Chapter 15: Humidity
The rain stopped on Saturday night. Sunday came up clear, the sky washed blue, and the family decided the day was made for a picnic. By eleven the plan was in trouble. The sun was no fiercer than usual, yet the air sat on the skin like a warm cloth. Shirts stuck. Sweat ran and stayed there. The washing pegged out at eight was still limp at noon, and no breeze moved it. By one o’clock the picnic had become a cinema, chosen for its cold dry air. The thermometer explains none of this. It read what it reads most Augusts. What had changed was invisible. The air was carrying nearly as much water vapour as it could hold, and the rain of the previous days had handed it a great deal of heat — heat that had been hidden inside the vapour and released when the droplets formed. Air that is already close to full cannot take much more. Sweat could not leave the skin, so the body could not cool itself. Washing could not dry. How much water vapour air holds, set against how much it could hold, runs through this whole chapter.
Diagram
Fig. 15.A1 Caption: Air at 25 °C that holds 20 g of water vapour per cubic metre still has room for 5 g more. Brief: One cubic metre of air drawn as a clear box at 25 °C, filled to four-fifths with water vapour labelled 20 g and the empty fifth labelled 5 g, with the full capacity 25 g marked at the top edge and a small figure below the box unable to dry, no rainfall or cloud shown.
15.1 Water In The Air
Moisture is never still. It enters the atmosphere, stays for a while, and leaves again, and while it is present it gives the air a completely different character. Two masses of air at the same temperature can feel nothing alike, and the water they carry is the reason.
Four processes carry that water around, and the whole chapter is built on them. Evaporation is the process by which water vapour enters the atmosphere on heating. Condensation is the process by which water vapour forms water droplets on cooling. Precipitation is the process by which those droplets fall to the ground in liquid, solid or frozen form. Humidity is the amount of water vapour present in the air.
The Water Is Never Lost
The total volume of water in the oceans and the seas stays constant. This seems odd, because water is leaving their surface every moment of every day, but every drop that evaporates is eventually returned to them.
The return happens along two routes. Water that condenses and falls back over the sea returns directly. Water that falls on land finds its way back indirectly, through streams and through overflow from land surfaces.
Flowchart
Fig. 15.A2 Caption: Water leaves the ocean and comes back to it, so the total volume in the oceans and seas does not change. Nodes: water evaporates from oceans, lakes and rivers → water vapour in the atmosphere → condensation → precipitation → falls over the sea? yes → returns directly to the ocean; no, falls on land → stream and overflow from land surfaces → back to the ocean
Where Evaporation Is Greatest
Evaporation is not spread evenly over the earth, because temperature and cloud cover differ from place to place. The chapter picks out three cases.
In low latitudes, the belt lying on either side of the equator, evaporation is greater, because temperatures there are relatively higher.
In Equatorial regions the sky often remains overcast with clouds, which cuts down the heat reaching the surface, so evaporation is relatively low.
In tropical deserts, surrounded by seas, evaporation is maximum, because high temperatures and clear skies work together.
The second and third cases are worth holding side by side. Heat alone does not settle the matter, because a permanently clouded sky can hold evaporation down in one of the hottest belts on earth.
What Makes Water Evaporate Faster
From all of this, the factors favouring evaporation are as follows.
Humidity: dry air promotes greater evaporation, because dry air has more capacity to hold water vapour than humid air has.
Supply of heat: the greater the heat of the water surface and of the air above it, the greater will be the rate of evaporation.
Winds: strong winds promote evaporation, which is why wet clothes dry more quickly on a windy day than when it is calm.
15.2 Humidity
Water is added to the atmosphere by the process of evaporation, changing from liquid to gaseous form as it goes. The water vapour held in the air is what we call its humidity.
Humidity is closely tied to temperature. The higher the temperature, the more water vapour the air holds; when the atmospheric temperature is low, the water vapour present is less. Warmth and moisture travel together, and that link is worth carrying through the rest of the chapter.
15.2.1 Absolute Humidity
The amount of water vapour held by air at a certain temperature and volume is called Absolute Humidity. It is a plain quantity of vapour, counted and stated, with no comparison built into it.
Absolute humidity is the mass or weight of vapour per unit volume of air, and it is usually calculated in grams per cubic metre. A cubic metre is a box of air one metre along each edge, so a reading of 20 grams per cubic metre means that box holds 20 grams of water vapour.
There is a difficulty with this measure. Since air rises upward, the absolute humidity of a parcel of air may change. For that reason, absolute humidity is generally not taken into account in weather calculations.
15.2.2 Relative Humidity
Air cannot hold unlimited vapour. At any given temperature there is a ceiling, and what matters for weather is how close the air has come to that ceiling. The ratio between the absolute humidity of a given mass of air and the maximum amount of water vapour that it can hold at the same temperature is called the relative humidity. It is generally expressed as a percentage.
\[\text{Relative Humidity (RH)} = \frac{\text{Actual amount}}{\text{Water holding capacity}} \times 100\]
The actual amount is the water vapour the air is carrying, in grams per cubic metre. The water holding capacity is the greatest amount the air could carry at that same temperature, also in grams per cubic metre. The two units cancel, so the answer is a percentage and carries no unit of its own.
The chapter works one sample right through.
The 80% is not a statement about how wet the air is. It is a statement about how nearly full it is. The same 20 g/cu.m in air that could hold far more would give a much lower percentage, and the air would feel dry. Relative humidity is a reading of the gap left, and the gap is what decides whether water can still evaporate into that air.
The same reasoning runs backwards. If the temperature of an air mass is 25 °C and its relative humidity is 60%, then it can still add 40% more water vapour at that temperature before it becomes saturated and the relative humidity reaches 100%.
Saturation And Dew Point
Relative humidity increases when more water vapour enters the atmosphere, and decreases when the vapour content is reduced. When the air is fully saturated, the relative humidity is 100%. Saturated air will not hold any more vapour at that temperature, and anything further that arrives has nowhere to go.
There is a second route to the same condition, and it does not involve adding vapour at all. Cool the air instead, and its capacity shrinks until the vapour already present is enough to fill it. Dew point is the temperature at which air gets fully saturated.
Telling The Two Apart
The two measures are easy to confuse, because both are counting the same water vapour in the same air. Absolute humidity is the measure of the actual amount of water vapour in the air, regardless of temperature. Relative humidity also measures the water vapour in the air, but always relative to the maximum vapour that the air can hold at that temperature.
They are therefore stated in different ways. Absolute humidity is expressed as grams of moisture per cubic metre of air, written \(\text{g/m}^3\). Relative humidity is expressed as a ratio, given as a percentage, between the absolute humidity of a given mass of air and the maximum amount of water vapour that it can hold at the same temperature.
Their behaviour differs too. The higher the amount of water vapour present, the higher is the absolute humidity, and temperature does not enter the sum at all. Warm air possesses more water vapour than cold air, so with the same amount of absolute humidity, air will have a different relative humidity depending on its temperature. This is exactly why absolute humidity is generally not taken into account in weather calculations, while relative humidity is the essential characteristic of weather forecasts, indicating the likelihood of precipitation, dew or fog.
Why Relative Humidity Matters
Relative humidity is the most essential characteristic of climate, and it generally helps to determine the amount of precipitation a place receives. A forecaster reading a rising relative humidity is watching the air close in on its ceiling.
Above all, human comfort depends on humidity, and highly humid air is more oppressive. The human body dissipates heat through perspiration and its evaporation. Under conditions of high relative humidity, the rate of evaporation of sweat from the skin decreases, and the human being feels warm and uncomfortable.
15.3 Measurement Of Humidity
Relative humidity decides how comfortable a day feels and how likely rain is, so it has to be measured rather than guessed. The humidity of the atmosphere is measured with the help of a hygrometer, an instrument also known as the dry and wet bulb thermometer. The second name describes it exactly, because it is two thermometers mounted side by side.
One of the two is left alone in the open air. In the dry and wet bulb thermometer, a piece of wet cloth is tied around the other bulb, and the free end of that cloth dips into a small container of water. The cloth therefore stays permanently damp, drawing water up as fast as it loses it.
Diagram
Fig. 15.3 Caption: Hygrometer Brief: Redraw the printed instrument — two vertical thermometers mounted on one board, labelled DRY and WET, each with °C and °F scales, the wet bulb wrapped in cloth whose lower end runs down into the small open water container at the base.
How The Two Bulbs Differ
Water evaporating from the wet cloth takes latent heat with it, exactly as it did from the ocean surface earlier in this chapter. That heat comes out of the bulb underneath. The rate of evaporation of water vapour from the wet bulb therefore keeps the temperature of this bulb lower than the temperature of the dry bulb.
How much lower depends on how readily the air will accept vapour. Dry air pulls moisture off the cloth quickly and cools the wet bulb sharply, so the gap between the two readings is wide. Damp air takes very little, and the gap narrows. A difference in temperature between the two thermometers therefore indicates the relative humidity.
The limiting case settles the pattern. Saturated air will not allow evaporation at all, and in that case the temperature reading on both the dry and the wet thermometers will be the same. This would mean that the relative humidity is 100%.
Flowchart
Fig. 15.A3 Caption: The gap between the two thermometer readings is what the instrument is really measuring. Nodes: wet cloth on one bulb → water evaporates from the cloth → latent heat is drawn out of that bulb → wet bulb reads lower than dry bulb → is there a difference between the two readings? wide difference → air is dry, relative humidity is low; narrow difference → air is damp, relative humidity is high; no difference → air is saturated, relative humidity is 100%
15.4 Condensation
Condensation is the reverse process of evaporation. In condensation, the water vapour in the atmosphere gets converted into water droplets or into ice, so water returns from the gaseous state to the liquid or the solid one.
The heat accounts move backwards as well. During condensation, the latent heat locked in the water vapour is released back into the atmosphere. Whatever the air had to spend to lift the water into vapour, it gets back the moment the vapour turns into droplets again. Fig. 15.2 sets the whole exchange in one landscape.
Diagram
Fig. 15.2 Caption: Precipitation Brief: Redraw the printed figure — a coastal landscape with the sea on the left and rising ground on the right, upward arrows off the sea labelled EVAPORATION, a cloud mass above labelled CONDENSATION, and downward arrows from the cloud to the land labelled PRECIPITATION, with a stream running back down the slope to the sea.
The Two Ways Air Sheds Its Water
Condensation can take place only under two circumstances. Either water vapour is added to air that is already saturated, so the extra has nowhere to sit, or the temperature falls below the temperature at which the air becomes saturated, so the capacity shrinks below what is already present. In both cases the outcome is the same — more vapour than the air can hold.
Flowchart
Fig. 15.A4 Caption: Condensation begins when the vapour present is more than the air can hold, and there are two ways of reaching that point. Nodes: air holding water vapour → is the vapour more than the capacity? add vapour to already saturated air → yes, the excess condenses; cool the air below its saturation temperature → capacity falls, yes, the excess condenses; neither happens → no, the vapour stays as vapour → back to air holding water vapour
The chapter cools one cubic metre of air to show what the second route does.
Notice that no water was added anywhere in that calculation. Not a single extra gram entered the air; the air simply lost the capacity to keep what it already had, and 6 g were pushed out as droplets. Cooling on its own is enough to make water appear, and that single idea explains every form of condensation in the next section.
Conditions Condensation Needs
Condensation takes place when the following atmospheric conditions exist.
There should be a high amount of water vapour present in the atmosphere, because air carrying very little has room to take more rather than shed any.
Minuscule particles of dust, salt and even smoke must be present, since these act as condensation nuclei — the particles around which the water vapour condenses.
The temperature of the air must be below dew point temperature, so as to encourage condensation.
15.5 Forms Of Condensation
Condensation is a single process, but it does not always look the same. What changes is where the cooling happens — high in the air, on a cold surface, or in the layer of air lying against the ground — and each of these gives a form with its own name. Fig. 15.4 sets several of them side by side.
Diagram
Fig. 15.4 Caption: Types of precipitation Brief: Redraw or source the printed six-panel strip — dew, mist, fog, rain, hail and snow shown as six vertical photographic panels of the same landscape, each with its name labelled in a tab at the top of its panel.
15.5.1 Clouds
Clouds are formed when minute droplets of water vapour condense on nuclei and remain suspended in air. Nothing about a cloud is solid; it is a crowd of droplets small enough for the air to carry.
The making of a cloud begins at the ground. Warm moist air near the ground rises into the sky, and since temperature decreases with height, the ascent causes a reduction in temperature below dew point. Moving air, on colliding with dust particles in the air, sheds its extra moisture. This leads to condensation in the form of tiny droplets of water. It is important to remember that condensation occurs only around tiny solid particles like dust, or carbon dioxide in smoke.
These impurities in the air are known as condensation nuclei, and the chain that depends on them is short and unforgiving. Without condensation nuclei, condensation does not take place. Without condensation, clouds do not form. Without clouds, precipitation cannot occur.
The products of condensation are so tiny that they remain in suspension and do not fall to the ground. The clouds then rise. As more and more droplets are added, the cloud slowly grows in size. When it cannot hold any more moisture, or when the air is fully saturated with relative humidity at 100%, the moisture comes down as rain.
Flowchart
Fig. 15.A5 Caption: A cloud is built from the ground upwards, and only rains when it can hold no more. Nodes: warm moist air near the ground rises → temperature decreases with height → air is cooled below dew point → vapour condenses on condensation nuclei → tiny droplets remain suspended, forming cloud → more droplets are added and the cloud grows → can the cloud hold more moisture? yes → the cloud keeps growing, back to more droplets are added; no, or relative humidity reaches 100% → moisture comes down as rain
Classification Of Clouds
When clouds rise, they take on different shapes, but not all clouds cause precipitation. Clouds may be grouped according to their formation in the atmosphere and their shape. Mainly, clouds can be of three types.
Cirrus clouds are fleecy like wool and are found generally at high altitude.
Cumulus clouds have a cauliflower-like shape.
Stratus clouds have a layered structure.
The cumulus type is worth a second look, because it is built differently from the others. The base of cumulus clouds is horizontal, but they have a vertical structure and their top is dome-shaped. They are generally termed as rain-bearing clouds.
15.5.2 Dew
When water vapour condenses on the surface, it forms tiny droplets of water called dew. Here the cooling is not done by rising air but by the ground itself, which chills the thin layer of air resting on it. Dew commonly occurs during winter, on account of the cooling of air below dew point.
15.5.3 Frost
Take the same surface cooling further and the water never becomes liquid at all. In very low temperature, when condensation occurs at zero metre or ground level, water vapour freezes into minute crystals of ice on objects near the ground, such as blades of grass, leaves and tiny rock particles. This is frost, and it is harmful to plants.
15.5.4 Fog And Mist
Fog and mist are two related terms. Droplets of water suspended in the atmosphere close to the surface of the earth are termed as fog. A fog is really a cloud that has formed at ground level, which is why walking through one feels the way it does.
The winter route to fog follows from the way the ground loses heat. On long winter nights, the ground cools more rapidly than the air above it. Such cooling reduces the temperature at the surface to below dew point, and water vapour at lower levels gets condensed around minute solid particles to form fog.
Cooling from below is not the only way to reach dew point near the surface. Where a warm ocean current runs alongside a cold one, air that has been warmed and moistened over the warm water is chilled sharply as it passes over the cold water, and fog forms over the sea. Fig. 15.5 shows this happening off the eastern coast of North America, where the cold Labrador Current flowing south meets the warm Gulf Stream carried north-east, with the Westerlies blowing across the meeting place.
Map
Fig. 15.5 Caption: The meeting of warm and cold currents gives rise to fog Brief: Redraw the printed map — the north-eastern coastline of North America labelled, the cold Labrador Current shown as an arrow running south along the coast, the warm Gulf Stream as an arrow running north-east, the Westerlies as broken arrows crossing them, and the fog patch shaded where the two currents meet; no other currents, coastlines or place names to be shown.
Mist is the lighter of the pair. It is less dense than fog and dissipates more quickly. The clearest way to tell them apart is by how far you can see through them: visibility during fog extends to less than one kilometre, while under conditions of mist it may go up to two kilometres.
15.6 Precipitation
Condensation puts water back into liquid or solid form, but it does not by itself bring the water down. The process by which the products of condensation — water droplets, ice crystals, sleet and the rest — fall to the ground is known as precipitation. This is the step that closes the circuit begun by evaporation, returning to the surface the water that left it.
There is a condition attached, and it is a matter of size. Precipitation takes place only when tiny particles of water join together to form large sized particles, which become too heavy to remain in suspension in the clouds. Until that happens the water stays where it is, however much of it there may be.
The Chief Forms
Rainfall, snowfall, drizzle, sleet and hail are the chief forms of precipitation, and what separates them is chiefly what happens to the water on its way down. Three of them are described here.
Rain is the most common form of precipitation. Raindrops of smaller size and less intensity are known as drizzle.
Snow forms from water droplets which rise higher and freeze on account of the drop in temperature. Snowfall usually occurs in winter in cold climates or on high mountains.
Hail forms when vertical air currents push water droplets or ice particles higher. They form into solid ice and fall as hail, and hailstones cause great damage to crops.
Flowchart
Fig. 15.A6 Caption: Everything that falls begins as the same cloud droplets, and the journey down decides the form it takes. Nodes: tiny droplets suspended in cloud → droplets join into larger particles → particles become too heavy to remain in suspension → they fall as precipitation → what happens on the way down? stays liquid → rain; stays liquid but in small drops of less intensity → drizzle; carried higher and frozen by falling temperature → snow; pushed higher by vertical air currents and formed into solid ice → hail
15.7 Types Of Rainfall
Deposition of moisture from the atmosphere on the earth’s surface is called precipitation, and one of the forms of precipitation is rainfall. Rain is not classified by how heavy it is or how long it lasts, but by what made the air cool in the first place.
Three types of cooling of air produce three different types of rainfall, known as Convectional Rainfall, Orographic Rainfall and Cyclonic Rainfall. In each of them air is lifted and cooled below dew point; what differs is the lifting agent — a mountain in the way, heat from below, or a colder air mass refusing to give way.
Flowchart
Fig. 15.A7 Caption: Three different ways of lifting and cooling air give three different types of rainfall. Nodes: warm moist air at the surface → what lifts it? a mountain barrier standing across the wind → relief or orographic rainfall; strong surface heating sending up convectional currents → convectional rainfall; a colder, heavier air mass forcing it to climb at a front → cyclonic or frontal rainfall; each branch → air cools below dew point → condensation → rainfall
15.7.1 Relief Rainfall
Relief Rainfall is also known as Orographic Rainfall. It occurs from the cooling of warm moist air which ascends above a mountain barrier lying in the direction of the prevailing winds — that is, a range standing across the path the winds are already taking.
The presence of mountains causes humid air to rise, because the air has nowhere else to go. The sudden ascent causes cooling of the air, leading to condensation and precipitation. Since it is caused by the relief of the land, meaning the shape and height of the ground itself, it is called relief rain. The slope facing the wind, which receives this rain, is the windward slope.
Diagram
Fig. 15.6 Caption: Orographic or Relief rainfall Brief: Redraw the printed figure — a mountain in section with the sea at its left foot, arrows of moist air ascending the left slope labelled ‘moist air ascending’ with rain falling on the windward slope, and arrows down the right slope labelled ‘dry air descending’ with the rain shadow area marked; all arrows pointing in the direction of air movement, left to right.
What Happens On The Far Side
The air that comes down the other slope is not the air that went up. On descending the leeward slope — the sheltered side, away from the wind — a decrease in altitude increases both the pressure and the temperature, leading the air to get compressed and warm. Warmer air has a larger capacity, so the relative humidity drops.
Consequently there is evaporation rather than condensation, and little or no precipitation in the rain shadow area. A place can therefore sit only a few kilometres from a drenched slope and receive almost nothing, purely because it stands on the wrong side of the ridge.
The monsoon winds, while ascending the Western Ghats along India’s western coast, cause heavy rainfall on the windward side. In the North-East, the Himalayan barrier makes the winds shed their moisture on the windward side, on the slopes facing south.
Other regions receiving orographic rainfall include Eastern Brazil on the east coast of South America, East China on the eastern edge of Asia, and the South Eastern United States. Orographic rainfall is especially heavy in the hot and humid areas bounded by tropical oceans, where the air arriving at the barrier is already carrying a great deal of vapour.
Within all these regions, rainfall is far from uniform, and three broad bands are recognised. Rainfall is measured as the depth of water that would collect on flat ground, in centimetres.
Regions of heavy rainfall receive 200 cm and above.
Regions of moderate rainfall receive between 100 cm and 200 cm.
Regions of very low rainfall receive less than 25 cm.
15.7.2 Convectional Rainfall
Convectional Rainfall occurs in the Equatorial regions, where the lifting is done by heat rather than by relief. The high temperature there leads to the rapid heating of the air near the surface.
Such heated air rises in convectional currents, leading to the development of clouds at about 10 km height. Ascending currents of hot and humid air cause condensation of the clouds, resulting in heavy rainfall. Because the heating has to build up first, this type of rainfall occurs in the afternoon at about 4 o’clock, and is known as the ‘4 O’clock Shower’.
Such rainfall is also accompanied by thunder and lightning. Convectional rainfall occurs daily in the Equatorial regions, which receive an annual rainfall of more than 200 cm.
Diagram
Fig. 15.7 Caption: Convectional rainfall Brief: Redraw the printed figure — a heated land surface at the base, red arrows of warm air rising in convectional currents on either side, green arrows of cold air descending, and a tall cloud mass above with rain falling vertically from it.
15.7.3 Cyclonic Or Frontal Rainfall
This type of rainfall is due to cyclones in the Tropical Latitudes and to depressions in the Temperate Latitudes, also called the mid-latitudes — the belts lying between the tropics and the polar regions. It happens irrespective of relief or convection, so neither a mountain nor strong surface heating is needed.
It is caused by convergence, meaning the meeting, of two different air masses with different temperatures and other physical properties. When warm and cold air masses confront each other, the warmer air, being lighter, generally climbs above the colder and heavier air.
The boundary zones between these air masses are called the fronts. The rising air is cooled while undergoing a frontal lift, and this causes precipitation. Such precipitation or rainfall is called Cyclonic or Frontal Rainfall.
Diagram
Fig. 15.8 Caption: Cyclonic or frontal rainfall Brief: Redraw the printed figure — a wedge of warm air labelled ‘warm air’ riding up over a block of cool air labelled ‘cool air’, red arrows showing the warm air climbing the sloping boundary, green arrows showing the cool air advancing beneath, and rain falling from the cloud along the boundary.
The same process gives very different weather in different latitudes, and the contrast is worth holding onto. In tropical cyclones the rainfall is very heavy but lasts for only a few hours. In temperate depressions, such as those of Western Europe on the western edge of the continent, the rainfall is much lighter though it lasts for several days, and there it falls in the form of a continuous drizzle.
15.8 Summary
- Moisture continuously enters and leaves the atmosphere, and while it is present it gives the air a different character.
- Four processes move water between the surface and the air.
- Evaporation is the process by which water vapour enters the atmosphere on heating.
- Condensation is the process by which water vapour forms water droplets on cooling.
- Precipitation is the process by which the droplets fall to the ground in liquid, solid or frozen form.
- Humidity is the amount of water vapour present in the air.
- The total volume of water in the oceans and seas remains constant, because everything that evaporates is returned to them directly by condensation and precipitation, and indirectly by stream and overflow from land surfaces.
- Generally 600 calories of heat are required to change one gram of liquid water into its vaporous state. This heat passes into the vapour in a hidden form called latent heat, and is released back into the atmosphere during condensation, causing a slight rise in temperature. This is why one feels more heat in the rainy season than under clear skies.
- Evaporation is not the same everywhere.
- In low latitudes it is greater, owing to relatively higher temperature.
- In Equatorial regions it is relatively low, because the sky often remains overcast with clouds.
- In tropical deserts, surrounded by seas, it is maximum, because of high temperatures and clear skies.
- Three factors favour evaporation.
- Humidity — dry air promotes greater evaporation, as it has more capacity to hold vapour than humid air.
- Supply of heat — the greater the heat of the water surface and the air above it, the greater the rate of evaporation.
- Winds — strong winds promote evaporation, so wet clothes dry faster on a windy day.
- Humidity is related to temperature: the higher the temperature, the more water vapour the air holds.
- Absolute humidity is the amount of water vapour held by air at a certain temperature and volume, expressed in grams per cubic metre. Since air rises upward it may change, so it is generally not taken into account in weather calculations.
- Relative humidity is the ratio between the absolute humidity of a given mass of air and the maximum amount of water vapour it can hold at the same temperature, expressed as a percentage. Air holding 20 g/cu.m at 25 °C when it could hold 25 g/cu.m has a relative humidity of 80%.
- Saturated air holds all it can at that temperature, so its relative humidity is 100%, and dew point is the temperature at which air gets fully saturated.
- Relative humidity is the most essential characteristic of climate, helps determine the amount of precipitation, and decides human comfort, because high relative humidity slows the evaporation of sweat from the skin.
- Humidity is measured with a hygrometer, also called the dry and wet bulb thermometer. Evaporation keeps the wet bulb cooler, and the difference between the two readings indicates the relative humidity; equal readings mean the air is saturated and the relative humidity is 100%.
- Condensation is the reverse process of evaporation, in which water vapour is converted into water droplets or ice and the locked latent heat is released.
- It occurs when vapour is added to already saturated air, or when the temperature falls below the temperature at which air becomes saturated.
- Air at 25 °C can hold 30 g of water vapour per cubic metre; cooled to 20 °C it can hold only 24 g, so the remaining 6 g gets condensed.
- It requires a high amount of water vapour, condensation nuclei of dust, salt or smoke to condense around, and air temperature below dew point.
- Condensation appears in four chief forms.
- Clouds form when minute droplets condense on nuclei in rising air and remain suspended; they are mainly cirrus, fleecy like wool at high altitude, cumulus, cauliflower-shaped and rain-bearing, and stratus, layered in structure.
- Dew forms when water vapour condenses into tiny droplets on a surface cooled below dew point, commonly in winter.
- Frost forms in very low temperature at ground level, when vapour freezes into minute ice crystals on grass, leaves and rock particles, and it is harmful to plants.
- Fog is droplets suspended close to the surface, formed when the ground cools faster than the air above on long winter nights; mist is less dense and clears more quickly, with visibility under one kilometre in fog and up to two kilometres in mist.
- Precipitation takes place only when tiny water particles join into large ones too heavy to remain suspended. Rainfall, snowfall, drizzle, sleet and hail are its chief forms, and hailstones cause great damage to crops.
- Three types of cooling of air produce three types of rainfall.
- Relief or orographic rainfall occurs when warm moist air is forced to ascend a mountain barrier lying across the prevailing winds, as the monsoon winds are by the Western Ghats and the Himalayan barrier.
- Descending air on the leeward slope is compressed and warmed, its relative humidity drops, and little or no rain falls in the rain shadow area.
- Convectional rainfall occurs in the Equatorial regions, where rapid heating sends air up in convectional currents to form cloud at about 10 km, giving the daily ‘4 O’clock Shower’ with thunder and lightning and more than 200 cm of rain a year.
- Cyclonic or frontal rainfall occurs where two air masses converge and the warmer, lighter air climbs over the colder, heavier air at a front — very heavy but brief in tropical cyclones, lighter but lasting for days as continuous drizzle in temperate depressions.
Flowchart
Fig. 15.A8 Caption: The whole chapter as one circuit — water leaves the surface, travels as vapour, condenses, falls, and returns. Nodes: water in oceans, lakes and rivers → evaporation, taking in latent heat → water vapour in air, measured as humidity → air lifted by a mountain barrier, by surface heating, or at a front → air cools below dew point → are condensation nuclei present? no → droplets do not form, back to water vapour in air; yes → condensation, latent heat released → clouds, dew, frost, fog or mist → droplets join and grow too heavy to stay suspended → precipitation as rain, drizzle, snow, sleet or hail → falls on the sea, returning directly; falls on land → stream and overflow from land surfaces → back to water in oceans, lakes and rivers
What This Chapter Cannot Tell You
The capacity figures used here are worked examples for one or two temperatures, not a complete table of how much vapour air can hold at every temperature. A relative humidity reading describes one spot at one moment, and says nothing about the next hillside or the next hour.
The three types of rainfall are a way of sorting causes, not a description of any particular storm, and a single spell of rain may owe something to more than one of them. The chapter also gives broad rainfall bands rather than the actual totals of the regions it names, so it can tell you where rain is heavy, but not how heavy.
15.9 Check Your Concepts
Look at Fig. 15.1.
- Name the changes of state in which heat is absorbed, and those in which heat is released.
- Using your answer to (a), explain why the air becomes slightly warmer when condensation takes place in it.
Look at Fig. 15.6.
- Name the slope on which the rain falls and the slope on which the rain shadow area lies.
- Explain, step by step, why the air that has crossed the mountain brings little or no rain to the far side.
This chapter states that air at 25 °C can hold 30 g of water vapour per cubic metre, and that at 20 °C it can hold only 24 g.
- A cubic metre of air at 25 °C is carrying 24 g of water vapour. Work out its relative humidity, showing every step.
- The same air is now cooled to 20 °C, with no vapour added or removed. State what its relative humidity becomes, and what happens to the water vapour it is carrying.
A cloud may be full of condensed water droplets and still give no rain at all.
- Explain what must happen to those droplets before precipitation can occur.
- Name the chief forms of precipitation, and state what decides whether the water reaches the ground as rain, as snow or as hail.
Distinguish between absolute humidity and relative humidity.
Give a geographical reason for each of the following.
- The total volume of water in the oceans and seas remains constant.
- In Equatorial regions the sky often remains overcast with clouds.
- When relative humidity is 100%, the temperature reading on both the wet and the dry thermometers is the same.
What is condensation? Explain briefly the process of condensation.
Draw a well labelled diagram showing convectional rainfall.
How is dew formed? What is the difference between fog and mist?
Condensation is regarded as the reverse process of evaporation. Imagine that this reverse process did not happen. What would its consequences be on climate and on human beings? Also state the conditions under which this reversal may not happen.