Humidity | ICSE Class 9 Geography Notes

This note covers humidity, absolute and relative humidity, saturation, dew point, evaporation, condensation, clouds, dew, frost, fog, mist, rain, snow, hail, convectional rainfall, relief rainfall, frontal rainfall and regional examples of rainfall processes.

What is humidity and how does water enter the atmosphere?

Humidity means the water vapour present in air. Water vapour is water in its gaseous state. Atmospheric water also occurs as liquid droplets and solid ice, so water vapour must be distinguished from the other forms of water in the air.

The proportion of water vapour varies from zero to four per cent by volume of the atmosphere. A percentage expresses a share out of a hundred. Water vapour plays an important part in weather, including the processes that produce clouds and rainfall.

Where does atmospheric moisture come from?

Evaporation is the conversion of liquid water into water vapour. Water bodies supply moisture to the atmosphere through this process. Transpiration is the release of water vapour from plants, which provides another source of atmospheric moisture.

Water is continually exchanged between the oceans, the continents and the atmosphere. Condensation, the conversion of water vapour into liquid water, removes vapour from the air. Precipitation is the release of condensed moisture that falls to the Earth's surface in liquid or solid form.

These processes connect the presence of moisture with changes in its physical state. Evaporation supplies vapour; condensation produces droplets; precipitation returns condensed water to the surface. They describe different stages rather than different names for the same event.

Definition: Humidity refers to water vapour in air. It does not mean rainfall, and a cloud consists of minute water droplets or ice crystals rather than water vapour alone.

To explain a weather process clearly, identify both the state of water and its location. Water droplets on a cold object, droplets suspended near the ground and drops falling from clouds require different descriptions.

How do absolute and relative humidity differ?

Absolute humidity is the actual quantity of water vapour per unit volume of air. It is expressed in grams per cubic metre, written as g/m³. Here g means gram, m means metre, and m³ means cubic metre, a unit of volume.

Relative humidity compares the moisture actually present with the full moisture capacity of air at the same temperature. It is expressed as a percentage, represented by the symbol %. Temperature therefore forms part of its definition, rather than being an optional detail.

What does each measurement tell us?

The moisture capacity of air depends on temperature. As air becomes warmer, its capacity to retain moisture increases. A change in temperature can therefore change relative humidity even when no additional moisture has entered the air.

If the actual moisture content is unchanged, warming lowers the proportion of capacity being used. Cooling has the opposite effect until saturation is reached. This relationship follows from comparing the same moisture amount with a changing capacity.

Note: A relative humidity comparison must refer to the temperature concerned. Absolute humidity describes an amount per unit volume; relative humidity describes that amount in relation to the capacity at a given temperature.

Absolute humidity differs from place to place. Relative humidity also reflects the temperature-dependent capacity of air. Do not treat these two measurements as interchangeable simply because both describe atmospheric moisture.

What are saturation and dew point?

Saturated air contains moisture to its full capacity at a given temperature. At that temperature, it cannot retain additional water vapour. Dew point is the temperature at which a particular sample of air becomes saturated.

The phrase “at a given temperature” is essential. Saturation is a condition of the air, while dew point is a temperature. The two ideas are connected, but they answer different questions about the same sample of moist air.

How does cooling lead towards condensation?

1. Begin with moist air that is not yet saturated, meaning that its moisture content is below its capacity at that temperature.
2. As the air cools, its capacity to retain water vapour decreases.
3. When the sample reaches its dew point, its moisture content equals its capacity and it becomes saturated.
4. With sufficient further cooling, excess water vapour condenses, forming liquid droplets under suitable temperature conditions.

Cooling does not need to add more vapour for saturation to occur. It changes the relationship between the existing moisture and the air's capacity. This explains why temperature is central to relative humidity and condensation.

The dew point also helps distinguish condensation products. The freezing point of water is 0°C, where °C means degrees Celsius. Dew requires a dew point above freezing, whereas frost forms under freezing conditions with moisture deposited as ice crystals.

Definition: Saturation describes air containing its full moisture capacity at a given temperature. Dew point identifies the temperature at which that condition is reached in a given sample.

Keep the sample of air in view when using this definition. Dew point concerns the moisture already present in that sample; it is not another name for the freezing point of water.

How do evaporation and condensation work?

Heat is the main cause of evaporation. As temperature increases, the ability of a parcel, or portion, of air to absorb and retain water vapour increases. Air with a low moisture content also has scope to take up additional moisture.

Air movement assists evaporation by replacing a saturated layer with unsaturated air. The replacement air can receive more vapour. Greater movement of air therefore increases evaporation by renewing the air in contact with the evaporating surface.

Why does water vapour condense?

Condensation results from loss of heat. When moist air cools sufficiently, it may reach the stage at which it cannot retain its existing vapour in gaseous form. Excess vapour then changes into liquid water.

In free air, condensation occurs around very small particles called hygroscopic condensation nuclei. These are moisture-absorbing particles that provide centres for condensation. Dust, smoke and salt particles from the ocean are particularly good nuclei because they absorb water.

Condensation can also occur when moist air touches a colder object. It may also take place when the temperature is close to the dew point. The amount of cooling and the relative humidity therefore help determine whether condensation occurs.

A decrease in air temperature is the most favourable condition for condensation. Its occurrence is also influenced by the volume, pressure and humidity of the air. These factors explain why the presence of vapour alone is not a complete explanation of droplet formation.

How do dew and frost form on surfaces?

Dew consists of water droplets deposited on cooler solid surfaces, such as stones, grass blades and plant leaves. The droplets form on the objects themselves, rather than around particles suspended in the air above them.

The ideal conditions are a clear sky, calm air, high relative humidity, and cold, long nights. For dew to form, the dew point must be above the freezing point. The deposited moisture is therefore liquid water.

What distinguishes frost from dew?

Frost forms on cold surfaces when moisture is deposited as minute ice crystals under freezing conditions. The dew point is at or below freezing. Its solid crystals distinguish frost from the liquid droplets of dew.

The ideal conditions for white frost resemble those for dew, except that the air temperature must be at or below freezing. A description of frost should therefore state both the cold conditions and the solid form of the deposited moisture.

Neither description requires water to fall from a cloud. That separates these surface deposits from precipitation. Although frost and snow both involve ice crystals, frost forms on a cold surface, while snow reaches the surface as precipitation.

Note: Do not define frost simply as frozen dew. The defining process here is the deposition of excess atmospheric moisture as minute ice crystals on a cold surface.

How do fog and mist differ from surface deposits?

Fog is a cloud with its base at or very near the ground. It forms when an air mass containing much water vapour cools suddenly and condensation occurs around fine particles within it.

An air mass is a large body of air with little horizontal variation in temperature and moisture. Unlike dew, fog consists of droplets suspended in the air. Its effect on visibility helps distinguish it from mist.

What visibility limits distinguish fog and mist?

Horizontal visibility is the distance over which objects can be seen horizontally near the surface. A kilometre, written as km, is a unit of distance. Mist limits visibility less severely than fog under the following visibility criteria.

Mists are frequent over mountains where warm air rising along slopes meets a cold surface. Fogs are prevalent where warm air currents meet cold currents. These conditions emphasise cooling and contact between air of different temperatures.

In urban and industrial centres, smoke supplies many nuclei that assist fog and mist formation. Smog describes the condition in which fog is mixed with smoke. Smoke particles provide condensation centres, while the fog contains condensed moisture.

To distinguish these forms, first ask where the water is. Droplets deposited on grass indicate dew. Droplets suspended near the ground indicate fog or mist, with visibility supplying a further distinction between those atmospheric forms.

What are clouds and how are the main types recognised?

A cloud is a mass of minute water droplets or tiny ice crystals formed through condensation of water vapour in free air at considerable elevations. Cloud classification considers height, extent, density and how readily light passes through the cloud.

What are the four basic cloud types?

• Cirrus: Thin, detached clouds with a feathery appearance. They form at high altitudes and are always white.
• Cumulus: Clouds resembling cotton wool, occurring in scattered patches and having flat bases.
• Stratus: Layered clouds covering large areas of the sky. They generally form through loss of heat or the mixing of air masses with different temperatures.
• Nimbus: Black or dark grey clouds forming at middle levels or very near the surface. They are extremely dense and opaque, meaning that sunlight does not pass through them.

Altitude means height above sea level. The following height ranges distinguish the high cirrus clouds from the generally lower cumulus clouds. Cumulus clouds generally form at heights of 4,000 to 7,000 m.

Nimbus clouds sometimes lie so low that they seem to touch the ground. Cloud appearance and cloud height are separate clues: a useful description combines the visible form with the appropriate height or level, rather than relying on colour alone.

These are photographs. Figure 10.1 shows separate bright cloud masses above a landscape. Figure 10.2 shows a darker, more extensive cloud cover above hills and water, with a brighter opening between clouds.

See Figs. 10.1 and 10.2 in your NCERT textbook

How do rain, snow and hail form as precipitation?

Continued condensation in free air allows condensed particles to grow. When air resistance cannot support them against gravity, the force drawing them towards the Earth, they fall to the surface. This falling moisture is precipitation.

Rainfall is precipitation in the form of liquid water. Snowfall consists of fine snowflakes under temperatures below freezing. The moisture forms hexagonal, or six-sided, crystals, which combine into flakes of snow.

How is hail different from snow?

Hailstones are small, rounded, solid pieces of ice reaching the Earth's surface. They have concentric layers, meaning layers arranged one around another. This layered, rounded structure differs from the flakes associated with snow.

Sometimes, raindrops become solid pieces of ice after leaving clouds and passing through colder layers. Thunderstorm clouds that extend into regions with temperatures below freezing can produce hail. Snow and hail are therefore both solid precipitation, but their form and formation should be distinguished.

Condensation and precipitation are connected but distinct. Condensation explains the formation of droplets from vapour. Precipitation requires condensed moisture to fall. The existence of a cloud should not be treated as a complete explanation of rain reaching the ground.

Keep forms of precipitation separate from types of rainfall. Rain, snow and hail describe the form of falling moisture. Convectional, relief and frontal rainfall describe the process responsible for lifting moist air and producing rain.

Note: Frost and snow both contain ice, but their locations and processes differ. Frost is deposited on cold objects; snow falls to the surface as precipitation.

How does convectional rainfall develop?

Convection is the upward movement of air after heating makes it lighter. Convectional rainfall begins with heating and rising air. As the air rises, it expands and loses heat, allowing moisture to condense.

What is the sequence from heating to rainfall?

1. Air is heated and becomes lighter than the surrounding cooler air.
2. The heated air rises in convection currents, which are upward movements produced by heating.
3. As it rises, the air expands and loses heat.
4. Water vapour condenses and cumulus clouds develop.
5. Heavy rain falls with thunder and lightning, but the rainfall does not last long.

This rainfall is common in summer or during the hotter part of the day. Its explanation must include the cooling of rising air: surface heating starts the uplift, but condensation occurs after the air has risen and lost heat.

Case study: Where is convectional rainfall very common?

Convectional rainfall is very common in equatorial regions, the areas around the Equator, which is the imaginary circle dividing the Earth into northern and southern halves. It is also very common in continental interiors, particularly in the northern hemisphere.

A hemisphere is half of the Earth. Here the northern hemisphere means the half north of the Equator. These regional examples connect convectional rainfall with strong heating without restricting it to a single continent or season.

Draw a heated ground surface with arrows showing air rising above it. Label rising warm air, expansion and cooling, condensation, cumulus clouds and rainfall. Keep the sequence from heating to cooling and then rain clear.

The identifying cause is uplift by heating. Heavy rain alone does not identify the rainfall type, because other mechanisms can also raise moist air and produce substantial rainfall.

How do mountains produce relief rainfall and rain shadows?

Orographic rainfall, also called relief rainfall, develops when moist air is forced to rise over a mountain barrier. Here relief refers to the height and shape of the land. The mountain provides the immediate cause of uplift.

The windward slope faces the approaching wind. The leeward slope is the sheltered slope on the opposite side. These terms refer to wind direction, so they should be linked to the approaching air in any diagram or explanation.

Why does rainfall differ across the mountain?

1. Moist air encounters the mountain and is forced upwards along its windward slope.
2. As the air rises, it expands and its temperature falls.
3. Moisture condenses, producing greater rainfall on the windward side.
4. After crossing the mountain, the air descends on the leeward side and becomes warmer.
5. Its capacity to retain moisture increases, leaving the leeward side with less rainfall.

A rain-shadow area is the area on the leeward side that receives less rainfall. Explain it through descending and warming air, rather than saying that the mountain simply blocks raindrops from travelling across it.

Case study: How do the Western Ghats illustrate relief rainfall?

A branch of the Arabian Sea monsoon, the seasonal moisture-bearing winds arriving from the Arabian Sea, encounters the Western Ghats. The winds climb the slopes, cool and give very heavy rainfall to the windward Sahyadris and Western Coastal Plain.

The rainfall range is 250 to 400 cm, where cm means centimetre, a unit used to express rainfall depth. After crossing the Western Ghats, the winds descend and warm. Areas to the east receive little rainfall and form a rain shadow.

On an outline map of India, label the Arabian Sea and Western Ghats. Add arrows towards the mountain barrier, label the windward Western Coastal Plain as very wet, and mark the rain-shadow area to the east.

How does cyclonic or frontal rainfall occur?

A front is the boundary zone where two different air masses meet. Frontal rainfall occurs as warm air is raised along such a boundary. Rising air cools, clouds form and precipitation follows.

Extra-tropical cyclones are weather systems developing in middle and high latitudes beyond the tropics. Latitude describes distance north or south of the Equator in angular terms. These cyclones have a clear frontal system, with warm and cold fronts.

How do warm and cold fronts lift air?

A warm front forms where warm air advances towards cold air. The warm air glides over the cold air, and a sequence of clouds develops ahead of the warm front, bringing precipitation.

A cold front forms where cold air advances towards warm air. The cold air pushes the warm air upwards, and cumulus clouds develop along the front. In both cases, the explanation depends on the uplift of warm air.

The cold front moves faster than the warm front and eventually overtakes it. The warm air is lifted completely above the land surface, forming an occluded front. The cyclone then dissipates, meaning that it weakens and dies away.

The warm-front section labels warm air above a gently sloping boundary and cold air below it. The cold-front section shows a steeper curved boundary, with cold air behind it and warm air ahead.

See Fig. 9.8 in your NCERT textbook

Case study: What brings winter cyclonic rain to northwestern India?

Some weak temperate cyclones from the Mediterranean Sea bring winter rainfall to Punjab, Haryana, Delhi and western Uttar Pradesh. The amount is meagre. The lower Himalayas receive precipitation as snow, illustrating how precipitation form also depends on temperature.

On an outline map covering the eastern Mediterranean and Asia, trace the route from the eastern Mediterranean across West Asia, Iran, Afghanistan and Pakistan to northwestern India. Label the route as that of winter cyclonic depressions.

How can the three rainfall mechanisms be compared?

The common link in these rainfall mechanisms is rising moist air. The air cools as it rises, condensation forms clouds, and condensed moisture can fall as precipitation. The distinction lies in what causes the air to rise.

What initiates the upward movement?

For convectional rain, begin the explanation with heating. For relief rain, begin with an approaching wind and a mountain barrier. For frontal rain, begin with contrasting air masses meeting along a front. Then connect the uplift to cooling, condensation and precipitation.

A geographical example should illustrate the mechanism. The Western Ghats example includes a mountain barrier and contrasting rainfall on its two sides. The winter cyclone example includes travelling weather systems bringing rain to northwestern India and snow to the lower Himalayas.

Likewise, a description of a rainfall event needs more than the word “heavy”. Identify the cause of rising air before naming the rainfall type. The intensity of rainfall and the process producing uplift are different aspects of the event.

Note: Compare like with like: absolute and relative humidity are measurements; dew, fog and clouds are forms of condensed moisture; rain, snow and hail are precipitation forms; convectional, relief and frontal describe rainfall mechanisms.

Glossary

• Humidity — Water vapour present in air, measured in different ways to describe atmospheric moisture.
• Absolute humidity — The actual amount of water vapour per unit volume of air, expressed in grams per cubic metre.
• Relative humidity — Actual atmospheric moisture expressed as a percentage of full capacity at a given temperature.
• Saturated air — Air containing moisture to its full capacity at the temperature being considered.
• Dew point — The temperature at which a given sample of moist air becomes saturated.
• Evaporation — The process in which liquid water changes into water vapour through the supply of heat.
• Condensation — The transformation of water vapour into liquid water as moist air loses heat.
• Condensation nuclei — Small particles, including dust, smoke and salt, around which condensation occurs in free air.
• Dew — Liquid water droplets deposited on cooler solid surfaces when the dew point is above freezing.
• Frost — Minute ice crystals deposited on cold solid surfaces under conditions at or below freezing.
• Fog — A cloud based at or very near the ground, reducing horizontal visibility below one kilometre.
• Precipitation — Condensed moisture released from the atmosphere and falling to the Earth's surface in liquid or solid form.
• Convection — The upward movement of air that has become lighter as a result of heating.
• Rain shadow — The area on the leeward side of a mountain that receives less rainfall.
• Front — The boundary zone formed where two air masses with different characteristics meet.

Common errors and misconceptions

• Misconception: Absolute and relative humidity measure the same thing. Correct: Absolute humidity measures vapour per unit volume; relative humidity compares actual moisture with capacity at a given temperature.
• Misconception: Relative humidity changes only when moisture is added or removed. Correct: Temperature changes affect moisture capacity and can alter relative humidity without adding water vapour.
• Misconception: Dew point is another name for freezing point. Correct: Dew point is the temperature at which a particular air sample becomes saturated.
• Misconception: Dew and frost fall from clouds. Correct: They form as deposits on cold solid surfaces; rain, snow and hail fall as precipitation.
• Misconception: A cloud is simply a visible mass of water vapour. Correct: Clouds consist of minute water droplets or tiny ice crystals produced by condensation.
• Misconception: Mountains create rain shadows by stopping raindrops. Correct: Air loses moisture on ascent, then descends and warms on the leeward side, where rainfall is lower.
• Misconception: Any heavy rain is convectional. Correct: Identify the cause of uplift: heating, a mountain barrier, or interaction along a front.

Exam-style questions with model answers

Q1. Distinguish absolute humidity from relative humidity, stating how each is expressed. [2 marks]

1. Absolute humidity is the actual water vapour per unit volume of air, expressed in grams per cubic metre.
2. Relative humidity is actual moisture compared with full capacity at a given temperature, expressed as a percentage.

Q2. A sample of moist air cools without additional moisture entering it. Explain how it reaches saturation and how further cooling can produce droplets. Define dew point in your answer. [3 marks]

1. Cooling reduces the sample's capacity to retain water vapour, so its existing moisture occupies a greater proportion of that capacity without extra vapour being added.
2. The dew point is the temperature at which this sample becomes saturated, with its actual moisture content equal to its full capacity.
3. With sufficient further cooling, excess water vapour condenses into liquid droplets under suitable temperature conditions.

Q3. Describe two characteristics of dew and two characteristics of frost, covering their physical form and temperature conditions. [4 marks]

1. Dew is liquid water deposited as droplets on cooler solid objects, such as grass blades, stones and plant leaves.
2. Dew requires the dew point to be above freezing; clear skies, calm air, high relative humidity and cold, long nights favour its formation.
3. Frost is deposited as minute ice crystals on cold surfaces rather than as liquid droplets.
4. Frost forms under freezing conditions, with the dew point at or below freezing and air temperature at or below freezing.

Q4. Fog reduces horizontal visibility below 1 km; mist limits it to between 1 km and 2 km. Here km means kilometre. Use these criteria to distinguish their effects on visibility in two points. [2 marks]

1. Fog restricts horizontal visibility to less than one kilometre, so objects are obscured over a shorter distance.
2. Mist allows visibility between one and two kilometres, making its restriction on visibility less severe than fog's.

Q5. Explain convectional rainfall in five stages, from heating of air to the resulting rainfall. [5 marks]

1. Heating makes air lighter, initiating the upward movement that distinguishes convectional rainfall from rainfall caused by mountain barriers or fronts.
2. The heated air rises in convection currents, carrying its moisture upwards from the heated area into the atmosphere above.
3. As this rising air expands, it loses heat. Cooling is the link between the initial upward movement and subsequent condensation.
4. Water vapour condenses as the rising air cools sufficiently, and cumulus clouds develop from the condensed moisture in the air.
5. Heavy rainfall follows with thunder and lightning. This rain does not last long and is common in the hotter part of the day or in summer.

Q6. Moist Arabian Sea monsoon winds encounter the Western Ghats. The windward Western Coastal Plain receives very heavy rain, while land east of the range receives little rain. Explain this contrast in five points. [5 marks]

1. The Western Ghats act as a barrier to the incoming moist winds. The winds are forced to ascend the windward slopes facing their approach.
2. As the air rises over the barrier, it expands and its temperature falls, reducing its capacity to retain water vapour.
3. Moisture condenses in the cooling air and produces rainfall. This explains the very heavy rain on the windward Western Coastal Plain.
4. After crossing the range, the air descends along the leeward slopes to the east and becomes warmer as it descends.
5. The warmer descending air has an increased capacity to retain moisture. The eastern leeward area consequently receives little rainfall and forms a rain shadow.

Q7. Define a front and explain how uplift occurs at a warm front and at a cold front to produce precipitation. [3 marks]

1. A front is the boundary zone between two different air masses. Their interaction can raise air and bring cloud formation and precipitation.
2. At a warm front, advancing warm air glides over colder air. It cools as it rises, producing clouds and precipitation ahead of the front.
3. At a cold front, advancing cold air pushes warm air upwards. The rising warm air cools and cumulus clouds develop along the front.

Q8. Distinguish rain, snow and hail by the physical form in which each reaches the surface. [3 marks]

1. Rain reaches the Earth's surface as liquid water drops. It is the liquid form of precipitation rather than a deposit formed on a cold object.
2. Snow falls as fine flakes made from ice crystals. These crystals are hexagonal, meaning six-sided, and occur under temperatures below freezing.
3. Hail reaches the surface as rounded solid pieces of ice. Hailstones have concentric layers of ice arranged one around another, unlike snowflakes.

Key takeaways

• Humidity concerns water vapour in air; absolute humidity measures its amount per unit volume, while relative humidity compares moisture with capacity.
• Relative humidity depends on temperature because the capacity of air to retain water vapour changes with temperature.
• Saturated air contains its full moisture capacity at a given temperature; dew point is the temperature at which saturation occurs.
• Dew and frost are surface deposits, whereas fog, mist and clouds contain condensed moisture suspended in air.
• Rain, snow and hail are precipitation forms, distinguished by whether falling moisture is liquid, flaky ice or rounded layered ice.
• Convectional rainfall follows heating, rising air, expansion, cooling and condensation; it is very common in equatorial regions.
• Relief rainfall is greater on windward slopes; descending, warming air helps explain the lower rainfall of leeward rain shadows.
• Frontal rainfall involves rising warm air where contrasting air masses meet, as illustrated by warm and cold fronts.

Test yourself

What does absolute humidity measure?

It measures actual water vapour per unit volume of air, expressed in grams per cubic metre.

Why must a relative humidity statement refer to temperature?

The moisture capacity used in the comparison depends on temperature, so a temperature change can alter relative humidity.

What is the difference between saturation and dew point?

Saturation is the condition of full moisture capacity; dew point is the temperature at which a given sample becomes saturated.

Name the ideal conditions for dew formation.

A clear sky, calm air, high relative humidity, and cold, long nights favour dew. The dew point must be above freezing.

How can frost be distinguished from snowfall?

Frost consists of ice crystals deposited on cold surfaces. Snowfall brings flakes of ice crystals down as precipitation.

Which cloud type has a cotton-wool appearance and flat base?

Cumulus clouds resemble cotton wool, have flat bases and occur in scattered patches.

Why is the leeward side of a mountain drier?

Air descends and warms after crossing the mountain, increasing its moisture capacity and helping create a region of lower rainfall.

What lifts warm air at a cold front?

Advancing cold air pushes warm air upwards, where cooling leads to condensation, cloud formation and precipitation.