CHAPTER NO.8 HUMIDITY AND PRECIPITATION

CHAPTER NO.8 HUMIDITY AND PRECIPIATION

Water is present in air in gaseous state like other gases while it enters into atmosphere from various water sources in the form of vapours because of evaporation which takes place because of the heat of Sun.

Amount of water vapours in atmosphere varies with time and place. Itis measured in comparison to volume on the scale from 0 to 4 percent but even small amount of variation is very important from the aspect of weather. Gaseous form of water present in airis known as humidity. Humidity is directly related with temperature and its amount

decreases as we move from Equator to Poles and moreover it also decreases with increase of height.

From the aspect of weather science humidty is measured and represented in various ways. Details about these are given below:

1. Absolute humidity : Absolute humidity is the actual amount of water vapour

present in a given volume of air. Itis represented in gram per cubic meter unit. This method is not commonly used by weather scientists because absolute humidity changes with the change in volume due to expansion and shrinking of air although the humidity in air may not change. Amount of absolute humidity decreases as we move from Equator to Poles and oceans to continents. Absolute humidity gives us information about the precipitation on a particular place.

2. Specific Humidity : Mass of the vapour in a unit mass of moist air is known as specific humidity. [tis usually expressed as grams of vapour per kilogram of air.Change in temperature and air pressure do not affect the specific humidity because of

which it is widely considered by weather scientists. Specific humidity changes with the change in amount of water vapours. Itis highest in Equatorial regions during summer i.e. near about 18 gram/kilogram and it is lowest in continental parts of Polar region

during winters i.e. 0.2 gram/kilogram. It is one of the best methods for measuring humidity with help of which we can make an idea about the amount of water which Earth receives through precipitation.

3. Relative Humidity : Ratio of amount of water vapours present in air at particular temperature and volume, and capacity of air to hold water vapours at same temperature is known as relative humidity. In other words we can say that the ratio between Absolute humidity and moisture holding capacity is known as Relative humidity. It is represented in percentage e.g. It temperature of air is 20°C then its capacity to carry humidity shall be 8 grams per degree celsius at such stage, 100% relative humidity means, air is completely saturated. It means that relat've humidity is

full to its capacity. This situation arises in following circumstances :—

(i) When absoulte humidity increases due to high evaporation.

(ii) When moisture holding capacity of air decreases due to low temperature.

Relative humidity decreases when the temperature of air increases. Relative humidity is very important for scientists working on atmosphere because information about precipitation is possible through various forms of humidity. In human life relative

humidity and comfort are related e.g. People donot like to live in regions having high relative humidity (i.e. more than 60 percent)Rainfall as agent of Precipitation

Water from our mother earth enters into atmosphere through evaporation and in the form of water vapours. These vapours move from one place to other with the help of winds. Water vapours get converted into solid or liquid state by condensation and they fall on Earth. This process is known as condensation. Precipitation takes place in various forms at different places of world. Its most important form is ‘Rainfall’.

Technically when condensation takes place at temperature above zero degree celcius then water vapours convert into water drops. Their size varies upto 10 micrometer (size of human hair is 75 micrometer) A droplet of rain water is of more than SOO microne and clouds are formed by lakhs of such drops. Hot and humid air and humidity holding centers are required in large amounts to form condition for Rainfall. Water

vapours get collected around these centers and clouds are formed when hot and humid air rises up and after cooling relative humidity comes to 100 percent. Formation of clouds does not necessarily mean that there will be rainfall because the size of drops of clouds is very small and their buoyancy in atmosphere does not allow them to fall as rain on Earth. Size of water drops suspended in air increases when they strike with

each other. When their increasing size breaks the holding capacity of air, they fall as rain on Earth.

1 micrometer = 10,00,000th part of 1 meter

Types of Rainfall

Rainall is caused by clouds while formation of clouds 1s due to condensation and saturation. Rainfall may be divided into three types on the basis of upward movement of air :-

1. Convectional Rainfall : C onvectional rainfall is caused by convection, where the surface layer of atmosphere is heated by the heat of sun causing the moisture laden air torise. As the air rises it cools down to form clouds by the process of condensation which results in rainfall. Rate of decrease in temperature which is known as adiabotic

lapse rate 1s 10°C per 1000 meter, causing condensation which further helps in formation of nimbus clouds. This type of rainfall occurs daily at the lower latitudes.Generally day time heating creates convection and by the afternoon, convectional winds get saturated.

On the other hand at higher lattitudes this type of rainfall occurs during summer season.

This rainfall lasts upto short time but brings lightening and thundering with itself.Because of conventional currents the size of clouds is large at height, which act as source of thundering and lightening.

2. Orographic Rainfall : This word has been derived from Greek word “OROS’

which means from mountain. Orographic rain is produced when air is lifted as it moves over a mountain range. As the air rises and cools, orographic clouds form and serve as the source of precipitation. Mostly this type of rainfall 1s received by the windward

side slopes of mountains which act as barrier in the movement of humid winds coming from sea. Low mountains situated near coasts and high mountains situated at inner parts of ocean Orographic rainfall. As the vapour holding capacity of hot air increases,relative humidity decreases because of which hot air moving down on the leeward slopes of mountains donot get rainfall. Most of the world’s rainfall falls under the

category of Orographic Rainfall. Difference between the rainfallreceived by eastern and western slopes of western ghats is very helpful in the concept of Orgraphic Rainfall.

3. Cyclonic Rainfall : When contrasting air masses enjoin, on abrupt zone or

boundary is formed which 1s called a front. Two air masses may be hot or cold, dry and wet or might be of any other type. Such contrasting air masses” contact creates uncertai nity in air because of which, hot and wet air rises up leaving below the cold and dry air.

This type of rainfall may further be divided into two types. Tropical regions receive cyclonic rainfall when humid air having different temperatures conjugate. Hot humid air starts rising resulting in formation of clouds which further leads to rainfall. In temperate regions, when hot and cold air masses make contact, an abrupt zone is formed. In this zone the hot air over rides the cold air resulting in the formation of cyclone. During Winters Punjab and North Western parts of India receive rainfall through cyclones arising from Gulf of Persia.

Distribution of Rainfall ; Distribution of rainfall is very complex in the world. Rainfall on a particular place depends on lattitude, humidity, winds, relief, atmospheric conditions etc. Because of all these there are spatial and temporal variations in the division of rainfall. Average rainfall on earth is 80 centimeter but large variations are found in this.

Equatorial regions receive (1000.cm) rainfall continuously throughtout the year. On the other hand tropical Desert regions remain dry throughtout the year and they receive rainfall upto 10cm only. In South American nation chile, Behia Faliksh region receives rian for 325 days each year while Erika region of Chile does not receive any rain

continuously for years. Monsoon regions like India receive 80 percent of their annual rainfall during four months i.e. June to Septemeber.

Rainfall pattern of the world may be divided into six major regions :—

1. High rainfall Equatorial region : Region between 10° N and 10° S lattitudes is known as Intertropical convergence. This region receives vertical sun rays because of which condensation of hot and humid air results in heavy rainfall continously throughout the year touching average of 150 to 200 cm annually

2. Trade wind region : Eastern parts of continents receive rainfall from the humid winds coming from sea because of the perennial winds blowing between 10° to 20°lattitudes in both the spheres. These winds become dry while reaching on the western parts of the continents because of this these parts donot receive any rainfall. That is why

deserts are situated on the western parts of continents. Basically, Trade wind region receives rainfall during summer season.

3. Sub Tropical Scanty rainfall region : High pressure regions are located (situated)between 20° to 30° lattitudes in both the spheres. In these regions air moves down due to anti cyclones because of which favourable conditions are not created for rainfall and these regions remain dry.

4. Mediterraneon regions : Regions situated between 30° to 40° lattitudes in both the spheres receive rainfall in winters from westerlies and they remian dry during summer under the effect of (eastern) trade winds.

5. Mid Latitudnal high rainfall regions : Western parts of continents situated

between 40° to 50’ latitudes in both the spheres receive heavy rainfall under the effect of westerlies blowing from sea to land however aAmount of rainfall decreases as winds proceed to eastern parts.

6. Low rainfall polar regions : Amount of rainfall decreases in the regions situated above 60° latitude in both the spheres because of low temperature and high air pressure. These regions receive average rainfall of 25 centimeters annually.

EXERCISE

1. Answer the following

{a) What name is given to ‘Specific Humidity’ in Punjabi?

(b) If a place faces high evaporation, what type of humidity shell

increase?

(d) At what name 'Hail'is known as gaseous form prior to solidifying?

(e) What is cyclonic rainfall known in Punjabi?

2 Answer the followings in 60 to 80 words.

(a) What is humidity? Name various types of humidity.

(b) What is rainfall? name various types of rainfall?

(d) What is Saturated air ? Explain.

(e) What relation humidity, winid and temperature have ? Write short note.