Three states of water namely gas, liquid, and solid are present in the atmosphere. Through the process of evaporation, water bodies transfer moisture in the atmosphere and plants through transpiration. Evaporation, transpiration, condensation, and precipitation are the processes that keep on taking place continuously along the atmosphere, the oceans, and the continents.

Humidity:

Humidity is known as the presence of water vapor in the air. Quantitatively, there areways to express it. 

Absolute Humidity:

• Absolute Humidity is defined as the actual amount of water vapor present in the atmosphere. 
• Its unit is grams per cubic meter and this is the weight of water vapor per unit volume of air. 
• It varies from place to place on the earth’s surface. 

Relative Humidity:

• The presence of moisture percentage in the atmosphere in comparison to its total capacity at a given temperature is defined as relative humidity. 
• The capacity to hold moisture varies with the change in temperature of air and also affects relative humidity. 
• It is higher when presenting over the oceans and least in the case of continents.

Evaporation and Condensation: 

The water vapor content in the atmosphere is supplemented or reduced as a result of evaporation and condensation. 

Evaporation:

• This is a process to transform water from liquid to gaseous state. 
• Heat is the main reason behind the evaporation process.
• Latent heat of vaporization is the temperature at which the water starts evaporating.
• There is a substitution of the saturated layer with the unsaturated layer along with the movement of air. Due to this reason, greater movement of air causes greater evaporation.

Condensation: 

• When water vapor transforms into water then it is called condensation. 
• It is due to the heat loss that the air becomes cool. 

• The direct conversion of water vapor into a solid form is known as sublimation.
• Hygroscopic condensation nuclei are the minute particles present in free air which form when condensation takes place by cooling around these minute particles. These particles are dust, smoke, and salt from the ocean and they are quite good nuclei because of their ability to absorb water. 
• When moist air comes in contact with some colder object, condensation again takes place and this phenomenon can also happen when the temperature is near the dew point. 
• The amount of cooling and the relative humidity of the air are the responsible factors.
• The volume of air, temperature, pressure, and humidity also affect it. 

Condensation happens when: 

• The air temperature is decreased to the dew point by keeping its volume constant.
• The temperature and volume are decreased.
• Moisture is supplemented to air via the evaporation process.
• Reduction in air temperature is a highly favorable condition for condensation to take place

• Dew, frost, fog, and clouds are the forms in which the water vapor or the moisture in the atmosphere exists after the condensation process.
• When the dew point is lesser than or higher than the freezing point, condensation takes place.
• Based on location and temperature, there can be different forms of condensation.

Dew: It is the moisture that is deposited as water droplets on cold solid surfaces (instead of nuclei in the air the surface) such as stones, grass blades, and plant leaves.

• Ideal conditions require to form: Clear sky, gentle air, high relative humidity, and long and chilly nights. 
• The dew point needs to be above the freezing point for its formation.

Frost: It is formed on cold surfaces when the temperature is below freezing point (0°C), i.e., the dew point is at or below the freezing point and that temperature condensation takes place.

• The excess moisture is deposited as small-sized ice crystals and not as water droplets.
• Ideal conditions require to form: The condition is similar to that of dew, but the temperature of air must be at or below the freezing point.

Fog and mist: These are formed when the air temperature mass possessing a huge amount of water vapor suddenly falls and the condensation automatically happens on fine dust particles.

•  Fog is a form of cloud which has its base at or very close to the ground. 
• They are mini clouds formed when condensation takes place around the dust, smoke, and salt particles nuclei.
• Due to fog and mist formation, the visibility drops to zero.
• Fog and mist are responsible for smoke formation in urban and industrial centers. Fog mixed with smoke is described as smog. 
• Difference between Fog and Mist:
  • Mist has more moisture content than fog.
  • A thicker layer of moisture is present in every nucleus in case of mist. They are found frequently over mountains because the rising warm air goes upwards on slopes and touches a cold surface. 
  • Mist has more moisture content than fog and fog is prevalent at those places where warm air currents meet cold currents.

Conclusion

Water, the universal solvent, exists in the atmosphere in different forms. Each of these forms have different characteristics and behave differently according to the change in the surroundings.