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Chapter 15

Humidity

Class 9 - Total Geography Morning Star


Short Answer Questions

Question 1

What determines the amount of water vapour in the air?

Answer

The amount of water vapour in the air is determined by temperature. The hotter the air, the more the water vapour present in it. When the atmospheric temperature is low, water vapour is less.

Question 2

How is relative humidity determined?

Answer

Relative humidity is expressed as 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.

Question 3

Name four chief forms of condensation.

Answer

Four chief forms of condensation are-

  1. Clouds
  2. Dew
  3. Frost
  4. Fog and mist

Question 4

Mention the different forms of precipitation.

Answer

The different forms of precipitation are-

  1. Rain
  2. Snow
  3. Hail

Question 5

What do you understand by 'condensation nuclei'?

Answer

Condensation occurs only around tiny solid particles like dust or carbon dioxide in smoke. These impurities in air are known as condensation nuclei.

Question 6

State the three types of rainfall.

Answer

The three types of rainfall are-

  1. Convectional rainfall
  2. Orographic rainfall
  3. Cyclonic rainfall

Question 7

What is cyclonic rainfall?

Answer

Cyclonic rainfall occurs due to cyclones and depressions irrespective of relief or convection. It is caused by convergence of two different air masses with different temperatures and other physical properties. When warm and cold air masses confront each other, the warmer air generally climbs above the colder air. The boundary zones of these air masses are called the fronts.

The rising air is cooled while undergoing a frontal lift. This causes precipitation. Such precipitation is called cyclonic rainfall.

Explain the following terms

Question 1

Precipitation

Answer

The process by which the droplets fall to ground in liquid, solid or frozen form is known as precipitation.

Question 2

Evaporation

Answer

The process by which water vapour enters the atmosphere on heating is known as evaporation.

Question 3

Condensation

Answer

The process by which water vapour forms water droplets on cooling is known as condensation.

Question 4

Humidity

Answer

The amount of water vapour present in air is known as humidity.

Structured Questions

Question 1(a)

What is 'humidity'? How is humidity measured?

Answer

The amount of water vapour present in air is known as humidity.

Humidity of the atmosphere is measured with the help of a hygrometer.

Question 1(b)

Distinguish between Absolute Humidity and Relative Humidity.

Answer

Absolute HumidityRelative Humidity
It is the measure of the actual amount of water vapour in the air, regardless of temperature.It measures water vapour in air but relative to maximum vapour that the air can hold at that temperature.
It is expressed as grams of moisture per cubic meter of air.It is expressed as 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.
The higher the amount of water vapour, the higher is the absolute humidity.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 temperature.
In weather calculations, absolute humidity is generally not taken into account.It is the essential characteristic of weather forecasts, which indicates the likelihood of precipitation, dew or fog.

Question 1(c)

Give a geographical reason for each of the following:

(i) The total volume of water in the oceans and seas remains constant.

(ii) In Equatorial regions the sky often remains overcast with clouds.

(iii) Human comfort depends on humidity.

Answer

(i) The total volume of water in the oceans and seas remains constant because all the water that evaporates from the earth's water bodies is eventually returned to it directly by the process of condensation and precipitation, and indirectly by stream and overflow from land surfaces.

(ii) In Equatorial regions the sky often remains overcast with clouds due to excessive heat. The high temperature leads to the rapid heating of air. Such heated air rises in convectional currents, leading to development of clouds at about 10 km height.

(iii) Human comfort depends on humidity because 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.

Question 1(d)

Draw a well labelled diagram showing orographic rainfall.

Answer

Below labelled diagram shows orographic rainfall:

Draw a well labelled diagram showing orographic rainfall. Humidity, Total Geography Solutions ICSE Class 9.

Question 2(a)

What is condensation? Explain briefly the process of condensation.

Answer

The process by which water vapour forms water droplets on cooling is known as condensation.

During condensation, the latent heat locked in water vapour is released back into the atmosphere. Condensation can take place only when water vapour is added to saturated air or when the temperature falls below the temperature at which air becomes saturated.

Question 2(b)

Explain the atmospheric conditions that favour condensation.

Answer

The atmospheric conditions that favour condensation are-

  1. There should be a high amount of water vapour present in atmosphere.
  2. Minuscule particles of dust, salt and even smoke act as condensation nuclei.
  3. The temperature of air must be below dew point temperature so as to encourage condensation.

Question 2(c)

Give a geographical reason for each of the following:

(i) Condensation is the reverse process of evaporation.

(ii) Coasts receive more rainfall than the interior of the continents.

(iii) The windward sides of mountains receive more rainfall than the leeward sides.

Answer

(i) Condensation is the reverse process of evaporation as evaporation is the process by which water vapour enters the atmosphere on heating while condensation is the process by which water vapour is converted back to water droplets on cooling.

(ii) Coasts receive more rainfall than the interior of the continents because the humidity in the clouds is high when they are at the coast. When they reach the interior of the continents, they have already shed water in the form of rain, and so, they do not have much humidity left to cause rainfall.

(iii) The windward sides of mountains receive more rainfall than the leeward sides because the sudden ascent of warm moist air on the windward sides causes cooling of air, leading to condensation and precipitation.
On the contrary, on descending the leeward slope, a decrease in altitude increases both the pressure and the temperature, leading the air to get compressed and warm. Consequently, the relative humidity drops and there is evaporation and little or no precipitation in the rain shadow area.

Question 2(d)

Draw a well labelled diagram showing convectional rainfall.

Answer

Below labelled diagram shows convectional rainfall:

Draw a well labelled diagram showing convectional rainfall. Humidity, Total Geography Solutions ICSE Class 9.

Question 3(a)

How is dew formed? What is the difference between fog and mist?

Answer

When water vapour condenses on the surface, it forms tiny droplets of water called dew.

The difference between fog and mist is as follows-

FogMist
Droplets of water suspended in the atmosphere close to the surface of the earth are termed as fog.It refers to the formation of clouds due to change in temperature or change in humidity.
The density of fog is quite high.The density of mist is quite low.
The visibility in fog is lower when compared to mist.The visibility in mist is higher when compared to fog.
The duration of fog is comparatively longer than mist.The duration of mists is comparatively shorter than fog.

Question 3(b)

Describe briefly the orographic and convectional rainfall.

Answer

Orographic rainfall — It occurs from the cooling of warm moist air which ascends above the mountain barrier lying in the direction of the prevailing winds. The presence of mountains causes humid air to rise. The sudden ascent causes cooling of air, leading to condensation and precipitation.

Convectional rainfall — This type of rainfall is common in equatorial regions. The high temperature leads to the rapid heating of air. Such heated air rises in convectional currents, leading to development of clouds at about 10 km height. Ascending currents of hot and humid air causes condensation of the clouds, resulting in heavy rainfall.

Question 3(c)

Give a geographical 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 called 4 o'clock rainfall.

Answer

(i) The sudden ascent of warm moist air on the windward sides causes cooling of air, leading to condensation and precipitation.
On the contrary, on descending the leeward slope, a decrease in altitude increases both the pressure and the temperature, leading the air to get compressed and warm. Consequently, the relative humidity drops and there is evaporation and little or no precipitation in the rain shadow area. Hence, a rain shadow area is generally dry.

(ii) Frontal rain is common in mid-latitudes due to the meeting of the sub-tropical and polar air masses. It is caused by convergence of two different air masses with different temperatures and other physical properties. When warm and cold air masses confront each other, the warmer air generally climbs above the colder air. The rising air is cooled while undergoing a frontal lift. This causes precipitation. Such precipitation is called cyclonic rainfall.

(iii) Convectional rainfall is called 4 o'clock rainfall because this type of rainfall occurs in the afternoon at about 4 o'clock.

Question 3(d)

Define precipitation. Explain briefly the three forms of precipitation.

Answer

The process by which products of condensation, water droplets, ice crystals, sleet, etc., fall to ground is known as precipitation.

The three forms of precipitation are-

  1. Rain — It is the most common form of precipitation. Raindrops of smaller size and less intensity are known as drizzle.

  2. Snow — Water droplets which rise higher and freeze on account of drop in temperature is called snow. Snowfall usually occurs in winters in cold climates or on high mountains.

  3. Hail — Sometimes, vertical air currents may push water droplets or ice particles higher. They form into solid ice and fall as hail. Hailstones cause great damage to crops.

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