Humidity, Evaporation and Condensation
Humidity -
- Water in gaseous form or the water in vapour in the atmosphere is known as Humidity.
- Humidity is the index of atmosphere's potential for yielding precipitation.
- It helps in the energy balance on the surface of the Earth through evaporation and condensation.
- In evaporation, heat is absorbed and in condensation, heat is released.
Measurement of humidity
1. Specific humidity
- It is the ratio of weight of water vapour in grams to the weight of air in Kilo grams.
- It is expressed as gm/kg of the air.
- It indicates the actual amount of moisture present in the air.
- It doesn't changes with the change in temperature.
2. Absolute humidity
- It is the ratio of weight of water vapour in grams to the volume of air in cubic metre.
- It is expressed in gm/cubic metre of the air.
- It increases with contraction of air (i.e., when temperature is decreased) and decreases with expansion (i.e., when temperature is increased).
3. Relative humidity
- It is the ratio between the amount of moisture air can hold at a given temperature and the amount it actually holds.
- It is expressed in percentage (%).
- It decreases with increase in temperature and increases with decrease in temperature.
- Air is said to be saturated when relative humidity is 100%.
Evaporation - The conversion of liquid water into water vapour is known as evaporation.
Factors affecting rate of evaporation -
1. Temperature
- Higher the temperature, higher the rate of evaporation.
2. Humidity
- Higher the humidity, lower will be the rate of evaporation due to saturation of air.
3. Wind speed
- Higher the wind speed, higher the rate of evaporation
4. Salinity
- Higher the salinity of the water, lower the evaporation rate will be
Distribution of evaporation -
- It is maximum along the Sub-tropical zone because of high temperature, low humidity and dry condition.
- It is moderate along the Equatorial region due to cloud cover and higher humidity.
- It is too low along the polar region due to very low temperature.
- Evaporation is greater over the ocean than that over the continents due to greater wind speed over the ocean.
- The rate of evaporation is higher where the cold and warm ocean current meets.
Condensation - Condensation is the conversion of water vapour into liquid form. It is the reversal of evaporation.
Dew Point -
- The temperature at which condensation takes place is known as dew point.
- At dew point the relative humidity of the air is 100%.
- Dew point varies with moisture content in the atmosphere.
- Higher the moisture content in the atmosphere, higher the dew point will be and vice-versa.
- If the condensation process happens below 0℃ the dew point is known as Frost point.
Factors affecting condensation
1. Temperature
- Lower the temperature, higher will be condensation
2. Humidity
- Higher the humidity level, higher the condensation will be
Various different kinds of condensation -
1. Dew
- The moisture deposited in the form of liquid droplets on the land surface is called dew.
- Conditions favourable for the formation of dew are - Long winter night, Clear cloudless sky and Calm windless air.
2. Frost
- When the temperature falls below the freezing point, a thin layer of ice is formed on solid surface which is known as frost.
- It is the deposition of neddle like white opaque ice crystals on the surfaces having temperature below 0℃.
4. Mist
- It is small water droplets suspended in the air.
- It reduces the visibility to 1 to 2 KM.
- It is most common in hilly areas and areas having water bodies.
5. Haze
- It is caused by the presence of more smoke and dust particles and less humidity level.
- The visibility level is upto 2 KM.
- It is more common in industrial areas where large number of industries are there.
6. Fog
- It is similar to a cloud near to the ground with visibility less than 1 KM.
- Fog is produced near to the surface when the temperature of the air drops suddenly.
- This conditions occurs in the region of temperature inversion.
- Where ever there is a temperature inversion, the chances of producing fog is more.
- Fogs are named as per the type of temperature inversion.
- For example - Fog created by Radiation inversion is known as Radiation fog, Fog created by Advection inversion is known as Advection fog, fog created by Frontal inversion is known as Frontal fog and the fog created by Air drainage inversion is known as Valley fog.
- Up slope fog - It is formed when a warm and moist air rises along the upslope where the temperature is low, causing it to cool down leading to the formation of fog.
- Smog - It is the mixture of smoke and fog.
Stability and instability in the atmosphere
When rises (convection, which may lead to precipitaiton) because of higher temperature, it results in instability.
When there is stable and calm condition (because of cooler surface or incase of subsiding air), it does not lead to precipitation, which results in stable condition.
Stability - It is the condition when air resists vertical movement and remains in original position. The air remains stable when it is cooled at its base or when the air is subsiding along the high pressure belt. In such condition the precipitation is unlikely to happen.
Instability - It is the condition where the air does not resist vertical movement and leads to cloud formation and precipitation. It is more common in the regions of high temperature, i.e., the air is heated from below and also in the regions of low pressure belt. Such condition may lead to precipitaion.
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Optional Notes
Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own.
Hope! It will help you to achieve your dream of getting selected in Civil Services Examination 👍
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