Water is a peculiar liquid as it can exist in three states namely solid state as ice, liquid states as water and gaseaus state as water vapour. In this chapter we discuss about humidity and rainfall.
Water Vapour : Water is continuosly evaporating from seas, rivers, lakes, ponds etc. due to the heat of the sun. This creates the moisture in the atmosphere and forms clouds.
These clouds provide rain to the earth. Humidity is produced due to the water vapours in atmosphere.
Oceans : Ocean currents transfer the sun’s heat from one place to the other and create world climate. Coasts and beaches are created by the ocean waves which strike against shore.
Rivers : Moving water through mountains and rocks creates valleys and gorges. The slits flows with them which gives rise to flood plains. They are making our landscapes. Moving water has been responsible for making transport routes and agriculture.
Glaciers : Glaciers are the slow moving rivers of ice under their own weight. The movement of glaciers curve huge rocks, gouge valleys into enormous troughs and cut off entire hillsides.
Evaporation : Evaporation is the process by which water of seas, rivers, lakes and ponds changes to water vapour due to the heat of the sun.
1. Evaporation takes place at all temperatures : With the rise of the temperature the rate of evaporation increases.
2. Hot air can hold more water vapour than cold air.
3. Evaporation causes cooling.
Factors which increases the Rate of Evaporation :
1. High temperatures increase the rate of evaporation.
2. Clear skies increase the rate of evaporation.
3. Winds increase the rate of evaporation. The wind carries away the moist air which is replaced with dry air.
4. Dry air increases the rate of evaporation.
Humidity
The amount of water vapour present in the air is called humidity. When the air contains more water vapour, the weather is said to be humid.
Plants grow well in high humidity, but it is uncomfortable for human beings. The body does not cool down because sweat cannot evaporate into the air.
Low humidity is better for humans but not so good for plants.
Absolute Humdity is defined as the the total amount of water vapour present in a given volume of air.
Specific Humidity is the actual amount of water vapour present in a given mass of air.
Relative Humidity expresses the amount of water vapour in the air.
Relative humidity is the ratio between the actual amount of water vapour present in the air and maximum amount of water vapour which the air can hold at that temperature.
It is expressed as a percentage.
Relative Humdity =
Measuring Humidity : Humidity can be measured with a hygrometer. Relative humidity can be measured by a instrument known as the Wet and Dry Bulb Thermometer.
Effects of Humidity
1. High relative humidity produces discomfort because sweat does not dry up easily.
2. Low relative humidity causes the skin to dry and makes it rough and sometimes the skin cracked.
3. In low humidity, the wood and glue dries out causing furniture to loosen and crack.
4. Mumbai is an important city for the cotton textile industry because of its humid climate which favours manufacture of high quality cotton cloth. In Ahmedabad, where humidity is low, humidifiers are used to regulate the humidity in cotton textile mills to produce good quality cloth.
Condensation
Condensation is the process by which water vapour is converted into water.
Condensation occurs only when the air becomes saturated.
Different forms of Condensation
1. Cloud : In atmosphere droplets of water suspend as clouds.
2. Mist : Mist are the droplets of water hanging in the atmosphere close to the surface of the Earth.
3. Fog : A dense mist is called fog.
4. Dew : Tiny drops of water when water vapour condenses on the ground or cool surfaces is called dew.
5. Frost : It is formed when moisture in the air freezes or dew freezes into ice.
Clouds : Mass of small water droplets or tiny ice crystals that floats in the air are called clouds.
Classification of Clouds : The shape and the height at which clouds are formed classify the clouds.
When the warm air rises rapidly from the Earth’s surface, ‘heaps’ or ‘bubbles’ of cloud are formed.
When the warm air rises slowly over a large area, it forms ‘layers’ or ‘strata’ of clouds.
Cirrus : These are wispy and feathery clouds.
Cumulus : Those are fluffy heaps with a flat base.
Stratus : These are stratified or layered clouds.
Low clouds are rain-bearing clouds.
The bases of these clouds are less than 2 km above sea-level.
Middle Clouds : These usually lie between 2 and 6 km above the Earth surface. Such as altocumulus and altostratus.
High clouds are those that lie more than 6 kms above the Earth. They are mainly made up of ice crystals.
Precipitation
It is the process by which water from the clouds falls down on the Earth as precipitation.
Forms of Precipitation
1. Rain : It is the common form of precipitation.
2. Drizzle : Less intense rain.
3. Snow : Snowflakes.
4. Sleet : Mixture of rain and ice.
5. Hail : Pellets and balls of ice, associated with thunderstorms.
Stages in the formation of Rain
1. The first stage is nucleation. In this stage, water collects and forms tiny cloud drops on dust particles. This type of nucleation occurs in the tropics.
Outside the tropics, water vapour condenses directly on ice crystals so that they grow bigger and bigger.
2. In the second stage, the droplets of crystals continue to grow in size and start falling due to gravity.
Factors determining Rainfall : The rainfall depends on the moisture content of the air which depends on three factors.
Land and sea contrast : More rainfall occurs on coastal regions when the sea, is closer than in the interior of continents.
Direction of the prevailing winds : Winds from the sea bring more rainfall than winds blowing from the land.
Presence of mountains : When mountains stop moist winds, they are forced to give rain on the windward side. The leeward side remains dry.
Special Effects
Rainbows are seen only when we are facing the rain and the sun is behind us.
Thunderstorms : These are severe storms accompanied with thunder and lightning.
Lightning : It occurs when the charge in the clouds is enough to overcome the resistance of air and a bolt of lightning is produced.
Thunder : The big sound produced by the rapid heating and expansion of the air through which lightning passes. First lightning takes place, then thunder follows. Light is seen first then sound is heard because the speed of light is very high as compared to speed of sound.
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