3. Humidity and Clouds
Complete solutions to the textbook exercise. Maharashtra State Board (Balbharati), Grade Eight Geography.
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Exercise
| A (Cloud Type) | B (Altitude) | C (Characteristics) |
|---|---|---|
| (A) Cirrus | (ii) higher altitude | (d) snow flake clouds |
| (B) Cumulonimbus | (i) Vertical extent in the sky | (a) Roaring clouds |
| (C) Nimbostratus | (iv) Low altitude | (c) continuous rainfall |
| (D) Alto-cumulus | (iii) Medium altitude | (b) Floating clouds |
(Cumulonimbus, Relative humidity, Absolute humidity, Condensation, Vapour-holding capacity)
(a) The Vapour-holding capacity of air is dependent on the temperature of air.
(b) The amount of vapour in 1 cu.m. of air shows the Absolute humidity.
(c) As Relative humidity is less in desert areas, the air is dry there.
(d) Cumulonimbus type of clouds are indicators of storm.
(e) In a free environment, the Condensation of the vapour present in the atmosphere takes place around dust particles.
(a) Humidity and clouds
| Humidity | Clouds |
|---|---|
| The proportion of water vapour present in the air is called humidity. It is an invisible gas. | Clouds are an accumulation of condensed water droplets or fine snow particles floating in the air at a greater height. |
| Humidity is found at all levels in the atmosphere near the earth's surface and dictates weather dampness. | Clouds are a visible form of condensation that occur specifically at higher elevations. |
(b) Relative humidity and Absolute humidity
| Relative Humidity | Absolute Humidity |
|---|---|
| The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature and volume. | The actual physical amount of water vapour present in 1 cubic meter (cu.m.) of air. |
| It is always expressed in percentage (%). | It is measured in grams per cubic meter (gm/cu.m.). |
(c) Cumulus clouds and cumulonimbus clouds
| Cumulus clouds | Cumulonimbus clouds |
|---|---|
| These clouds are huge, dome-shaped, white and cotton-like, forming between 500 m to 6000 m. | These are dense, dark clouds that look like huge mountains with an anvil-like shape at the top. |
| They are an indicator of pleasant weather. | They are indicators of thunderstorms, bringing heavy rain, lightning, and sometimes hail. |
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(a) Why is the air in a region dry?
The dryness or dampness of the air depends on the proportion of water vapour present in it. In a region where there is very little moisture or water vapour in the air, the relative humidity is extremely low. Consequently, the air in such a region (like deserts or cold mountainous regions) is considered dry.
(b) How is humidity measured?
Humidity of the air is generally measured in grams per cubic meter (gm/cu.m.). Absolute humidity is determined by finding out the exact mass of water vapour in a single cubic meter volume of air. Relative humidity is calculated mathematically by taking the ratio of absolute humidity to the air's vapour-holding capacity and multiplying it by 100 to get a percentage.
(c) What are the prerequisites for condensation?
Condensation requires two main prerequisites: low temperature and high relative humidity. As the air's temperature drops, its vapour-holding capacity reduces. When the relative humidity reaches 100% (the dewpoint), the air becomes saturated and the water vapour starts condensing into water droplets around fine dust or salt particles in the environment.
(d) What is a cloud? Write its types.
A cloud is an accumulation of very fine condensed water droplets or snow particles that have gathered around microscopic dust particles and float in the atmosphere at a greater height. Based on their altitude from the base, clouds are divided into three main types:
- High Clouds (7000m to 14000m): Cirrus, Cirro-cumulus, Cirro-stratus.
- Medium Clouds (2000m to 7000m): Alto-stratus, Alto-cumulus.
- Low Clouds (Less than 2000m): Strato-cumulus, Stratus, Nimbostratus, Cumulus, and Cumulonimbus (which can have a variable vertical extent).
(e) Which type of clouds give rain?
Nimbo-stratus clouds cause continuous rainfall or snowfall. Cumulonimbus clouds bring heavy rain with thunderstorms and occasionally hail.
(f) On what does the percentage of relative humidity depend?
The percentage of relative humidity depends on two primary factors: the absolute humidity (the actual amount of water vapour currently present in the air) and the vapour-holding capacity of the air, which in turn is strictly dependent on the air's temperature.
(a) Clouds float in the sky.
When condensation occurs at higher elevations, the resulting condensed water droplets or fine snow particles are extremely small and almost weightless. Because of their minimal weight and the constant upward vertical flow of air (winds), they are continuously pushed upward, allowing them to float in the sky rather than immediately falling to the ground.
(b) The proportion of relative humidity changes according to altitude.
The vapour-holding capacity of air is directly dependent on its temperature. As we move to higher altitudes away from the sea level, the temperature of the air decreases. Because cold air can hold less moisture than warm air, its vapour-holding capacity drops. Therefore, for the same amount of actual water vapour present, the proportion of relative humidity increases as the altitude increases and temperature falls.
(c) Air becomes saturated.
Air can hold moisture only in specific quantities at specific temperatures. As air cools down, its moisture-holding capacity reduces. At a certain point, the lowered moisture-holding capacity of the air becomes exactly equal to the actual proportion of moisture already present in it. When these two amounts match (100% relative humidity), the air cannot hold any more vapour and is said to be saturated.
(d) Cumulus clouds change into cumulonimbus clouds.
Cumulus clouds are formed extensively due to the vertical flow of air. Under certain weather conditions, this strong vertical flow pushes the air higher and higher. This adds to the continuous formation and accumulation of the clouds, causing their vertical expanse to increase drastically. As they grow immensely into huge, mountain-like dense structures, the cumulus clouds transform into dark cumulonimbus storm clouds.
(a) When the temperature of the air is 30° C, its vapour-holding capacity is 30.37 gms/cu.m. If absolute humidity is 18 gms/cu.m. then what would be the relative humidity?
Relative Humidity (%) = (Absolute humidity ÷ Vapour holding capacity) × 100
Relative Humidity = (18 ÷ 30.37) × 100
Relative Humidity = 0.59269 × 100 = 59.27%
Answer: The relative humidity of the air would be 59.27%.
(b) What would be the absolute humidity of air if 1 cu.m. air contains 4.08 gms of vapour at 0°C temperature?
Absolute humidity is defined as the amount of water vapour physically present in exactly 1 cubic meter of air.
Since 1 cu.m. of this air contains exactly 4.08 grams of water vapour, the absolute humidity is simply that value per cubic meter.
Answer: The absolute humidity of the air would be 4.08 gms/cu.m.
Note: This is an ongoing student activity meant to be carried out over a month. Generally, you will observe that when the difference between maximum and minimum temperature is small (meaning it stays warm/mild), it is often heavily overcast and humidity is high (very damp). When the difference is large, the air is usually clearer and less humid.