Atmospheric Pressure
Notes
Atmospheric pressure is the weight of the air column above a unit area. The mean sea-level pressure is 1,013.25 millibars (hPa). It is measured with a barometer (mercury barometer by Torricelli, or aneroid). Isobars join places of equal pressure; the pressure gradient is the rate of change of pressure over distance, and it drives winds.
Vertical variation: pressure falls with height, fastest near the surface (roughly 1 mb for every 10 m in the lower atmosphere).
Factors affecting pressure: temperature (warm air rises, giving low pressure), altitude, water vapour (moist air is lighter), and the Earth's rotation and gravity.
Pressure belts of the world
Belt | Latitude | Origin | Features |
|---|---|---|---|
Equatorial Low | 5°N-5°S | Thermal | Doldrums; rising air, calms, convectional rain; the ITCZ |
Subtropical High | about 30°N/S | Dynamic (sinking air) | Horse latitudes; hot deserts |
Subpolar Low | about 60°N/S | Dynamic | Where westerlies meet polar easterlies; cyclonic storms |
Polar High | 90°N/S | Thermal (cold) | Cold, dense, sinking air |
- The belts shift north and south with the Sun's apparent movement (seasonal shift).
Pressure systems: cyclone (a low-pressure centre, air converges and rises; winds spiral anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere; stormy weather) and anticyclone (high pressure, air diverges and sinks; clear skies).
Coriolis force: the effect of the Earth's rotation, deflecting winds to the right in the Northern Hemisphere and left in the Southern Hemisphere; it is zero at the equator and maximum at the poles. Buys Ballot's law: in the Northern Hemisphere, with your back to the wind, low pressure is to your left.
Southern Oscillation: seesaw in pressure between the eastern and western Pacific (Tahiti and Darwin); with El Nino it forms ENSO, which affects the Indian monsoon.
Key Points
- Mean sea-level pressure is 1,013.25 mb; falls with altitude.
- Pressure belts: equatorial low, subtropical high, subpolar low, polar high.
- Equatorial low and polar high are thermal; subtropical high and subpolar low are dynamic.
- Coriolis force: right in the NH, left in the SH; zero at the equator.
- Cyclone: low, convergence; anticyclone: high, divergence.
MCQs
Q1. The average sea-level atmospheric pressure is:
(a) 760 mb
(b) 1,013.25 mb
(c) 1,500 mb
(d) 500 mb
Ans: (b).
Q2. The "doldrums" are associated with the:
(a) Subtropical high
(b) Equatorial low
(c) Subpolar low
(d) Polar high
Ans: (b).
Q3. The subtropical high-pressure belts are also known as:
(a) Roaring forties
(b) Horse latitudes
(c) Doldrums
(d) Polar front
Ans: (b).