Humidity

ICSE Class 9 · Geography · Chapter 15

What this chapter covers

Meaning of humidity
Absolute humidity and relative humidity
Condensation
Clouds
Dew and frost
Fog and mist
Precipitation
Types of rainfall
Relief or orographic rainfall
Convectional rainfall
Cyclonic or frontal rainfall

Key definitions

Humidity is the amount of water vapour present in the air at a given time and place.

Evaporation is the process by which water changes from a liquid into water vapour at a temperature below its boiling point.

Saturated air is air that holds the greatest amount of water vapour it can hold at its present temperature.

Hygrometer is the general name for an instrument that measures the humidity of the air.

Absolute humidity is the actual mass of water vapour present in a given volume of air, usually expressed in grams per cubic metre (g/m³).

Relative humidity is the ratio of the amount of water vapour actually present in the air to the greatest amount the air could hold at the same temperature, expressed as a percentage.

Dew point is the temperature at which a given mass of air becomes saturated when it is cooled without any change in the amount of water vapour it contains.

Condensation is the process by which water vapour in the air changes into liquid water or, below freezing point, into ice, when the air is cooled to its dew point.

Condensation nuclei are the minute particles of dust, smoke, salt and other matter in the air around which water vapour condenses to form droplets.

A cloud is a visible mass of very small water droplets, ice crystals, or both, suspended in the atmosphere well above the ground, formed by the condensation of water vapour around condensation nuclei.

Dew is the moisture that condenses in the form of tiny water droplets on cool surfaces near the ground, such as grass, leaves and the roofs of cars, when the air in contact with them is cooled below its dew point and the dew point is above freezing point.

Frost is the thin, white, feathery layer of ice crystals that forms on the ground and on exposed objects when the air in contact with them is cooled to a dew point below 0 °C, so that water vapour changes directly into ice.

Key points

Humidity is the amount of water vapour in the air; it enters the air mainly by evaporation from the oceans and by transpiration from plants.
Warm air can hold far more water vapour than cold air; air holding as much as it can is saturated.
Absolute humidity is the actual mass of vapour in a given volume of air, in g/m³.
Relative humidity is the vapour present as a percentage of what the air could hold at that temperature; it rises when air cools and falls when it warms.
Dew point is the temperature at which air becomes saturated on cooling; below it, condensation begins.
Condensation needs cooling to the dew point and surfaces or condensation nuclei, and it releases latent heat.
Clouds form when rising air cools to its dew point; the basic kinds are cirrus, cumulus, stratus and nimbus.
Dew is water and frost is ice formed on cold surfaces on clear, calm nights; fog and mist form in the air near the ground, with fog reducing visibility below 1 km.
Precipitation is water falling from clouds as rain, snow or hail; hail forms only in cumulonimbus clouds.
Relief rainfall falls on the windward slopes of mountains, leaving a rain shadow on the leeward side, as with Mumbai and Pune.
Convectional rainfall comes from strong heating of the ground and gives heavy afternoon showers, typical of equatorial regions.
Cyclonic or frontal rainfall comes from air rising in tropical cyclones or along the fronts of temperate cyclones, as in north-western Europe and with India's western disturbances.

Questions answered

Define humidity. Name the main source of water vapour in the atmosphere.

Humidity is the amount of water vapour present in the air at a given time and place. The oceans are the main source, through evaporation, with smaller amounts from lakes, rivers, wet soil and transpiration by plants.

Why can warm air hold more water vapour than cold air, and what happens to unsaturated air when it is cooled enough?

The capacity of air to hold water vapour depends on its temperature and roughly doubles for every 10 °C rise. When unsaturated air is cooled, its capacity falls until it equals the vapour present; the air is then saturated at its dew point, and further cooling causes condensation.

Distinguish between absolute humidity and relative humidity.

Absolute humidity is the actual mass of water vapour in a given volume of air, in grams per cubic metre. Relative humidity is the amount present expressed as a percentage of the greatest amount the air could hold at that temperature. Absolute humidity does not change when air is simply warmed or cooled; relative humidity falls on warming and rises on cooling.

Air at 20 °C contains 12 g of water vapour per cubic metre. Air at 20 °C can hold at most 17 g per cubic metre. Calculate the relative humidity.

RH = (12)/(17) × 100% ≈ 71%.

Give a reason why relative humidity is usually highest just before sunrise.

The air is coolest just before sunrise. Its capacity to hold vapour is lowest then, while the actual amount of vapour has changed little, so the ratio of the two, the relative humidity, is highest.

What is meant by dew point? Why does a cold glass of water "sweat" on a humid day?

Dew point is the temperature at which air becomes saturated when cooled without change in its water vapour. The air touching the cold glass is cooled below its dew point, and the excess vapour condenses on the glass as droplets.

State the two conditions necessary for condensation in the atmosphere.

The air must be cooled to its dew point, and there must be surfaces or condensation nuclei, such as particles of dust, smoke or salt, on which the vapour can condense.

Give two differences between dew and frost.

Dew is made of liquid water droplets, while frost is made of ice crystals. Dew forms when the dew point is above 0 °C, frost when it is below 0 °C.

Past board exam questions

ICSE 2019 sample paper · 2 marks

Draw a well labelled diagram of orographic rainfall.

Show answer

Diagram: a mountain range with the sea on one side. Moisture-laden winds blow from the sea and are forced to rise along the windward slope; as they rise they cool, condense into clouds and give heavy rain on the windward slope. Labels: sea, moist winds, windward slope, rising air cools, condensation/clouds, heavy rainfall, mountain, descending dry warm winds, leeward slope, rain-shadow area (little rain).

ICSE 2019 sample paper · 2 marks

Distinguish between absolute and relative humidity.

Show answer

Absolute humidity is the actual amount of water vapour present in a given volume of air, expressed in grams per cubic metre. Relative humidity is the ratio of the actual amount of water vapour present in the air to the maximum amount the air can hold at that temperature, expressed as a percentage.

ICSE 2019 sample paper · 2 marks

(i) Name the type of rainfall experienced in the Equatorial region. (ii) State two characteristic features of the rainfall mentioned by you.

Show answer

(i) Convectional rainfall. (ii) Any two: it occurs almost daily in the afternoon (about 4 p.m.); it is heavy and short-lived, accompanied by thunder and lightning; it falls throughout the year; the sky clears by evening.

ICSE 2019 sample paper · 3 marks

(i) How is dew formed? (ii) How is frost different from dew? (iii) Why is fog commonly formed in the winter season?

Show answer

(i) On clear, calm nights the ground loses heat quickly by radiation; the air in contact with the cold surfaces is cooled below its dew point, and the water vapour condenses as tiny water droplets on grass, leaves, etc. - this is dew. (ii) Frost forms when the dew point is below freezing point (0°C): the water vapour turns directly into tiny ice crystals instead of water droplets; dew is water droplets formed above 0°C. (iii) In winter the nights are long and the air near the ground cools rapidly below its dew point; the water vapour condenses on dust and smoke particles near the ground, forming fog.

ICSE 2019 sample paper · 3 marks

Give a reason for each of the following: (i) A rain shadow area is generally dry. (ii) Frontal rain is common in mid latitudes. (iii) Convectional rainfall is also called 4 o’clock rainfall.

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(i) The winds shed most of their moisture on the windward slope; when they cross the mountain and descend the leeward slope they get warmer and drier, so they can hold more moisture and give little rain - the rain shadow area remains dry. (ii) In the mid latitudes the warm, moist air masses from the tropics meet the cold polar air masses (fronts are formed); the warm air rises over the cold air, cools and condenses, causing frontal (cyclonic) rain. (iii) In the equatorial region the land is heated through the morning; by afternoon the warm moist air rises, cools and condenses, and rain falls in the late afternoon, around 4 p.m., almost every day.