Atmospheric Pressure

What is Atmospheric Pressure?

Atmospheric Pressure refers to the pressure exerted by the weight of the air in the atmosphere on the surface of the Earth. It is measured using a barometer and is commonly expressed in millibars (mb) or hectopascals (hPa).

Units of Atmospheric Pressure

Standard atmospheric pressure at sea level: 1013.25 mb or 1 atm
In SI units: 1 atmosphere (atm) = 101325 Pascals (Pa)
Measured using: Barometer

Factors Affecting Atmospheric Pressure

Altitude: Pressure decreases with height.
Temperature: Warm air is lighter, hence lower pressure.
Water Vapor: Moist air is lighter than dry air, so more humidity means lower pressure.
Earth’s Rotation: Causes deflection and pressure variations.

Major Pressure Belts of the Earth

Equatorial Low (0°): Hot rising air creates a low-pressure zone.
Subtropical High (30° N and S): Cool sinking air forms high-pressure zones.
Subpolar Low (60° N and S): Rising air again causes low pressure.
Polar High (90° N and S): Cold sinking air forms high-pressure areas.

Atmospheric Circulation and Pressure

Pressure differences drive wind and circulation patterns. Air moves from high to low pressure, and Earth's rotation causes deflection (Coriolis Effect).

Importance of Atmospheric Pressure

Determines wind patterns and direction.
Influences weather and climate.
Essential for understanding monsoons and storms.

Previous Year Questions (PYQs)

Q: What is the standard atmospheric pressure at sea level?
Ans: 1013.25 millibars (mb) or 1 atm.
Q: Which instrument is used to measure atmospheric pressure?
Ans: Barometer.
Q: Atmospheric pressure is highest at?
Ans: Sea level.
Q: What happens to atmospheric pressure with increase in altitude?
Ans: It decreases.
Q: Which pressure belt is found at 30° N and 30° S?
Ans: Subtropical High Pressure Belt.
Q: In which direction does air move in pressure systems?
Ans: From high pressure to low pressure areas.
Q: What is the unit of atmospheric pressure in SI?
Ans: Pascal (Pa).
Q: Which belt is called the doldrums?
Ans: Equatorial Low Pressure Belt.
Q: What is the main reason for variation in atmospheric pressure?
Ans: Temperature differences.
Q: Which layer of atmosphere has the highest pressure?
Ans: Troposphere (near Earth's surface).
Q: How does humidity affect air pressure?
Ans: Moist air reduces atmospheric pressure.
Q: What is Coriolis Effect?
Ans: Deflection of moving air due to Earth’s rotation.
Q: Low pressure is associated with which type of weather?
Ans: Cloudy and rainy weather.
Q: What causes winds to change direction?
Ans: Pressure gradient and Coriolis effect.
Q: Which belt is responsible for tropical deserts?
Ans: Subtropical High Pressure Belt.
Q: Which pressure belt lies at the poles?
Ans: Polar High Pressure Belt.
Q: What is the approximate drop in pressure per 10 meters altitude?
Ans: 1.2 mb approximately.
Q: Which gas contributes most to atmospheric pressure?
Ans: Nitrogen (about 78%).
Q: Which belt is associated with calm air and heavy rainfall?
Ans: Equatorial Low Pressure Belt.
Q: How do jet streams relate to pressure?
Ans: They form along boundaries of pressure zones in upper atmosphere.
Q: What is isobar?
Ans: A line on a map connecting points of equal atmospheric pressure.