Notes-Class 8-Geography-Chapter-3-Humidity and Cloud-Maharashtra Board

Topics to be learn :

Weather conditions :

Evaporation :

The process of converting water into steam or water vapour is called evaporation.

The factors like dryness of air, temperature and speed of wind affects evaporation as follows :

As dryness of air increases rate of evaporation increases.
At more temperature rate of evaporation is more.
Rate of evaporation is more if wind speed is more
The rate of evaporation is high in the coastal regions. Therefore, humid climate is found in the coastal regions.
The equatorial region on the earth receives perpendicular sunrays. Therefore, the temperature of the air in the equatorial region is found to be high. Due to high temperature, the rate of evaporation is more in the equatorial region. Therefore, humid climate is found in the equatorial region.

Humidity in the air :

Moisture-holding capacity of the air :

The capacity of the air to hold water vapour is called moisture-holding capacity of the air.
Warmer air’s moisture-holding capacity is comparatively more. On the other hand, cold air’s moisture-holding capacity is comparatively low.
The condition of air at a certain temperature, in which the moisture holding capacity of air becomes equal to the proportion of moisture present in it, is called saturation of the air.
Saturation of the air leads to precipitation or snowfall.

Absolute humidity :

Absolute humidity :

The amount of water vapour in 1 cu.m. of air is called the absolute humidity.
Generally, humidity of the air is measured in grams per cubic meter.
Absolute humidity is higher in the coastal areas and the equatorial regions. On the other hand, absolute humidity is lower in the inland areas and the polar regions.
The distribution of land and water on the earth and the seasons affect absolute humidity.

Absolute humidity is measured with the help of the following formula :

Absolute humidity = \(\frac{\text{Mass of water vapour}}{\text{Volume of air}}\)

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Relative humidity :

Relative humidity :

The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature is called relative humidity.
The change in temperature of the air leads to change in moisture-holding capacity of the air. This in turn, leads to change in the relative humidity of the air.
The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature is called relative humidity.

Relative humidity is measured with the help of the following formula :

Relative humidity (%) = \(\frac{\text{Absolute humidity}}{\text{Vapour holding capacity}}\) x 100

Relative humidity is found to be comparatively high in the morning and at night. It is found to be comparatively low in afternoon.
Relative humidity is found to be more in the coastal areas and it is found to be less in desert areas.

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Condensation and Sublimation :

The process of changing water vapour in the air into water is called condensation. It is also called densification.
The process of changing water vapour in the air into snow is called sublimation.
Due to fall in temperature, the relative humidity of the air increases. This in turn leads to condensation or sublimation.
Condensation of vapour in the open air occurs around the dust or salt particles in the air.
Dew, frost and fog are the forms of condensation at ground level.
Clouds are a form of condensation at higher altitude.

Clouds and types of clouds :

A visible mass of fine particles of ice and water accumulated around the dust particles in the air at high altitude is called a cloud.
Clouds causes rainfall or snowfall on the earth.
The proportion of water vapour in the air near the surface of the earth is more.
Therefore, clouds of larger size are seen at a lower altitude. The proportion of water vapour in the air at higher altitude is less.
Therefore, clouds of smaller size are seen at a higher altitude.

According to international classification, clouds are classified into following types :

(i) High clouds :

(i) High clouds :

Features of high clouds :

Cirrus : Cirrus are wispy.
Cirro-Stratus : Cirro-Stratus look like bed sheet with wrinkles. Av halo is generally seen around these clouds. .
Cirro-Cumulus clouds : Cirro-Cumulus look like groups of small waves.

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(ii) Medium clouds :

(ii) Medium clouds :

Clouds at an altitude of 2000 m to 7000 m are considered as medium clouds.

Alto-Stratus and Alto- Cumulus are medium clouds.

Features of medium clouds :

Alto-Stratus : Alto-Stratus are comparatively thin. The sun is visible through these clouds as if seen through a milky glass.
Alto-Cumulus : Alto-Cumulus are in the form of layers. They have wave like structure. They are white in colour and have a grey shade.

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(iii) Low clouds :

(iii) Low clouds :

Features of low clouds :

Strato-Cumulus : Strato-Cumulus have layers. These clouds are mostly seen in round clusters. They are white to earthy in colour.
Stratus : Stratus are found in layers They have uniform base. They are ash coloured.
Nimbostratus : Nimbostratus have thick layers. They are greyish in colour. They cause continuous rainfall. They are also responsible for snowfall.
Cumulus : Cumulus are found between 500 m to 6000 m altitude. They are mostly formed due to the vertical ﬂow of the air. They have huge size and dome-like shape. They are grey in colour. They indicate pleasant atmosphere. With an increase in the vertical expanse, these clouds turn into cumulonimbus clouds and bring rain.
Cumulonimbus : Cumulonimbus appear like huge mountain. They are dense and dark in colour. They have anvil-like shape at the top. They are the indicators of thunder, lightning and storm. They bring rain with storm. Sometimes they also bring hailstones. The raindrops of these clouds are found to be larger in size.