WBBSE Notes For Class 8 Geography Chapter 5 Cloud And Rain

Chapter 5 Cloud And Rain

What is Cloud:

Cloud Meaning 

Clouds, which are condensed water vapour, are one of the most visible marks (elements) of the weather. They form in distinctive patterns and often give a quick clue as to what weather might happen in the near future.

Meteorologically clouds are very significant because all forms of precipitation occur from them. It may be mentioned that not all clouds yield precipitation but no precipitation is possible without clouds.

Read and Learn all WBBSE Notes For 8 Class Middle School Geography

Chapter 5 Cloud And Rain: The Cloud Family

Clouds can be classified in two ways—
(1)According to height and
(2)According to shape and formation.

(1) Classification according to height: By height, clouds can be divided into three types:
1. High clouds
2. Medium clouds
3. Low clouds

(2) Classification according to shape and formation:

Clouds can be classified into four main types which are given Latin names. They are

1. Cirrus—Looks like a feather or fine scales,
2. Stratus—they have a layered structure
3. Cumulus—they look like a mound of fully cotton and
4. Nimbus—they are rain clouds, giving plenty of rainfall.

 

 

High clouds: The lower limit of this cloud is 6000 to 12000 metres or above 20000 feet. There are mainly three types of clouds in the High cloud family. These are as follows:

1. Circus: 3 The high altitude detached clouds having feather-like or fibrous appearance are called ‘cirrus clouds’ They are composed of tiny ice crystals and are transparent and white in colour but have brilliant colour at sunset and sunrise. When the weather is clear these clouds are seen and-no rain occurs. So this cloud generally indicates fair weather.

 

2. Cirrocumulus: Cirrocumulus clouds are white-coloured clouds having cirri form layers or wave-like forms. They generally appear as the scaled body of a mackerel fish. So, the cirrocumulus cloud ‘Mackerel Sky’.This cloud usually indicates fair weather because from these clouds rain occurs.

3. Cirrostratus: Cirrostratus clouds are generally white in colour and spread in the sky like milky thin sheets. The sun and moon seen through this cloud have a halo. This cloud forms a rainbow. It signals storms.

Medium clouds: The lower limit of this cloud in between 2100 mts to 6000 metres (6500 to 20000 feet). There are mainly two types of clouds in the Medium Cloud falily. These are:
(1)Alto-cumulus and
(2)Alto-stratus

(1) Alto-cumulus: Alto-cumulus clouds are characterized by wavy layers of globular form. Sometimes these clouds are called as ‘sheep clouds’ or ‘wool- pack clouds because they are looking like scattered mounds of wool.

They appear white or grey in colour. In between the clouds blue sky is seen These clouds generally herald clear weather but when they accumulate in large amounts rain may occur.

(2) Alto-stratus: Alto-stratus clouds are thin sheets of grey or blue colour having fibrous or uniform appearance. The sun looks very hazy through this cloud. Rain occurs for a long duration and over large areas from this cloud.

Low clouds: The lower limit of this cloud is less than 2100 m. or 6500 feet. Most clouds are stratus or layered. There are mainly three types of clouds in the Low clouds family. These are
(1) Strato cumulus,
(2) Stratusand
(3) Nimbostratus.

(1) Strato-cumulus: Strato-cumulus clouds are found in rounded patches between the height of 2500 m to 3000 m. They are of grey or whitish clour. They are composed of globular masses or rolls which are generally arranged in lines, waves or groups. So, they are also called ‘Bumpy clouds’.

They are generally associated with fair or clear weather but occasional rain on snow is not ruled out. During winter in temperate regions, often the sky is covered by this cloud giving heavy rainfall.

Humidity And Clouds

(2) Stratus: Stratus clouds are dense, low-lying fog-like clouds of dark grey colour but are seldom close to the ground surface. They are composed of several uniform layers and cover the entire sky. It does not extend high from the surface so creates invisibility at low levels. It is a great problem for mountaineers and air pilots. Often this cloud occurs drizzle.’

(3) Nimbostratus: Nimbostratus clouds are low clouds of dark grey to black colour, very close to the ground surface. They are so compact and thick (hundreds of metres) that there is complete darkness and there is copious precipitation.

‘Nimbo’ is derived from the Latin word ‘nimbo’ meaning thereby ‘rainstorm’, it indicates bad weather because it causes heavy rainfall. During cyclones it covers the whole sky and gives rainfall.

Vertical clouds or Clouds of great vertical extent: The height of the clouds extend from 1500 m to 9000 m, resembling a tree from below to above. There are mainly two types of vertical clouds. These are—

(1)Cumulus and
(2)Cumulonimbus.

(1) Cumulus: Cumulus clouds are very dense, widespread, dome-shaped and have flat bases. They start from low levels but reach high above. The bottom of the cloud is flat but its top resembles a cauliflower.

The lower part of the cloud is dark grey or black but the higher part is white and the sun looks bright through it. It usually indicates clear and fair weather.

(2) Cumulonimbus: Cumulonimbus clouds are thunderstorm clouds. It is a huge dense cloud. The bottom and middle of the cloud is black in colour, the sides are white or grey.

It has a great vertical extent of about 4000 m upward from the layer of air adjacent to the earth’s surface. During ‘Kalbaisakhi’ these clouds are seen in the North-west corner of the sky.

These clouds cause heavy rain, storms or hailstorms accompanied by lightning, thunder and gusty winds. So it-is also called ‘Thunder Cloud’.

The origin of clouds: Cloud is the most common form of ‘Condensation’. When moist air rises, it cools down. The water vapour present in the air also cools and changed into minute droplets of water or crystals.

These droplets of water or ice crystals stick to dust and salt particles, floating in the atmosphere. These small water droplets or ice crystals form a floating mass called cloud.

Thus, a cloud is nothing but a collection of countless droplets of water or the tiniest crystals of ice suspended in the atmosphere. These droplets or crystals are so small that they are blown about and carried with the slightest movement of air.

Evaporation: The process where by a liquid (water) changes into a gas (water vapour) is called evaporation. The water of the water bodies is changed into water vapour by solar energy. It mixes with other gases of the atmosphere. At 10°C. temperature, on cubic metre air can hold 11-4 gm water vapour. Warm air can absorb more water vapour than cold air.

Saturated air: When a specific quantity of airmass, at a particular temperature holds the maximum amount of water vapour as it can possibly hold is called ‘saturated air’. The amount of water vapour required to saturate the air depends on temperature and pressure.

Dew point: The temperature of air at which it becomes saturated with water vapour starts to condense to form water droplets is called ‘dew point’. The upper level of saturation is known as dew point.

Condensation: The physical process of transformation from the vapour to the liquid state is called ‘condensation’. In the atmosphere, condensation occurs either when the temperature drops sufficiently for moisture to be cooled to its dew point, or when there is enough water vapour within an airmass for it to reach saturation point.

Distinguish between Evaporation and Condensation:

Evaporation	Condensation
1. Evaporation is a process by which water changes into a vapour.	1. Condensation is a process by which water vapour present in the air changes into minute droplets of water.
2. It takes place more when the weather is hot, dry and windy.	2.  It takes place when the temperature of the saturated air goes down below the dew point.
3. It also takes place more quickly in the lower latitudes.	3.  It also takes place more quickly at higher latitudes than at lower latitudes.
4. As a result of evaporation the amount of water on the surface of the earth goes on decreasing and decreasing	4. As a result of our censer or the amount of water goes on increasing arc increasing.

 

Some forms of condensation:
(1) Fog: Droplets of water suspended in the lower layers of the atmosphere resulting from the condensation of water vapour around nuclei of floating dust or smoke particles is called ‘fog’. Its visibility is less than 1 km.

(2) Mist: A reduction of visibility within the lower atmosphere to 1—2 km caused by condensation producing v/ater droplets within the lower layers of the atmosphere is called ‘mist’. It is intermediate between fog and haze.

 

(3) Haze: An obscurity of the lower atmosphere that limits visibility to under 2 km, but over 1 km, is called ‘haze’. It is normally formed by water particles that have condensed around nuclei to the atmosphere but may also be a result of particles of smoke, dust or salt in the air.

 

 

(4) Smog: A form of fog that occurs in areas where the air contains a large amount of smoke is called ‘smog’ (Smoke + fog).

Humidity: Humidity means the amount of water vapour present in the air. Water vapour is always present in the air. About 2% of the atmosphere consists of water vapour. Most of the water vapour of the atmosphere comes from the oceans, lakes, and rivers through evaporation.

(1) Absolute Humidity: Total amount of water vapour present in air at a particular temperature is absolute humidity. It is defined as the weight of water vapour per unit volume of air. It is expressed as grams per cubic metre of air.

Distribution:
1)Absolute humidity is maximum in the equatorial region,
2)It is more in summer than in winter, similarly, it is higher during the day than at night.

(2) Relative Humidity: Relative humidity is expressed as a percentage. It is the ratio between the actual amount of the water vapour present in the air at a temperature and the maximum amount of water vapour that the same volume of air can hold at a given temperature.

⇒ \(\text { R.H. }=\frac{\text { Actual amount of water vapour present in air }}{\begin{array}{l}
\text { The maximum amount of water.vapour present in the } \\
\text { same air can hold at given temperature. }
\end{array}} \times 100\)

In other words, it is a ratio, expressed in percentage, between the absolute humidity and the maximum vapour capacity of the air.

⇒ \(\text { R.H. }=\frac{\text { Absolute humidity }}{\text { Vapour capacity }} \times 100\)

Example: An air at 20°C temperature has 8 grams of water vapour per cubic metre actually present. But the air at 20°C temperature can hold 16 (gms) of water vapour as per cubic metre.

∴ \({ R.H. }=\frac{8}{16} \times 100=50 \%\)

When the relative humidity is 100%, the air is fully saturated.

Distribution:
1) Relative humidity is maximum at the Equator and decreases polewards,
2) It is low in hot deserts, in continental interiors, as in areas of anti-cyclonic conditions.

3)Specific Humidity: The humidity of the atmosphere expressed as the ratio of the weight of water vapour (in grams) to the total weight (in kilograms) of a given volume of air is called Specific humidity. This varies from about 0-2 gm/Kg in very dry cold arctic air to over 18-0 gm/Kg in hot humid tropical air.

Distinguish between Relative humidity and Absolute Humidity:

Relative Humidity	Absolute Humidity
1. It is a ratio between the actual amount of water vapour present In the air at a particular temperature and the maximum amount of water vapour that the air can hold at that temperature.	1. The total amount of water vapour present in the air at a particular temperature is absolute humidity.
2. It is a ratio between the absolute humidity and vapour capacity of the air.	2. It is defined as the weight of water vapour per unit volume of water.
3. It is maximum In the equatorial region, but is lowest in hot deserts.	3. It Is maximum over oceans and lowest in high-pressure areas.
4. It is expressed as a percentage.	4.  It is expressed as grams per cubic metre of air.

 

The Importance of Relative Humidity: Relative humidity is daily life. Some important points have been mentioned here.

(1) It is possible to find out the possibility of rainfall with the help of relative humidity.
(2) Relative humidity is essential to agricultural operations.
(3) Many chemicals and medicines deteriorate in high relative humidity.
(4) It is necessary to take into account the relative humidity before constructing buildings.
(5) Radio, Television, electric instruments and other scientific instruments are affected by relative humidity.
(6) High relative humidity do not enjoy good health. Even very low relative humidity is not conducive to health. It has been found that a relative humidity of 60% is most suitable for human health.

The Proportion Of Relative Humidity Changes According To Altitude

The Measurement of Relative Humidity: Relative humidity is measured with the help of various types of Hygrometers. The most common Hygrometer is known as ‘Dry and West Bulb Thermometer’.

Precipitation: The particles of water or ice that form within clouds and fall towards the earth surface is Precipitation. According to famous climatologist H. J. Critehfield, “Precipitation is defined as water in liquid or solid forms falling to the earth.” Precipitation includes all forms in which moisture falls on the earth’s surface.

It is the process by which condensed water from clouds falls on the earth’s surface. It can be solid or liquid. Precipitation is a complex process when millions of drops of water combine together and fall on the earth.

Forms of Precipitation: Different forms of precipitation are:
1. Rainfall,
2. Snowfall,
3. Hail,
4. Dew,
5. Frost,
6. Sleet,
7. Drizzle,
8. Glaze.

1. Rainfall: The release of moisture in the form of drops of water is called ‘rainfall’.After condensation of moist laden air, clouds are formed. Teh drops of water become so heavy that air cannot hold these. The falling of these drops of water from the clouds is called ‘rainfall’.

2. Snowfall: A from of precipitation consisting of crystals of ice is called ‘snow’.it is produced when condensation takes place at a temperature below freezing point, (0°C), so that the minute crystals of ice form directly from the water vapour.

These small crystals then unite to form flakes of ice called snow.The coming down of snow flakes towards the earth’s surface is known as ‘snowfall’.It is common in higher latitudes and high mountain regions.

 

3. Hall: Precipitation in the form of pellets of ice (hailstone) that develop in and fall from a cumulonimbus cloud, either at a cold front or where intense heating of the surface causes rapidly ascending convection currents is called hail.

When 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. Hailstones cause damage to crops and buildings.

 

4. Dew: When the temperature for the air is higher than the dew point and a cool object having a lower temperature comes in contact with such air. This causes condensation in the air The drops of water thus formed rest on the cold object like grass, leaves, rocks, etc.

These drops of water on the cold objects are called dew. The conditions favouring dew formations are moist air, light winds, and clear night skies to ensure maximum cooling by radiation.

 

5. Frost: A weather condition that occurs when the air temperature is at or below the freezing point (0°C), the water vapour will condense to form an icy deposit. It is called frost.

6. Sleet: In the U. K., Sleet is described as a form of precipitation consisting of either partly melted snow-flakes or rain and snow falling together. In the.U.S.A., Sleet is described as a form of precipitation consisting of frozen raindrops that have subsequently partially remelted. So Sleet is considered as a mixture of rain and snow.

7. Drizzle: Light rainfall, consisting of particles of size less than 0-5 mm in diameter and are close together is called ‘drizzle’. Normally drizzle is produced by stratus and strato- cumulus clouds.

8. Glaze: A covering of smooth clear ice that coats objects and surfaces is called ‘glaze’. It occurs when supercooled water droplets freeze on contact with a surface and when a fall in temperature causes wet surface to freeze.

Distinguish between Precipitation and Rainfall:

Precipitation	Rainfall
(1) The particles of water or ice that form within clouds and fall towards the earth is precipitation.	(1)The release of moisture in the form of drops of water is called rainfall.
(2) It is the collective name given to different forms of moisture after condensation.	(2)It is a type of precipitation when moisture falls on the earth in the form of droplets of water.
(3) Precipitation has two forms—liquid and solid.	(3)Rainfall has only liquid form.
(4) Rainfall, snow, hail, drizzle, sleet, dew, frost and glaze are the common forms of precipitation.	(4) Three major types of rainfall are—(1)Convectional,(2)Relief or Orographic and (3)Frontal or cyclonic rainfall.

 

How is Rainfall caused: Humidity is the basis of rain. The general cause of rainfall is the cooling of saturated air. The mechanism of rainfall passes through a number of stages:
(1)Air should have plenty of water vapour,
(2)That air should be saturated with moisture,
(3)The air should get cooled or get the chance to come in contact with the cold air for condensation and
(4)Condensation of water vapour makes droplets. Thus the release of moisture in the form of drops of water is called ‘rainfall’.

Types of Rainfall: On the basis of mode of formation and process of cooling of air, there are three types of rain.
(1) When the air is cooled by rising convection currents (Convectional rainfall).
(2) When moist air is cooled due to ascent along a mountain barrier (Orographic or Relief rain).
(3) When air is cooled due to the meeting of warm and cold air masses.(Frontal or Cyclonic rain).

(1) Convectional Rainfall: Convectional rainfall is formed when the air is cooled by the rise of the convectional current. It happens when the land is intensely heated, the hot and moist air rises up vertically as convection currents.

As it rises up, it expands and cools due to the release of pressure. Hence, condensation takes place resulting in a cumulus or cumulonimbus clouds.

Consequently, it results in torrential rainfall. The equatorial region of low latitude experiences convectional rainfall. In this region, the convectional system is best developed because daily heating of the ground surface up to noon causes convection currents.

Consequently, the sky becomes overcast by 2—3 p.m. daily causing pitch darkness and heavy rains and the sky becomes clear by 4 p.m. So it is called 4 O’ clock rain. Thus, the convectional rainfall in the equatorial region is a daily regular feature.

 

Characteristic features of convectional rainfall:
(1) It occurs daily in the afternoon in the equatorial region.
(2) It is of very short duration but occurs in the form to heavy showers.
(3) It occurs through thick dark and extensive cumulonimbus clouds.
(4) It is accompanied by cloud thunder and lighting.

(2) Orographic or Relief Rainfall: Orographic rainfall occurs when moist air is cooled due to ascending along a mountain slope or a plateau barrier. The moist wind coming from the sea rises up along the slope of the land and comes in contact with cool upper atmosphere.

As a result the moist air condenses and rainfall occurs. Since this type of rainfall is caused by the relief of the land, it Is known as relief rainfall’. The the windward slopes’ gets maximum rainfall.

 

Distinguish between “Leward slope’ and ‘Windward slope”:

Windward Slope	Leward Slope
(1) Windward side is the side of the mountain which faces the moisture (rain-bearing winds).	(1) The Leward slope/side is the side of the mountain which is opposite the windward side.
(2) On the windward side, the air ascends and becomes cool.	(2) On the Leeward side the air descends and gets warm.
(3) When the moisture-bearing winds climb the windward side of the mountain, they cool down and bring heavy rainfall.	(3) When these air cross over to the other side (Leward side) they have already lost much of their moisture. While descending they further become warm and dry so they give less rainfall on the Leeward side.
(4) The western slope of the Western Ghats gets more rain because they form the windward side.	(4) The Eastern slopes of the Western Ghats get lesser rain because they form the Leeward side.

Chapter 5 Cloud And Rain: Characteristic Features Of Orographic Rainfall

 

• The windward slope, also called as rain slope, receives the maximum amount of rainfall whereas the leeward side of the mountain gets vey low rainfall. .
• There is maximum rainfall near the mountain slopes and it decreases away from the foothills.
• Orographic rainfall may occurs in any season’ Unlike other types of rainfall it is more widespread and of long duration.
• The amount of rainfall increases with increasing height along the windward slopes of the mountains.

Distinguish between Convectional Rainfall and Orographic Rainfall:

Convectional Rainfall	Relief/Orographic Rainfall
1. This rainfall is caused by convection currents.	1. This rainfall is caused when a mountain forces the winds to rise.
2. The rising air expands and is cooled to give rainfall.	2. It occurs in mountainous regions.
3. It gives heavy showers for a short period.	3. It gives heavy rainfall on a windward slope, but Leeward slope is dry and is called Rainshadow
4. The equatorial region gets heavy showers in the afternoon daily.	4. South-West Monsoons give heavy rainfall on the Western Ghats, but the Deccan plateau lies in rain-shadow.

 

(3) Cyclonic or Frontal Rain: When hot and air masses move towards each other, parallel to the earth’s surface along a front, the cold air being heavy flows near the earth’s surface, while the hot air blows above it, rises and expands to cool and condense to form rainfall.

This is called “Cyclonic Rain’ because as this rain is associated with cyclones. This rain is common in North-West Europe in winter, Punjab plains experience rainfall in winter from cyclones coming from the Mediterranean Sea.

Chapter 5 Cloud And Rain: Characteristics of Cyclonic Frontal Rain

1. This rainfall is slow, continuous and extensive.
   The mechanism of cyclonic rainfall is of two types:
   1)Temperate cyclone and 2) Tropical cyclone.
2. This rainfall associated with a warm front is widespread and long-duration.
3. Sometimes this rainfall- occurs in the form of snowfall and hailstorms.
4. Most of the rains of temperate regions are received through cyclones.
5. If tropical cyclones are full of moisture become saturated and yield heavy showers characterized by lightning and thunder.

Raingauge: The amount of rainfall is measured by an instrument called ‘Raingauge’.

Importance of Precipitation: Precipitation is an important factor of weather and climate. Rainfall often brings a cooling effect on the weather. During intense summer rainfall offers a great relief of the people.

Temperature goes down due to heavy showers. Snowfall often brings very chilled weather and people suffer from shivering cold.

WBBSE Notes For 8 Class Middle School Geography