Chapter - 11
Water in the Atmosphere
️ Humidity :-
The water vapor present in the atmosphere is called the humidity of the atmosphere. Humidity is measured in grams per cubic metre.
️ Types of Humidity :-
There are three types of humidity :-
Absolute humidity: The amount of water vapor present per unit volume of air is called absolute humidity. It is expressed in per cubic metre.
Specific humidity: The weight of water vapor per unit weight of air is called specific humidity. It is expressed in grams per kilogram.
Relative humidity: The ratio of the water vapor present in the air at any temperature and the capacity of the same air to hold water vapor at the same temperature is called relative humidity. It is expressed in percentage quantity.
️ Formula to find relative humidity :-
Relative Humidity = Absolute Humidity – Humidity Tolerance x100
️ Saturated Air :-
When an air contains water vapor equal to its capacity, it is called saturated air.
️ Evaporation :-
The process of changing water from liquid to gaseous state is called evaporation.
On what does evaporation depend?
The amount of evaporation depends on the temperature, expansion and wind speed etc.
️ Latent heat of vaporization :-
About 600 calories of energy is used to convert one gram of water into water vapor. This is called the latent heat of vaporization.
️ Compaction :-
The process of changing the gaseous state of water into liquid or solid state is called condensation. Dew, frost, fog and cloud are forms of condensation.
️ Satisfied :-
The air with 100% relative humidity is saturated.
️ Dewpoint :-
The temperature at which air becomes saturated is called dew point.
️ Dew :-
Dew is a form of condensation. The earth heats up during the day and cools down at night. Sometimes the earth's surface becomes so cold that the temperature of the air touching it becomes so low that the water vapor present in the air gets condensed and it forms small droplets on the leaves of plants and other types of surfaces. But it accumulates. It is called dew.
️ Favorable conditions for becoming dew :-
The following conditions are favorable for the formation of dew –
- Long nights
- Cloudless sky.
- cool air.
- High relative humidity.
- Dew point being higher than freezing point.
️ Tushar :-
When condensation occurs when the temperature drops below the freezing point (i.e. below 0°C), ie, the dew point is at or below the freezing point, then frosts are formed on cold surfaces.
️ Fog :-
When the air mass filled with a lot of water vapor suddenly falls downwards, then the process of condensation takes place on the small dust particles.
Therefore fog is a cloud whose base is on the surface or very near to the surface.
️ Difference between fog and mist :-
Fog :-
- Fog is drier than fog.
- Fogs are small clouds that contain dust particles and smoke particles.
Fog :-
- 8 There is more moisture in fog than in fog.
- Fog is found more in the mountains.
️ Cloud :-
Clouds are formed by the condensation of water vapor around the nuclei of fine dust particles present in the air.
️ Forms of clouds :-
Since clouds are formed at some height above the surface of the earth, on the basis of their expansion, density and transparency or opacity, clouds are classified into four forms:-
Cirrus Clouds :- Their formation is 8000-12000 m. Of . Happens at altitude. These are thin and scattered clouds, which appear like feathers. They are always white in colour.
Cotton Clouds (Cumulus Clouds) :- They look like cotton. Usually formed at an altitude of 4000-7000 meters. These can be seen scattered and scattered here and there. They are of flat base.
Stratus Clouds: - These are layered clouds which are spread over a large area in the sky. These clouds are usually formed either by loss of heat or by mixing of air at different temperatures.
Rain clouds (Nimbus Clouds): - They are black or dark gray in color. They are formed very close to the middle levels or the surface of the earth. They are opaque to the sun's rays. Rain clouds are thick, shapeless masses of water vapor.
These four basic forms of clouds together form clouds of the following forms:
- High cloud (5 to 14 km)
- Mid-altitude clouds (2 to 7 km) between strata and cotton.
- Low altitude clouds (less than 2 km) layered rain clouds, layered rain clouds and cotton rain clouds.
️ Rain :-
When air laden with water vapor rises for some reason, it cools down and condensation of water vapor takes place. In this way water particles are born and they collect on the dust particles present in the atmosphere and start floating in the air itself. Hence clouds are formed. Clouds collide with an obstacle and drop their moisture in the form of water on the surface of the earth. This is called water rain.
️ Types of Rain :-
It is of three types :-
- Convection Rainfall
- Orographic Rainfall
- Cyclonic Rainfall
️ Convection Rainfall :-
When the surface becomes very hot, the air surrounding it also becomes hot. The air heats up and expands and lighter air rises up and convective currents are formed. By going up, this air cools down and the water vapor present in it starts condensing. Cotton clouds are formed by condensation. Which brings heavy rain. This is called convective rain.
Orographic Rainfall: -
When hot air laden with water vapor has to climb up the slope of a mountain or plateau, then this air becomes cold. As it cools, it becomes saturated and on climbing up the water vapor starts condensing, causing rain, this is called mountainous rain.
️ Cyclonic Rainfall :-
Rainfall caused by cyclones is also called cyclonic rain or retrograde rain.
