Water in the Atmosphere Notes in English Class 11 Geography Chapter-10 Book-Fundamental of Physical Geography

• Water vapor is present in the air, which can be from 0 to 4 percent of the atmosphere's volume. It plays an important role in weather-related events. Water is found in three forms in the atmosphere—gas, liquid and solid. 

• This cycle of water continues through evaporation (conversion of water into steam), transpiration (conversion of water into steam from plants), condensation (conversion of steam into water) and rainfall.

• This water vapor in the air is called humidity, which is measured in different ways. For example:

• Absolute humidity: It is the weight of water vapor per cubic meter of air.

• Relative humidity: This compares the amount of water vapor present in the air at a given temperature to its maximum capacity, as a percentage.

• The humidity of air depends on temperature, and when the water vapor in the air reaches its full capacity, it is said to be saturated. The temperature at which this saturation occurs is called the dew point.

Evaporation and Condensation

The amount of water vapor in the atmosphere varies due to the processes of evaporation and condensation.

Evaporation

• Evaporation is the process in which water changes from liquid state to gas (water vapor). The main factor in this process is temperature. The temperature at which water evaporates is called latent heat of vaporization. Apart from this, humidity and speed of air also affect evaporation. Faster speed of wind speeds up the process of evaporation.

Condensation

• Condensation is the process in which water vapor cools and turns back into liquid (water). The main reason for this is heat loss. Cold air reduces the capacity to hold water vapor, resulting in excess water vapor turning into liquid. This process occurs around small particles, such as dust, smoke, or salt, called condensation nuclei.

Types of condensation

After condensation, water vapor can change into different forms, such as:

1. Dew

2. Fog

3. Frost

4. Clouds

Conditions Necessary for Condensation

The major factors influencing the process of condensation include the air temperature falling to the dew point, the volume of the air decreasing, and the entry of additional water vapor into the air. Due to these factors, the water vapor present in the air starts turning into liquid.

1. Dew

• Dew forms when moisture in the air condenses as water droplets on cold surfaces, such as grass, plant leaves, or stones. It occurs when moisture begins to freeze on solid surfaces rather than condensing on condensation nuclei.

Favourable conditions for dew formation

1. Clear sky.

2. Calm and slow wind.

3. High relative humidity.

4. Cold and long nights.

• Dew is formed only when the dew point temperature is above the freezing point.

2. Frost

• Frost forms when moisture on cold surfaces freezes as tiny ice crystals. It occurs when the air temperature reaches the freezing point (0°C) or below.

Conditions for Frost Formation

1. Clear and cool nights.

2. Calm and slow wind.

3. High relative humidity.

4. Air temperature 0°C or less.

difference between dew and frost

• Dew freezes as water droplets, while frost freezes as ice crystals. Both are formed as a result of condensation, but their formation differs according to the temperature.

3. Fog and mist

• Fog and mist are small clouds that form near cold surfaces. They form when water vapor present in the atmosphere condenses on dust or smoke particles.

Fog

• Fog is a type of cloud that forms near the surface, which reduces or reduces visibility to zero. Fog becomes more dense due to smoke in urban and industrial areas. When a mixture of fog and smoke forms, it is called 'smoke fog'.

mist

• Mist contains more moisture than fog and is more likely to form over mountains when warm air comes into contact with a cold surface. Mist is wetter and moister than fog.

Difference between fog and mist

• Fog is drier and forms near the surface, while mist is more moist and forms mainly in mountainous areas.

5. Clouds

• Clouds are formed by the condensation of water vapor in the air and contain water droplets or ice particles. They appear in different heights and shapes.

Types of clouds

• Cirrus Clouds are formed at altitudes of 8,000-12,000 meters. These are thin, scattered, and feather-like white clouds, which appear light and always white in color. 

• Cumulus Clouds are found at an altitude of 4,000-7,000 meters and are scattered clouds resembling cotton wool. They have a flat base and look very beautiful. 

• Stratus Clouds are layered clouds that cover a large portion of the sky. They are formed by a mixture of cold air or wind and are usually white or light gray in color. 

• Nimbus Clouds are thick dark grey or black clouds that are responsible for heavy rainfall and sometimes appear very close to the surface.

precipitation

Precipitation is the process when water vapour falls to the earth after condensing in the atmosphere. It can be in the form of liquid (water) or solid (ice).

Types of precipitation

• Rain:  When water vapour falls to the ground in the form of water droplets, it is a natural process that occurs at normal temperatures.

• Snowfall:  When the temperature is below 0°C, water vapor falls to the ground as snow. This snow forms hexagonal crystals, which take on unique and beautiful shapes.

• Sleet:  Frozen water droplets fall into cold air and are formed when rain passes through cold layers. This process occurs in cold weather and the water freezes in the form of ice.

• Hail:  Round pieces of ice are formed when cold air passes through them and are covered with layers of ice. These usually fall in the form of hail and are the result of a special process that takes place in cold environments.

Types of Rain (Based on Origin)

• Convectional Rainfall:  When warm air rises, cools and condenses, causing rain. This rain is heavy but short-lived and is found mainly in tropical regions.

• Orographic Rainfall:  When air rises up the slope of a mountain, it cools, condenses and rains. The leeward (windward) slope receives more rain, while the other slope receives less rain, which is called the rain shadow region.

• Cyclonic Rainfall:  Rainfall caused by cyclones occurs when cold and warm air meet. This process causes cyclonic rainfall, which can be heavy and widespread.

Distribution of rainfall in the world

The amount of rainfall can be divided into different categories. 

• Heavy rainfall (more than 200 cm) occurs in the equatorial regions, monsoon coastal areas, and the western coasts of temperate regions. 

• Moderate rainfall (100-200 cm) occurs in the interior parts of the continents and in the coastal areas. Low rainfall (50-100 cm) is found in the interior parts of the tropical and temperate regions. 

• Very little rainfall (less than 50 cm) occurs in rain shadow regions and areas at higher latitudes.