Water in Atmosphere- 1 | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
WATER IN THE ATMOSPHERE – PART - 1 
 
INTRODUCTION: 
Water is present in the atmosphere in three forms namely – gaseous, 
liquid and solid.  
 
The amount of water vapour in the atmosphere is added or 
withdrawn due to evaporation and condensation respectively. 
 
EVAPORATION 
Evaporation is a process by which water is transformed from liquid to 
gaseous state.  
 
Heat is the main cause for evaporation. 
 
 
 
CONDENSATION 
 
 
 
Page 2


 
WATER IN THE ATMOSPHERE – PART - 1 
 
INTRODUCTION: 
Water is present in the atmosphere in three forms namely – gaseous, 
liquid and solid.  
 
The amount of water vapour in the atmosphere is added or 
withdrawn due to evaporation and condensation respectively. 
 
EVAPORATION 
Evaporation is a process by which water is transformed from liquid to 
gaseous state.  
 
Heat is the main cause for evaporation. 
 
 
 
CONDENSATION 
 
 
 
 
The transformation of water vapour into water is called 
condensation.  
When moist air is cooled, it may reach a level when its capacity to 
hold water vapour ceases.  
 
Then, the excess water vapour condenses into liquid form. 
 
Condensation is caused by the loss of heat. 
 
LATENT HEAT 
The amount of heat required to change the state of a unit mass of a 
substance from solid to liquid or from liquid to vapour without 
changing its temperature is known as the latent heat. 
 
Or 
 
The amount of heat that changes the state of the material without 
changing its temperature. 
 
EXAMPLE: 
There comes a point while heating a cube of ice when there is no 
change in temperature in spite of continuous heating.  
 
There will be absolutely no rise in the temperature till the ice cube 
has melted entirely. 
 
LATENT HEAT OF VAPORISATION 
Latent Heat of Vapourisation is defined as the amount of heat required 
to change the unit mass of liquid at boiling point into vapour under 
constant temperature.  
 
Page 3


 
WATER IN THE ATMOSPHERE – PART - 1 
 
INTRODUCTION: 
Water is present in the atmosphere in three forms namely – gaseous, 
liquid and solid.  
 
The amount of water vapour in the atmosphere is added or 
withdrawn due to evaporation and condensation respectively. 
 
EVAPORATION 
Evaporation is a process by which water is transformed from liquid to 
gaseous state.  
 
Heat is the main cause for evaporation. 
 
 
 
CONDENSATION 
 
 
 
 
The transformation of water vapour into water is called 
condensation.  
When moist air is cooled, it may reach a level when its capacity to 
hold water vapour ceases.  
 
Then, the excess water vapour condenses into liquid form. 
 
Condensation is caused by the loss of heat. 
 
LATENT HEAT 
The amount of heat required to change the state of a unit mass of a 
substance from solid to liquid or from liquid to vapour without 
changing its temperature is known as the latent heat. 
 
Or 
 
The amount of heat that changes the state of the material without 
changing its temperature. 
 
EXAMPLE: 
There comes a point while heating a cube of ice when there is no 
change in temperature in spite of continuous heating.  
 
There will be absolutely no rise in the temperature till the ice cube 
has melted entirely. 
 
LATENT HEAT OF VAPORISATION 
Latent Heat of Vapourisation is defined as the amount of heat required 
to change the unit mass of liquid at boiling point into vapour under 
constant temperature.  
 
 
 
LATENT HEAT OF FUSION 
Latent Heat of Fusion as the amount of heat required to change a unit 
mass from solid to liquid at a constant temperature.  
 
 
NOTE: 
 
SUBLIMATION: 
Sublimation is the transition of a substance directly from the solid to 
the gas phase, without passing through the intermediate liquid phase. 
 
In case of sublimation, the amount of heat energy involved is far 
greater as it includes both the latent heats of vaporisation and fusion.  
HUMIDITY  
Water vapour present in the air is known as humidity.   
ABSOLUTE HUMIDITY 
The actual amount of the water vapour present in the atmosphere is 
known as the absolute humidity. 
The absolute humidity differs from place to place on the surface of the 
earth. 
The ability of the air to hold water vapour depends entirely on its 
temperature.  
RELATIVE HUMDITY 
The percentage of moisture present in the atmosphere as compared to 
its full capacity at a given temperature is known as the relative 
humidity.  
 
Page 4


 
WATER IN THE ATMOSPHERE – PART - 1 
 
INTRODUCTION: 
Water is present in the atmosphere in three forms namely – gaseous, 
liquid and solid.  
 
The amount of water vapour in the atmosphere is added or 
withdrawn due to evaporation and condensation respectively. 
 
EVAPORATION 
Evaporation is a process by which water is transformed from liquid to 
gaseous state.  
 
Heat is the main cause for evaporation. 
 
 
 
CONDENSATION 
 
 
 
 
The transformation of water vapour into water is called 
condensation.  
When moist air is cooled, it may reach a level when its capacity to 
hold water vapour ceases.  
 
Then, the excess water vapour condenses into liquid form. 
 
Condensation is caused by the loss of heat. 
 
LATENT HEAT 
The amount of heat required to change the state of a unit mass of a 
substance from solid to liquid or from liquid to vapour without 
changing its temperature is known as the latent heat. 
 
Or 
 
The amount of heat that changes the state of the material without 
changing its temperature. 
 
EXAMPLE: 
There comes a point while heating a cube of ice when there is no 
change in temperature in spite of continuous heating.  
 
There will be absolutely no rise in the temperature till the ice cube 
has melted entirely. 
 
LATENT HEAT OF VAPORISATION 
Latent Heat of Vapourisation is defined as the amount of heat required 
to change the unit mass of liquid at boiling point into vapour under 
constant temperature.  
 
 
 
LATENT HEAT OF FUSION 
Latent Heat of Fusion as the amount of heat required to change a unit 
mass from solid to liquid at a constant temperature.  
 
 
NOTE: 
 
SUBLIMATION: 
Sublimation is the transition of a substance directly from the solid to 
the gas phase, without passing through the intermediate liquid phase. 
 
In case of sublimation, the amount of heat energy involved is far 
greater as it includes both the latent heats of vaporisation and fusion.  
HUMIDITY  
Water vapour present in the air is known as humidity.   
ABSOLUTE HUMIDITY 
The actual amount of the water vapour present in the atmosphere is 
known as the absolute humidity. 
The absolute humidity differs from place to place on the surface of the 
earth. 
The ability of the air to hold water vapour depends entirely on its 
temperature.  
RELATIVE HUMDITY 
The percentage of moisture present in the atmosphere as compared to 
its full capacity at a given temperature is known as the relative 
humidity.  
 
 
With the change of air temperature, the capacity to retain moisture 
increases or decreases and the relative humidity is also affected.  
 
It is greater over the oceans and least over the continents. 
 
SATURATION POINT 
The air containing moisture to its full capacity at a given temperature 
is said to be saturated. 
 It means that the air at the given temperature is incapable of holding 
any additional amount of moisture at that stage 
 
DEW POINT 
The temperature at which saturation occurs in a given sample of air 
is known as dew point 
  
SPECIFIC HUMIDITY: 
 The term Specific Humidity is applied to express the ratio of weight of 
water vapour to the weight of moist air (including water vapour). 
 
NOTE: 
Humidity is measured by an instrument called the hygrometer.  
Hygrograph is an instrument which plots a graph of relative humidity 
round the clock.