Chapter 9 — Atmospheric Circulation and Weather Systems

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Atmospheric pressure is the weight of a column of air per unit area from mean sea level to the top of the atmosphere, expressed in millibars. It is measured with a mercury barometer or an aneroid barometer. The average sea-level pressure is 1,013.2 mb.

Surface winds are controlled by (1) the pressure gradient force, which drives air from high to low pressure; (2) the frictional force, which slows wind speed and is strongest near the surface up to 1–3 km altitude; and (3) the Coriolis force, which deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The Coriolis force is a deflecting force caused by Earth's rotation, named after the French physicist who described it in 1844. It is directly proportional to the angle of latitude — maximum at the poles and zero at the equator. Because it is absent at the equator, tropical cyclones cannot form near the equator.

Geostrophic wind blows in the upper atmosphere (2–3 km above the surface) where friction is absent. When isobars are straight, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar — this is the geostrophic wind.

Near the equator is the equatorial low (Inter Tropical Convergence Zone). Along 30°N and 30°S are the subtropical highs. Along 60°N and 60°S are the sub-polar lows. Near the poles are the polar highs. These belts oscillate with the apparent movement of the sun.

These are the three circulation cells in the general circulation of the atmosphere. The Hadley cell operates in the tropics — air rises at the ITCZ and sinks at about 30°N and S. The Ferrel cell operates in middle latitudes with surface westerlies. The Polar cell has cold air subsiding near the poles flowing towards middle latitudes as polar easterlies.

During the day, land heats faster than the sea, creating low pressure over land; wind blows from sea to land as the sea breeze. At night the land cools faster, creating higher pressure over land; wind blows from land to sea as the land breeze.

Conditions favourable for tropical cyclones include: large sea surface with temperature above 27°C, presence of the Coriolis force, small variations in vertical wind speed, a pre-existing weak low-pressure area or low-level cyclonic circulation, and upper divergence above the sea-level system.

Tropical cyclones originate only over warm seas, move east to west, have higher wind velocity, are more destructive, and lack a frontal system. Extra-tropical cyclones form along the polar front, have a clear frontal system, cover a larger area, can originate over land or sea, and move west to east.

The eye is the calm centre of a tropical cyclone where air subsides. Around the eye is the eye wall, where spiralling air ascends to the tropopause, wind reaches maximum velocity (up to 250 km per hour), and torrential rain occurs. The diameter of the circulating system varies between 150 and 250 km.

ENSO (El Niño–Southern Oscillation) is the combined phenomenon of the El Niño (warm water appearing off the coast of Peru replacing the cool Peruvian current) and the Southern Oscillation (pressure changes in the central Pacific and Australia). Strong ENSO years bring heavy rainfall to the arid west coast of South America, drought in Australia and sometimes India, and floods in China.

Air masses are classified by source region into five types: Maritime tropical (mT) — warm and moist; Continental tropical (cT) — warm and dry; Maritime polar (mP) — cold and moist; Continental polar (cP) — cold and dry; and Continental arctic (cA) — very cold.

A tornado is a spiralling wind that descends like a trunk of an elephant with very low pressure at its centre, causing massive destruction. It can develop from severe thunderstorms and generally occurs in middle latitudes. A tornado over the sea is called a waterspout.