Chapter 9 — 2.5 Humidity and Precipitation

The atmosphere is a mixture of gases, and among them water vapour is the only one whose quantity changes greatly from place to place and from hour to hour. Near the equator over a warm ocean the air may carry water vapour amounting to four per cent of its volume; over a cold desert in winter it may carry almost none. This changeable gas enters the air by evaporation from oceans, rivers, lakes, wet soil and the leaves of plants, and it leaves the air by condensation and precipitation. The whole of the water cycle passes through this one invisible stage.

Humidity is the term for the amount of water vapour present in the air at a given place and time. The word comes from the Latin humidus, meaning moist. Humidity cannot be seen, but it is felt: on a sultry afternoon in Kolkata in June the air feels heavy and sweat does not dry, because the air is already nearly full of vapour; on a dry January morning in Rajasthan the lips crack because the air is thirsty for moisture.

Water vapour is important in three ways. First, it is the source of all clouds and precipitation, and therefore of all fresh water on land. Second, it is a powerful greenhouse gas; it absorbs the long-wave heat radiated by the earth and keeps the lower atmosphere warm. Third, it carries latent heat. When one gram of water evaporates it absorbs about 600 calories of heat from its surroundings, and when that vapour condenses again the same heat is released. This is how the energy of the tropical oceans is carried to higher latitudes and how a thunderstorm finds the energy to grow to a height of fifteen kilometres.

Warm air can hold much more water vapour than cold air. Air at 30 degrees Celsius can hold roughly 30 grams of vapour in each cubic metre, while air at 0 degrees can hold only about 5 grams. This single fact explains most of what follows in the section: when moist air is cooled it soon reaches the limit of what it can hold and the surplus vapour must condense. The three ways of stating humidity, which we take up next, are simply three ways of comparing how much vapour the air has with how much it could have.

Absolute humidity is the actual mass of water vapour present in a unit volume of air. It is expressed in grams of vapour per cubic metre of air. If a sample of one cubic metre of air is found to contain 12 grams of water vapour, its absolute humidity is 12 grams per cubic metre. This is the most direct measure of moisture, and it is high over warm seas and low over cold or dry land. Its weakness is that the volume of a parcel of air changes with temperature and pressure. When air rises it expands, and the same grams of vapour are spread over a larger volume, so absolute humidity falls even though not a single molecule of water has been lost. For this reason absolute humidity is not convenient for studying air that moves up and down.

Specific humidity removes this difficulty by comparing mass with mass. It is the mass of water vapour in a unit mass of moist air, expressed in grams of vapour per kilogram of air. Because mass does not change when air expands or contracts, the specific humidity of a rising or sinking parcel stays constant until vapour is actually added by evaporation or removed by condensation. Meteorologists therefore prefer it for tracing air masses. Specific humidity is highest, around 18 to 20 grams per kilogram, in the equatorial belt, and it decreases towards the poles, falling to under 1 gram per kilogram over Antarctica. A closely related measure is the mixing ratio, the mass of vapour per kilogram of dry air; for ordinary purposes the two are nearly equal.

Both measures tell how much water is present but neither tells how close the air is to saturation. Air with 10 grams per cubic metre is comfortably dry at 35 degrees but is completely saturated at about 11 degrees. To know whether dew or fog will form, we need the third measure, relative humidity, which compares the moisture present with the moisture the air could hold at its temperature.

A simple way to remember the three: absolute humidity asks 'how much vapour in this box of air', specific humidity asks 'how much vapour in this weight of air', and relative humidity asks 'how full is the air'.

Air at a given temperature can hold only a certain maximum quantity of water vapour. When it holds that maximum it is said to be saturated. Relative humidity is the ratio of the water vapour actually present in the air to the maximum the air could hold at the same temperature, expressed as a percentage. If air at 30 degrees, which can hold about 30 grams per cubic metre, in fact holds 15 grams, its relative humidity is 15 divided by 30, that is 50 per cent. Saturated air has a relative humidity of 100 per cent.

Relative humidity changes in two ways. It rises when more vapour is added by evaporation, and it also rises when the air is cooled, because cooling reduces the capacity of the air while the vapour present stays the same. In the same way it falls when the air is warmed. This is why relative humidity is usually highest just before sunrise, when the air is coldest, and lowest in the early afternoon, even though the actual amount of vapour may not have changed at all through the day.

The dew point is the temperature to which a parcel of air must be cooled, without changing its moisture content, for it to become saturated. Below the dew point the surplus vapour condenses. If evening air at 25 degrees contains 17 grams per cubic metre, and 17 grams is the capacity of air at 20 degrees, then the dew point of that air is 20 degrees. Once the night temperature falls to 20 degrees dew will begin to form on grass and metal roofs. When the dew point is below 0 degrees the vapour turns directly into ice crystals, and the temperature is then called the frost point.

Relative humidity matters for comfort and health. At high relative humidity sweat cannot evaporate, the body cannot cool itself and the heat feels oppressive; this is the discomfort of the pre-monsoon months in Bengal. At very low relative humidity the skin and throat dry out. For weather forecasting the dew point is the more useful figure, because the difference between air temperature and dew point tells the forecaster how much cooling is needed before fog or cloud will appear.

Evaporation is the process by which liquid water changes into water vapour at temperatures below the boiling point. It takes place from every free water surface, from the oceans, which supply about 85 per cent of all vapour, from lakes, rivers and tanks, from wet soil and from the tiny pores of plant leaves, where the process is called transpiration. Together, evaporation and transpiration from a vegetated surface are called evapotranspiration. Evaporation is a cooling process: the escaping molecules are the fastest ones and they carry away about 600 calories of heat for every gram that leaves, which is why a wet cloth feels cold and why an earthen pot keeps water cool.

The rate of evaporation depends on several factors.

• Temperature. Warmer water and warmer air mean faster molecules and a larger capacity of the air to receive vapour. Evaporation is highest in the hot tropical oceans and in the afternoons.
• Humidity of the air. Dry air accepts vapour readily; air that is already near saturation accepts almost none. Clothes dry slowly on a humid monsoon day.
• Wind. Moving air carries away the vapour-laden layer just above the water and replaces it with drier air, so a breeze speeds evaporation. On a still day the layer above a pond soon becomes saturated and evaporation slows.
• Area of the water surface. A wider surface exposes more molecules to the air. Water spread in a shallow tray dries faster than the same water in a bottle.
• Air pressure. Low pressure allows molecules to escape more easily; evaporation is somewhat faster at high altitude.
• Salinity. Dissolved salts hold the water molecules more firmly, so sea water evaporates about five per cent more slowly than fresh water at the same temperature.

Evaporation is the first step of the hydrological cycle and the source of all humidity. Over the oceans between 10 and 30 degrees latitude, under the clear skies of the trade-wind belts, evaporation exceeds precipitation and the sea surface is saltiest. Over land the total evaporation is limited by the water available, and in a desert it may be less than the rainfall simply because there is no water left to evaporate. In West Bengal the rate of evaporation is highest in April and May, before the monsoon, when temperatures are high and the air is still relatively dry.

Condensation is the change of water vapour into liquid water. It is the opposite of evaporation and it releases the latent heat that evaporation absorbed. Condensation happens when air is cooled to its dew point, and it needs a surface on which the droplets can gather. In the free air that surface is provided by tiny particles of dust, salt, smoke and pollen, called hygroscopic nuclei or condensation nuclei; near the ground it may be the surface of a leaf, a stone or a window pane. Air is cooled to the dew point in four common ways: by losing heat at night through radiation, by touching a cold surface, by mixing with colder air and by rising and expanding. The first three produce the forms of condensation near the ground; the fourth produces clouds.

Dew forms on clear, calm nights when the ground cools rapidly by radiation and the thin layer of air touching grass, leaves and metal roofs is chilled below its dew point, which must be above freezing. The vapour condenses into droplets on the surfaces. Clouds prevent dew by slowing the loss of heat, and wind prevents it by mixing the cold layer with warmer air above. Dew is common in Bengal in November and December.

Frost forms in the same way when the dew point is below 0 degrees Celsius; the vapour then passes directly into feathery ice crystals on the surface. Frost in the plains of north India in January damages potato and vegetable crops, and in Darjeeling it whitens the tea gardens.

Fog is a cloud resting on the ground. It forms when a thick layer of moist air near the surface is cooled below its dew point and the condensed droplets remain suspended, reducing visibility to below one kilometre. Radiation fog forms on long, clear winter nights over moist ground and is the fog of the Gangetic plain from December to February; advection fog forms when warm moist air moves over a cold surface, as over the Grand Banks off Newfoundland. Mist is a thinner fog in which visibility is more than one kilometre. Smog is fog in which smoke and industrial gases are trapped; the word joins smoke and fog. Smog is dangerous to the lungs, and the winter smog of Delhi and the Howrah industrial belt is a serious health problem. Hoar frost, haze and rime are other minor forms of condensation described in the section.

A cloud is a visible mass of tiny water droplets or ice crystals suspended in the air at some height above the ground. Clouds form when air rises, expands and cools adiabatically to its dew point; the vapour then condenses on hygroscopic nuclei into droplets so small, about one hundredth of a millimetre, that they float. The height at which a rising parcel reaches its dew point is the cloud base, and it is flat because all the air in the parcel reaches saturation at about the same level.

Clouds are classified by two features, their height and their appearance. Three words describe the shapes: cirrus means a curl of hair and describes wispy, fibrous clouds; cumulus means a heap and describes clouds with a flat base and cauliflower-like domes; stratus means a layer and describes sheet clouds spread across the sky. The word nimbus means rain-bearing and the prefix alto means middle.

By height, in the middle latitudes, the families are:

• High clouds, above 6 km, made entirely of ice crystals: cirrus, delicate white feathers; cirrocumulus, small white ripples, the mackerel sky; cirrostratus, a thin milky veil that produces a halo around the sun or moon and often warns of an approaching depression.
• Middle clouds, 2 to 6 km: altocumulus, grey or white rounded patches; altostratus, a grey sheet through which the sun shows as if through frosted glass, often giving light rain.
• Low clouds, below 2 km: stratus, a uniform grey layer like lifted fog, giving drizzle; stratocumulus, rolls and patches of grey; nimbostratus, a thick, dark, shapeless rain cloud that brings steady, prolonged rain of the kind Bengal receives in the monsoon.
• Clouds of vertical development: cumulus, the fair-weather cloud of the afternoon with a flat base and bright domed top; and cumulonimbus, the towering thunder cloud whose base may be at 500 metres and whose anvil-shaped top of ice reaches 12 to 15 km, bringing heavy showers, thunder, lightning and hail. The Kalbaisakhi storms of April and May in Bengal come from cumulonimbus clouds.

Clouds affect the earth's heat budget in two opposite ways: by day they reflect sunlight and cool the surface, by night they trap outgoing heat and keep the surface warm. This is why cloudy nights are warmer than clear ones and why frost and dew need a clear sky.

Precipitation is the falling of condensed water from clouds to the surface of the earth in any liquid or solid form. Cloud droplets are far too small to fall; they must grow to about a millimetre across, a million times their volume, before gravity can bring them down through the rising air. Two processes make them grow. In warm clouds droplets of different sizes collide and merge, a process called coalescence. In cold clouds, where ice crystals and supercooled droplets exist together, the crystals grow at the expense of the droplets, because the vapour pressure over ice is lower than over water; the crystals become heavy, fall and melt into rain on the way down. This is the ice-crystal or Bergeron process, and it produces most of the rain of the middle latitudes.

The forms of precipitation are:

• Rain: liquid drops larger than 0.5 mm in diameter. It is the commonest form and the only one of importance in the plains of West Bengal.
• Drizzle: very fine drops, less than 0.5 mm, falling slowly and close together from low stratus cloud.
• Snow: white, six-sided ice crystals formed when vapour turns directly into ice in clouds below freezing point and the air below the cloud is also cold enough for the flakes to reach the ground. Snow falls in the Himalaya above about 2500 m in winter and occasionally at Darjeeling.
• Sleet: a mixture of rain and snow, or rain drops that freeze into small ice pellets while falling through a layer of cold air near the ground.
• Hail: hard, rounded lumps of ice, from 5 mm to several centimetres across, produced only in cumulonimbus clouds. A frozen droplet is carried up and down by violent currents, gathering a fresh coat of ice on each journey, until it is too heavy to be held up. A cut hailstone shows onion-like layers of clear and cloudy ice. Hail during the Kalbaisakhi flattens standing rice and mango blossom in Bengal.

Precipitation is measured as the depth of water that would cover a flat surface if none ran off or soaked in, expressed in millimetres or centimetres. Snow is either melted and measured as water or measured as depth of snow, roughly ten centimetres of fresh snow equalling one centimetre of rain. Precipitation is the source of all river flow, groundwater and soil moisture, so its quantity, seasonality and reliability shape agriculture and settlement everywhere.

All rain requires moist air to be lifted and cooled to its dew point, and the three types of rainfall are named for the three ways in which the air is lifted. In convectional rainfall the lifting is done by heat. When the ground is strongly heated by the sun, the air resting on it is warmed, expands, becomes lighter and rises in a column, while cooler air flows in from the sides to take its place; this circulation is convection, the same movement that stirs water in a heated pot. The rising air cools at the dry adiabatic rate of about 10 degrees per kilometre until it reaches its dew point, where a flat-based cumulus cloud forms. The latent heat released by condensation warms the parcel, keeps it lighter than its surroundings and drives it higher, so the cumulus swells into a cumulonimbus reaching 10 km or more. Within an hour or two of the cloud forming, heavy rain falls in large drops, usually with thunder and lightning and sometimes with hail.

Convectional rain has clear characteristics. It occurs in the afternoon or early evening, after the ground has been heated through the morning. It is heavy but short, lasting perhaps thirty minutes to two hours, and it falls over a small area, so one village may be flooded while the next stays dry. It needs a high surface temperature and a plentiful supply of moisture, and so it is the typical rain of the equatorial belt, where it falls almost every afternoon throughout the year, giving the Amazon and Congo basins and the islands of Indonesia over 2000 mm a year with no dry season. This daily rhythm of the equatorial climate is often called the 4 o'clock rain.

In India convectional rain is the rain of the hot season. In West Bengal the Kalbaisakhi or Nor'wester of April and May is a convectional thunderstorm: the Chota Nagpur plateau and the plains of Bengal are intensely heated, moist air from the Bay of Bengal is drawn in, and towering cumulonimbus clouds build up in the afternoon and sweep south-eastwards with violent squalls, hail and a brief, welcome downpour that cools the air and helps the jute and the summer rice. Similar pre-monsoon showers are called Mango showers in Kerala and Karnataka, because they help the mango to ripen. In the middle latitudes convectional showers occur on hot summer afternoons.

Orographic rainfall or relief rainfall is produced when moist wind is forced to rise over a mountain range that lies across its path. The word comes from the Greek oros, a mountain. The wind cannot pass through the barrier, so it climbs the slope facing it, the windward slope. As it rises it expands and cools, first at the dry adiabatic rate and then, once its dew point is reached, at the slower wet rate; clouds gather along the slope and heavy rain falls on the windward side. Having crossed the crest, the air descends the far slope, the leeward slope. Descending air is compressed and warms at the dry adiabatic rate, its relative humidity falls sharply, the clouds evaporate and little or no rain falls. This dry belt on the leeward side is the rain shadow, and the warm dry descending wind is often given a local name, such as the Foehn of the Alps or the Chinook of the Rockies.

The amount of orographic rain depends on the moisture of the wind, the height and steepness of the range and the angle at which the wind meets it. A range at right angles to a moist sea wind receives the heaviest rain. Rainfall generally increases with height up to a certain level, usually between 1500 and 2500 metres in the tropics, beyond which the air has lost most of its moisture and rainfall decreases again; this is called the inversion of rainfall.

The Indian monsoon provides the world's best examples. The Arabian Sea branch of the south-west monsoon strikes the Western Ghats almost at right angles; the windward western slopes and the Konkan coast receive 250 to 400 cm of rain, while Pune and the Deccan plateau, barely 100 km east in the rain shadow, get 50 to 70 cm. The Bay of Bengal branch is turned north by the Arakan Yoma and then west by the Himalaya; where it enters the funnel-shaped Meghalaya hills it is forced up the Khasi scarp and Mawsynram and Cherrapunji receive over 1100 cm a year, the highest in the world, while Shillong on the plateau behind gets about 200 cm and the Brahmaputra valley beyond receives even less. In West Bengal the Bay branch rising over the Darjeeling Himalaya gives the foothills and the Terai between 300 and 400 cm. Outside India, the Cascade Range in the north-west of the United States and the Andes in Chile show the same contrast between a wet windward coast and a dry leeward interior.

Cyclonic rainfall is produced when air is lifted within a cyclone, a region of low pressure into which winds blow spirally. The lifting happens in two different ways in the two kinds of cyclone, so this type of rain is described separately for the middle latitudes and the tropics.

In the temperate cyclone or depression of the middle latitudes, between about 35 and 65 degrees, a warm, moist air mass from the subtropics meets a cold, dry air mass from the poles. The two masses do not mix at once; the boundary between them is called a front, and the rain that falls along it is called frontal rainfall. Along the warm front, where warm air advances over the retreating cold air, the warm air slides gently up the long wedge of cold air, cooling slowly and producing a sequence of clouds, first cirrus, then cirrostratus, altostratus and finally nimbostratus, from which steady, gentle rain falls over a wide area for many hours. Along the cold front, where cold air advances and pushes under the warm air like a wedge, the warm air is forced up steeply and quickly, cumulonimbus clouds form and there are short, heavy showers with thunder, followed by clearing skies. A depression may be 1500 km across and its rain belt moves with it from west to east; the rain of Britain, western Europe and the winter rain of the Mediterranean and of north-west India are of this kind. The western disturbances that bring winter rain to Punjab, Haryana and snow to Kashmir between December and February are temperate cyclones travelling east from the Mediterranean.

In the tropical cyclone, the hurricane, typhoon or the cyclone of the Bay of Bengal, there are no fronts. Warm, extremely moist air over a sea surface warmer than 27 degrees spirals inward and rises violently in a ring of cumulonimbus around a calm central eye. The latent heat released feeds the storm. Rainfall is torrential, often 200 to 500 mm in a day, accompanied by winds above 120 km per hour and a storm surge of the sea. The Bay of Bengal cyclones of October-November and April-May, such as Aila in 2009 and Amphan in 2020, bring this rain to the coastal districts of West Bengal, South 24 Parganas, North 24 Parganas and Purba Medinipur, with destructive floods in the Sundarbans.

Cyclonic rain differs from convectional rain in falling over a much larger area, in lasting longer and in not being tied to the afternoon; and it differs from orographic rain in needing no mountain barrier.

Students of this section are regularly asked to compare the three types of rainfall, so it is worth setting the differences out clearly. The common factor in all three is that moist air must rise and cool to its dew point; they differ in the cause of the rising, the kind of cloud, the duration and intensity of the rain, the area covered and the region where each is typical.

Several points deserve comment. Convectional rain is unreliable for agriculture because it is patchy; a farmer cannot depend on his field being under the cloud. Orographic rain is highly reliable wherever a moist wind blows steadily against a range, which is why the windward slopes of the Ghats and the Himalayan foothills carry dense forests and plantations of tea and rubber. Cyclonic rain of the temperate kind is the gentlest and most useful, soaking into the soil rather than running off, which is one reason the farmland of western Europe is so productive; the tropical variety, however, is the most destructive rain on earth.

Real rainfall is often a mixture. The monsoon rain of the plains of West Bengal is partly cyclonic, because it is brought by the monsoon depressions that form at the head of the Bay of Bengal and travel north-west along the Ganga valley, and partly convectional, because the humid air breaks into thunderstorms in the heat of the afternoon. The rain of the Darjeeling hills adds the orographic element. A Madhyamik answer that recognises this combination shows real understanding.

The average yearly rainfall of the whole earth is about 100 cm, but it is spread very unevenly. Rainfall is controlled by latitude and the pressure belts, by the direction of the prevailing winds and whether they blow from the sea or the land, by the position of mountain barriers and by the temperature of the nearby ocean currents. Reading a world rainfall map with these controls in mind, five broad belts stand out.

The equatorial belt of heavy rainfall, roughly 10 degrees north to 10 degrees south, receives over 200 cm a year, distributed through all months. High temperature, low pressure, rising air and abundant evaporation from oceans and forests give daily convectional rain to the Amazon basin, the Congo basin, the Guinea coast and the islands of Malaysia and Indonesia.

The trade-wind belts, between 10 and 30 degrees, show a sharp contrast between the two sides of the continents. The trade winds blow from the north-east and south-east, so the eastern coasts, such as the east coast of Brazil, the West Indies, eastern Madagascar and the Queensland coast, receive heavy orographic rain, while the western sides of the continents in the same latitudes, under the subsiding air of the subtropical high pressure and beside cold currents, are the great hot deserts: the Sahara, Arabia, Thar, Kalahari, Atacama and the deserts of western Australia, with under 25 cm.

The monsoon lands of south and south-east Asia, also within the tropics, receive heavy rain, 100 to 300 cm or more, but concentrated in the summer months when the wind blows from the sea; the world's rainiest places, Mawsynram and Cherrapunji, lie here.

The Mediterranean margins, on the western sides of continents between 30 and 40 degrees, have moderate winter rain of 40 to 75 cm from temperate cyclones and dry summers under the subtropical high.

The westerly belt, between 40 and 60 degrees, receives cyclonic rain throughout the year, heaviest on the western coasts facing the ocean, such as western Britain, Norway, British Columbia, southern Chile and New Zealand, with 100 to 250 cm, decreasing rapidly inland so that the interiors of Asia and North America, far from any sea and behind mountains, are cold deserts such as the Gobi with under 25 cm.

The polar regions receive very little precipitation, mostly as snow, because the frozen air holds almost no vapour; Antarctica with under 5 cm is the driest continent. In short, rain is heaviest where the air is warm, moist and rising, and least where the air is cold, dry or sinking.

Two instruments belong to this section. The instrument that measures humidity is the hygrometer, and the commonest kind is the wet and dry bulb hygrometer or psychrometer. It consists of two identical mercury thermometers fixed side by side on a stand. The bulb of one is left bare and reads the air temperature; the bulb of the other is wrapped in a piece of muslin whose end dips in a small vessel of distilled water, so that the bulb is kept wet. Water evaporating from the muslin takes latent heat from the bulb and lowers its reading. The drier the air, the faster the evaporation and the greater the cooling, so the difference between the dry-bulb and wet-bulb readings, called the depression of the wet bulb, is large when the air is dry and zero when the air is saturated. A printed table gives the relative humidity and the dew point for any pair of readings. The instrument is kept in the louvred Stevenson screen with the other thermometers so that it reads the shaded, ventilated air. Other hygrometers use human hair, which lengthens as humidity rises, or an electronic sensor.

Rainfall is measured with the rain gauge. The ordinary Symons rain gauge is a cylinder, usually of copper or plastic, about 12.7 cm in internal diameter, with a funnel of exactly the same diameter fitted into its top. The funnel leads into a bottle or inner receiver where the water collects; the narrow neck reduces loss by evaporation. The gauge is set in open ground away from trees and buildings, at a distance of at least twice the height of any obstacle, with its rim 30 cm above the ground so that rain splashing from the soil cannot enter. At a fixed hour every day, 8.30 in the morning at Indian stations, the collected water is poured into a graduated measuring glass whose scale is calibrated for the funnel's area and reads the depth directly in millimetres. A depth of 1 mm means that 1 litre of water fell on every square metre. Snow caught in the gauge is melted before measurement. Self-recording gauges of the tipping-bucket or float type trace the rainfall continuously on a chart, showing the intensity of each shower.

The daily readings are added to give monthly and annual totals, and the average of thirty years of records gives the normal rainfall used to judge whether a monsoon is deficient or excessive. A line joining places with equal rainfall on a map is an isohyet, and the isohyet map of India shows at a glance the wet coast, the dry rain shadow and the desert.