is cooled due to its mixing with colder air.
But the effect of cooling produced by radiation, conduction and mixing is confined to a thin layer of the atmosphere. The non-adiabatic processes of cooling produce only dew, fog or frost. They are incapable of producing a substantial amount of precipitation.
The only process capable of reducing the temperature of deep and extensive air masses, so that cloud-formation and appreciable precipitation may be possible, is the expansion associated with rising air currents or the adiabatic cooling.
Forms of Condensation the 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— (i) lower than the freezing point, (ii) higher than the freezing point. Whereas white frost, snow and some clouds are produced when the temperature is lower than the freezing point, dew, fog and clouds result even when the temperature is higher than the freezing point. Forms of condensation may also be classified on the basis of their location, i.e. at or near the earth’s surface and in free air. Dew, white frost, fog and mist come in the first category, whereas clouds are in the second category.
Various forms of condensation are discussed below.
Dew When moisture is deposited in form of water droplets on cooler surface of solid objects such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions -for its formation are a clear sky, little or no wind, high relative humidity and long, cold nights leading to greater radiation of heat from the earth for its cooling. For the formation of dew, it is necessary that the dew point is above freezing point.
When condensation takes place at a dew point which is at or below freezing point (0°C), excess moisture is deposited in the form of minute ice crystals instead of droplets of water. It is called white frost. The ideal conditions for formation of white frost are the same as those for formation of dew, except that the air temperature must be at or below freezing point.
Fog is defined as a cloud with its base at or very near the ground. Fogs are of different kinds depending upon the nature of the cooling process. Radiation fog results from radiation, cooling of the ground and adjacent air. These fogs are not very thick. Fogs formed by condensation of warm air when it moves horizontally over a cold surface, are known as advectional fog.
These fogs are thick and persistent. Sometimes, due to convergence of warm and cold air masses, the warm air mass is pushed up by the heavier cold air mass. Then, if the warm air reaches saturation and some moisture falls down as precipitation, this moisture falling as precipitation may condense to produce fog at the boundary of the two air masses. These are called frontal or precipitation fog.
This is also a kind of fog in which the visibility is more than one kilometre but less than two kilometres.
A cloud is a mass of minute droplets of water or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. Clouds are caused mainly by the adiabatic cooling of air below its dew point.
Clouds can be classified on the basis of— (a) their appearance, i.e. general shape, structure and vertical extent, and (b) their height or altitude.
On the basis or the appearance, the following cloud types may be identified.
Cirrus clouds are high, white and thin. They are composed of ice crystals. They have a fibrous and feathery appearance. Cumulus clouds exhibit a flat base and have the appearance of rising domes. These clouds have a cauliflower structure. Stratus clouds can be described as sheets of layers that cover much or all of the sky. All the clouds either reflect one of these three basic forms or are combinations or modifications of them.
On the basis of height, following categories of clouds can be identified.
(i) Low Clouds (ground level to 2000 metres height); these include stratocumulus, stratus, nimbostratus, cumulus and cumulonimbus, (ii) Medium Clouds (2000-6000 metres height) include altocumulus and altostratus. (iii) High Clouds (6000-12,000 metres height) include cirrus, cirrostratus and cirrocumulus. The term ‘alto’ is added to signify height; the term ‘nimbus’ is added to signify rain. The International Cloud Code lists 28 types but ten fundamental genera are recognised.
Condensation of water vapour in the air in the form of water droplets and ice another falling on the ground is called precipitation. This may take place in liquid or solid forms of water. It is only when the raindrops (i.e. cloud particles) or ice pellets become large enough, through continuous condensation, to overcome prevalent buoyant force and updraft, that precipitation occurs. Precipitation is perhaps the most important stage of the hydrological cycle.
Forms of Precipitation:
Precipitation in the form of drops of water is called rainfall, when the drop size is more than 0.5 mm. It is called virage when raindrops evaporate before reaching the earth while passing through dry air. Drizzle is light rainfall with drop size being less than 0.5 mm, and when evaporation occurs before reaching the ground, it is referred to as mist. When the temperature is less than 0°C, precipitation takes place in the form of fine flakes of snow and is called snowfall.
Sleet is frozen raindrops and refrozen melted snow water. It may be a mixture of snow and rain or merely partially melted snow. When a layer of air with temperature above freezing point overlies a sub-freezing layer near the ground, precipitation takes place in the form of sleet. The 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. Precipitation in the form of hard rounded pellets is known as hail.
The drop size in this case is between 5 mm and 50 mm. The hail drops are nothing but hygroscopic nuclei like dust, salt, smoke particles which accumulate around them coating after coating of ice during their continuous rise and fall in strong turbulent winds in the upper layers. Once they become heavy enough to defy the buoyant force, they fall down as hail. In India and in mid-latitudes, this type of precipitation is common during March-May period. Hailstones have a typical structure of several concentric layers of ice one over the other.
Types On the basis of its origin, precipitation may be classified into various types. In general, only rain and snow make significant contribution to precipitation totals. Hence, in many parts of the world, the terms rainfall and precipitation are used interchangeably, although snowfall is less easily measured with the same degree of accuracy.
Various types of precipitation are discussed below.
When heated air becomes light, it is carried upwards, where, due to low temperature, it cools down and in presence of moisture, condensation occurs. The condensed particles fall down as precipitation. Since this involves an upward movement of air, this type is referred to as convectional precipitation. This process releases latent heat of condensation which further heats the air and forces the air to go further up. Convectional precipitation is heavy but of short duration, highly localised and is associated with minimum amount of cloudiness. It occurs mainly during summer and is common over equatorial doldrums in the Congo basin, the Amazon basin and the islands of south-east Asia. (Fig. 2.28)
This type of precipitation occurs when warm, humid air strikes an orographic barrier (a mountain range) head on. Because of the initial momentum, the air is forced to rise. As the moisture laden air gains height, condensation sets in, and soon saturation is reached. The surplus moisture falls down as orographic precipitation along the windward slopes. Besides taking the moist winds aloft, the orographic barriers (i) chill moist winds by contact with snow capped summits, (ii) obstruct the path of low pressure areas, (iii) cause convection along the slope by differential heating.
The windward slope of a mountain range gets more precipitation than the leeward slope because the; air moves down the slope and gets warmed up. Hence, the leeward slope is drier and is known as the rain-shadow area. The wide variation in the amount of rainfall at Mahabaleshwar and Pune, only a few kilometres away from each other, is due to the orographic nature of rainfall. Mahabaleshwar, situated on the Western Ghats, receives more than 600 cm of rainfall, whereas Pune, lying in the rainshadow area, has only about 70 cm. (Fig. 2.28)
Frontal Precipitation When two air masses with different temperatures meet, turbulent conditions are produced. Along the front convection occurs and causes precipitation. For instance, in north-west Europe, cold continental air and warm oceanic air converge to produce heavy rainfall in adjacent areas. (Fig. 2.28)
Cyclonic Precipitation This type of precipitation is caused when ascent of air takes place due to horizontal convergence of air streams in low pressure cells within a cyclone. The precipitation in a tropical cyclone is of convectional type while that in a temperate cyclone is because of frontal activity.
Monsoonal Precipitation This type of precipitation is characterised by seasonal reversal of winds which carry oceanic moisture (especially the south-west monsoon) with them and cause extensive rainfall in south and Southeast Asia.
Global Distribution of Precipitation:
Different places on the earth’s surface receive different amounts of precipitation in a year, and that too, in different seasons. Nevertheless, the main features can be explained with the help of global wind and pressure systems, land-water distribution and nature of the relief features.
In general, high latitudes having high pressure associated with subsiding and diverging winds’ experience rather dry conditions. On the other hand, the equatorial belt with high temperature, low pressure, high humidity, converging and unstable winds and ascending air receives ample precipitation.
Besides wind pressure systems, the inherent nature of the air involved is also an important factor in determining the potential for precipitation. Since Cold air has lower capacity to hold moisture than warm air, a general decrease in precipitation is revealed with the increasing distance of latitude from the equator towards the poles.
Besides the latitudinal variation, the land- water distribution complicates the global precipitation pattern. Large land masses in middle latitudes generally experience a decrease in precipitation towards their interiors. Further, the mountain barriers alter the ideal precipitation pattern that one would expect from the global wind systems. Windward mountain slopes receive abundant precipitation, while the leeward slopes and adjacent lowlands fall in the rain-shadow. Fig. 2.29 shows major precipitation regimes of the world.
Areas receiving heavy precipitation of 200 cm per annum include the windward slopes of the mountains along the western coasts in the cool temperate zone.
A moderate rainfall of 100 to 200 cm occurs over the areas adjacent to the high precipitation regime. The coastal areas in the warm temperate zone also receive moderate amount of rainfall.
The central parts of the tropical lands and the eastern and interior parts of the temperate lands receive inadequate precipitation varying between 50 and 100 cm per annum.
Areas lying in the rain-shadows, the interior of the continents and high latitudes receive low precipitation of less than 50 cm per annum. The western margins of the continents in the tropical lands, and the arid deserts come under this category.
Seasonal Variation of Precipitation:
The equatorial regime has two maxima after summer and winter solstices. The tropical regime has a maximum after the summer solstice. The monsoonal regime has a maximum during summer. The Mediterranean regime has a winter maxima and a summer minima. The continental regime has a summer maxima and winter. Minima. The maritime regime along western coasts in temperate zones has a winter maxima.
The continents have a late morning and early afternoon maxima. The tropics have an afternoon maximum. The .maritime regime has a maximum during night and early morning due to nocturnal convection.