Atmosphere – Pressure Belts of the World

Atmospheric Pressure

• Defined as weight of column of mercury 76 cm / 760 mm high at sea level
• Measured by Barometer

Isobar

• An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea level.
• Spacing of isobars represent pressure gradient i.e., rate of change of pressure

Variation in Atmospheric Pressure

• Variation in atmospheric pressure is responsible for horizontal movement of winds
• Some of the factors affecting atmospheric pressure are “Temp. of air, Altitude, Water Vapour, Gravitational force & Rotation of earth

Pressure Belts

• Horizontal distribution of pressure is expressed in terms of pressure belts
• Run in E – W direction along certain specific latitudes
• Total seven pressure belts

Thermally induced pressure belts

• Equatorial low & polar high

Dynamically induced pressure belts

• Subtropical high & sub polar low

Equatorial Low

• Low pressure belt extending up to 10 * N & S of equator
• Low pressure prevails due to high temperature, high water vapour in air & max. speed of earth’s rotation at the equator
• Surface winds are generally absent as winds rises vertically near this region, hence only vertical currents are found

Polar High

• From 80 – 90* N – S of equator → Polar Region
• Caused by extremely low temp. at poles, which remains below freezing point even during summers
• Air remains cold & dense resulting in high pressure

Subtropical High

• From 23 ½ * – 35 * N – S of equator
• A region of high pressure caused due to descent of cool air from high altitudes as shown back
• Descending air creates calm conditions with feeble winds in this high pressure belt
• Also known as Belt of Calm or Horse Latitude

Subpolar low

• 45 – 60 * N – S of equator
• Due to blowing away of air vertically up from the land surface mainly because of

• Cold air from poles becomes comparatively hot on reaching circles & rises vertically up
• Due to comparatively greater axial rotation of earth at 45 – 60* N – S than the poles

Wind

• Horizontal movement of air caused by horizontal atmospheric pressure difference
• Greater the pressure gradient, greater the velocity of wind
• Deflection of winds is caused by coriolis effect i.e., Winds get deflected to their right in N – Hemisphere & to their left in S – Hemisphere → Ferrels law
• Coriolis force changes only direction of wind not its speed
• Coriolis force speed is 0 at the equator & max at poles
• That means only horizontal component of coriolis force is considered