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Summary
Geography Class 25

Revision of the previous class (05:12 PM) 

Humidity, Evaporation, and Condensation (05:20 PM)

  • The amount of water Vapour in the atmosphere is called Humidity, It helps in energy balance on the surface of Earth through Evaporation and Condensation.
  • Humidity is the index of the atmosphere's potential for Yielding precipitation.
  • Measurement of Humidity in the atmosphere
  • 3 ways:
  • a) Specific humidity
  • It is the ratio of the weight of water vapor in grams to the weight of air in kgs expressed as grams/kg of Air.
  • It indicates the actual amount of Moisture present in the air.
  • Here amount of water vapor remains the same, It is not showing the effect of temperature.
  • b)Absolute humidity
  • It is the ratio of the weight of water vapor in grams to the volume of air in a meter cube
  • Expressed as grams per meter cube of air
  • Measures humidity in grams/m3 i.e 100 gms/m3
  • It decreases with the expansion of air and increases with the contraction 
  • c)Relative humidity
  • Measures Humidity in percentage 
  • It is the ratio between the amount of water vapor present in the air to the maximum amount of water vapor it can hold at a given temperature.
  • Relative humidity seen just with temperature.
  • If the temperature is increased the maximum capacity increases and relative humidity decreases.
  • If the temperature is decreased maximum capacity decreases Relative humidity increases.
  • Air is said to be saturated when Relative humidity reaches 100%.

Evaporation (05:58 PM) 

  • The conversion of liquid water into water vapor is called Evaporation
  • 3 important factors
  • a) Higher the temperature, the Higher the rate of evaporation.
  • b) Humidity: Higher the humidity, the Lower the rate of evaporation.Due to the saturation of Air.
  • c) Wind speed: Higher the wind speed, the Higher the rate of evaporation as Air movement provides a constant supply of fresh air.
  • Distribution
  • Highest in 30 degrees 
  • Lowest in the polar region.

Condensation (06:07 PM)

  • The process of conversion of water vapor into liquid water is called as condensation.
  • It is the reversal of evaporation.
  • The temperature at which condensation takes place is called a dew point.
  • If condensation happens below zero degree celsius it is called a frost point.
  • Condensation forms
  • Dew
  • It is the moisture deposited in the form of liquid water droplets on land surfaces.
  • The conditions favorable for the formation of dew are long winter nights, Calm air, and cloudless sky.
  • Frost
  • It is a thin layer of ice on a solid surface.
  • It is formed when the temperature of the surface is below the freezing point and the water droplets are falling on such a cold surface.
  • Rime
  • The deposition of needle-like white opaque icy crystals on the surfaces with temperature below zero degrees Celcius.
  • It occurs when supercooled water droplets in the air come in contact with the surface below the freezing point.
  • Fog (07:05 PM)
  • It is produced near the surface when the temperature of the air drops suddenly.
  • The visibility will be less than 1 km.
  • The condition required for the formation of fog is the same as the condition of temperature inversion
  • Following conditions favor fog formation commonly
  • Long winter night, Calm air, and cloudless sky.
  • Types of Fog:
  • Radiation fog, Valley fog, Frontal fog, Advection fog
  • Mist
  • It consists of small droplets of water suspended in the air.
  • It is similar to fog but with a lower density of water vapor.
  • Visibility in Mist is between 1-2 km.
  • Haze
  •  It is caused by smoke and dust particles with humidity.
  • It is generally observed in Urban areas.
  • Visibility in Haze is up to 2 km.

Clouds and precipitation (07:13 PM)

  • Stability condition
  • It is the condition when air resists vertical movement and remains in its original position.
  • Stability is when the air is cooled at its base or when air subsides along high-pressure belts.
  • Pressure is unlikely in this condition.
  • Instability condition
  • It is a condition where air does not resist vertical movement and leads to cloud formation and precipitation. 
  • Instability occurs along the regions of High temperature and low pressure.

Types of Clouds (07:26 PM) 

  • Cirrus
  • Cirrus means Thin, High altitude, and feathery appearance
  • White in color and indicates fair weather.
  • Ciro stratus
  • Layered and High altitude with a milky appearance.
  • Stratus
  • Stratus means layered.
  •  Alto
  • Alto means middle altitude.
  • cumulus
  • Cumulus clouds have a cotton wool-like appearance with dome-shaped cauliflower top
  • Also called globular masses
  • Nimbus
  • Nimbus means Rain bearing
  • Alto Cumulus
  • Globular masses of clouds with a cotton wool-like appearance at middle altitude.
  • Altostratus
  • A layer of clouds in sheets along the middle altitude.
  • Stratus
  • A low uniform layer of cloud near the ground level produces light drizzle. 
  • Cumulonimbus clouds
  • Overgrown cumulus clouds are very dark heavy and dense with an anvil top.
  • It causes heavy rainfall with thunder and lightning.

The Topic for the next class: Precipitation, Type of Rainfall.