What is Atmospheric Pressure & Humidity and Rainfall

Definition Of Atmospheric Pressure

The air has weight and occupies space. It exerts its weight as pressure on the surface of the earth which is called Atmospheric pressure. Atmospheric pressure is maximum at sea level.

Factor Affecting Atmospheric Pressure


  • When air is heated it expands, become light and rises vertically.
  • Rising warm air exerts less pressure on the surface and causes a low pressure.
  • When air is cooled it becomes heavy and sinks vertically and exerts greater pressure on the surface creating high pressure.


atmospheric pressure barometer

  • The upper atmosphere is very thin and less dense than the lower atmosphere.
  • Hence, the pressure is also less and keeps decreasing with increasing altitude or height.
  • Atmospheric pressure is measured in millibars with the help of an instrument called a barometer


Pressure Belts

Atmospheric pressure belts


  • On the basis of heat zones, we observe alternating High pressure (HP) and Low pressure (LP) belts on the surface of this earth, due to rising.
  • Warm air in certain regions and sinking cold air in others.
  • The pressure belts are not permanent and keep shifting north and south with the movement of the sun by 10 degrees.

           The pressure belts are :

  • Equatorial low – This belt extends 5°N and 5°S of the equator.
  • Sub-tropical highs – These are located beyond tropics around 30°N and 30°S latitudes.
  • Sub-polar lows -This belt is located around 60°N and 60°S latitude.
  • Polar highs – The poles are zones of intense cold throughout the year. Hence, these remain areas of high pressure.

Planetary Winds

atmospheric pressure plentary wind

  • Air in motion is called winds.
  • on the surface of the earth, winds blow from high pressure to low pressure.
  • They do not blow straight but are deflected from their path due to the rotation of the earth.
  • The rotation of the earth creates the Coriolis effect, which causes the deflection of winds from their direct path to flow.
  • The Coriolis effects are explained in Ferrel’s law which states that wind and water get deflected to the right in the northern hemisphere and to in the northern to the left in the southern hemisphere.
  • There is three permanent wind system on the surface of the earth:
  1. Tradewinds
  2. Westerlies
  3. Polar Winds

Tradewinds blow from the sub-tropical high-pressure belts to the equatorial low-pressure belt. These winds get deflected from their path due to the rotation of the earth. These winds become warmer when they blow towards the equator, hence, their capacity to hold moisture increases and they keep drying the surface over which they blow.

These winds blow from sub-tropical high-pressure belts to the sub-polar low pressure. They blow from the south-west in the northern hemisphere and from the north-west in the southern hemisphere. They blow from a warmer region to a cooler region, they cause plenty of rain to the western coast of the continents.

Polar Winds
These are cold and dense winds that blow from the polar high-pressure to sub-polar low-pressure belts. they blow in the same directions as the trade winds in their respective hemisphere.


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Humidity and Rainfall

The amount of water vapour in the atmosphere is called humidity. It is the temperature that controls the amount of humidity. The higher temperatures cause more evaporation and thus the greater humidity.  The three processes through which water enters the atmosphere and return to the earth and this called water cycle

The Process of Water Cycle

Evaporation – Evaporation is the changing of water into vapour due to heating. It depends on temperature,  hence, evaporation is higher in summers than winters.

Condensation – It is the process in which water vapour in the air changes into water. It happens when warm, moist air rises and cools in the upper atmosphere. The water vapour turns to minute water droplets that cling together forming clouds. The condensation takes places at the temperature which is called the dew point.

precipitation – Precipitation is the process in which condensed water vapour falls down on the surface of the earth in various forms such as rain, snow, sleet and hall.

Rain – When the air reaches the saturation point, the rainfall and other forms of perception take place. It refers that the air is full of moisture and is unable to hold it at that temperature. There are three types of rainfall

  • Relief Rainfall
  • Convectional Rainfall
  • Cyclonic Rainfall

Snow – Snow is formed when water vapour in the atmosphere turns directly into very small ice crystals due to the extremely low temperature. The precipitation comes down as tiny, soft flakes called snow. Cold regions and high mountains experience heavy snowfall in winter seasons.

Sleet – This form of precipitation is a combination of snowflakes and rain falling together.

Hail – When falling raindrops are thrown back high above the clouds by rising air currents, they freeze and become ice pellets. When these pellets become heavy they fall down as hailstones.


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