Which one of the following is the most important constituent of the atmosphere for human beings?
- Water vapour
- Dust particle
Which one of the following process is responsible for transforming liquid into vapour?
The air that contains moisture to its full capacity :
- Relative humidity
- Absolute humidity
- Specific humidity
- Saturated air
(c). Saturated air
Which one of the following is the highest cloud in the sky?
Name the three types of precipitation.
Why does the amount of water vapour decreases rapidlly with altitude?
There is absence of sources of water at greater heights. The temperature also decreases with heights. So the evaporation is also low.
How are clouds formed? Classify them.
Clouds are formed by condensation of water
Clouds are classified into three groups according to their height :
(a) High Clouds. (Above 6 km.) Cirrus clouds are high clouds. It has a feathery appearance.
(b) Medium Clouds. (2 km. to 6 km.) Cumulus clouds are medium clouds. These are wooly and whitish clouds.
(c) Low Clouds. (Below 2 km). Nimbus clouds are low clouds. These dark grey clouds give rain.
Discuss the salient features of the world distribution of precipitation.
Distribution of rainfall. The distribution of
rainfall is shown according to latitudes. This is known
as zonal distribution.
(1) The average annual precipitation for the whole earth is about 975 m.m. On the land it is about 660 m.m., but on the ocean it is about 1100 m.m.
(2) The maximum annual rainfall is about 1900 m.m. and it occurs in the equatorial zone between 10° N — 10° S latitudes.
(3) Between 20° — 30° latitudes, the average annual rainfall is 800 m.m.
(4) Between 45° — 55° latitudes, the average annual rainfall is minimum about 1000 m.m.
(5) Beyond 75° North and South latitude, the average rainfall is minimum about 250 m.m. only.
(6) The continental interiors get low rainfall as compared to coastal areas.
(7) Trade winds give rainfall in eastern parts of the tropics.
(8) The westerlies give rainfall in the west in the temperate latitudes.
What are forms of condensation? Describe the process of dew and frost formation.
Condensation in the process by which water vapour of the atmosphere is changed to water (from gaseous to liquid state). With the fall of temperature or cooling of air, its capacity to hold water vapour is lowered. The air is saturated when dew-point is reached. Condensation starts when the temperature falls below dew-point. Then the excess of water is released or condensed into a liquid form.
Condensation takes place :
(i) When the air rises, expands and cools due to adiabatic temperature changes.
(ii) When moist laden wind strikes against a mountain and is forced to ascend.
(iii) When two warm and cold air masses meet.
Forms of Condensation. Condensation takes place in different forms such as :
(i) frost and snow.
(ii) dew, fog, mist and smog.
(iii) clouds and haze.
(i) Clouds : When the air usually at a considerable height above the earth’s surface is cooled, condensation takes place in it. As a result, a mass of small water drops is formed. This is called cloud. Clouds are formed when air carrying moisture is lifted upwards and cooled as it rises. The condensed water droplets cling to the dust particles in the air and thus reduce the visibility. clouds assume an almost infinite variety of forms. They have been classified according to their shape and altitude.
(ii) Fog : When the air near the earth’s surface is cooled throughout, condensation takes place in it. Small drops of water are formed and they cling to the dust particles which float in the air and reduce the visibility. This is called mist and if it is very thick then it is known as fog. The visibility in the mist is more than one kilometre, while in fog it is less than one kilometre. Fog when formed at a considerable height above the earth’s surface is termed as cloud. Fog formed on smoke is called smog.
(iii) Rain : When moisture-laden air rises up, condensation takes place and clouds are formed. If the clouds are further cooled, there is more condesation and drops of water that are formed are big in size. These big drops are too heavy to remain hanging in the air and fall down towards the earth’s surface. The falling of these drops of water from the clouds is known as rain.
(iv) Snowfall : If condnsation takes place at a temperature below freezing point (0° Celsius), the water-vapour condenses into small crystals of ice instead of drops of water. These small crystals then unite to form flakes of ice called snow. The coming down of snowflakes towards the earth’s surface is known as snowfall.
(v) Hail : Sometimes rain-drops on their way to the earth’s surface are carried by strong air currents upward into colder regions. This causes the rain-drops to freeze. After freezing, they collect more water which too freezes. Thus they grow in size and become heavy. They become so big and heavy that the air can no longer hold them. Therefore, they fall back to the earth’s surface as hailstones.
(vi) Dew : When the temperature of the air is higher