Jet Streams and Cyclones

High above our heads, rivers of air race around the planet, steering storms and shaping the weather. Closer to the ground, great whirling systems of low pressure (cyclones) bring rain and, at their fiercest, devastation. Understanding jet streams and the two families of cyclones is essential to physical geography and frequently tested.

Jet streams are narrow belts of very fast wind near the top of the troposphere, often described as "high-altitude rivers of air." Their key features:

• they blow at altitudes of about 5–12 km, generally from west to east,
• they flow in both hemispheres, the Southern as well as the Northern,
• their speeds are very high, about 300–500 km per hour, faster in winter than summer, and
• they meander in great waves and steer surface weather systems, including cyclones.

The main types are the polar front jet stream (where polar and tropical air meet, around 40°–60° latitude), the westerly sub-tropical jet stream (important for India and the monsoon), and the easterly tropical jet stream. The sub-tropical jet shifts its position just before the Indian monsoon begins. Its movement is closely linked to both the onset and the retreat of the monsoon.

A cyclone is a system of low pressure with winds spiralling inward: anticlockwise in the Northern Hemisphere and clockwise in the Southern (due to the Coriolis effect).

Temperate (extra-tropical) cyclones form in the middle latitudes (about 35°–65°) along the boundary (front) between warm and cold air masses. They are:

• large (up to ~1000 km across) but generally less violent than tropical cyclones,
• more active in winter, and
• they move from west to east, bringing widespread, gentle-to-moderate rain.

Tropical cyclones are intense, violent storms that form over warm tropical oceans and strike coasts with ferocious winds, torrential rain and storm surges. The same storm has different names in different regions:

• Cyclones in the Indian Ocean,
• Hurricanes in the Atlantic,
• Typhoons in the western Pacific and South China Sea, and
• Willy-willies in western Australia.

Atlantic hurricanes carry names from an official list. Hurricane Gustav and Hurricane Ike, for example, struck the United States Gulf Coast in 2008.

They draw their energy from the warm sea's moisture. Once they cross the coast (landfall), that moisture supply cuts off and they weaken quickly.

Several conditions must come together before a tropical cyclone can be born:

• Warm ocean water: sea-surface temperatures of roughly 27°C or more supply the heat and moisture that fuel the storm.
• Coriolis force: the Earth's rotation gives the storm its spin. It is too weak very close to the equator, so cyclones do not form there.
• A low-pressure convergence zone: the Inter-Tropical Convergence Zone (ITCZ), the belt where the trade winds of the two hemispheres meet, provides the converging, rising air that seeds cyclone formation.

The last condition explains a famous puzzle. Tropical cyclones do not originate in the South Atlantic or the South-Eastern Pacific. Those seas can be warm enough, and the Coriolis force operates there. The missing ingredient is the ITCZ, which seldom shifts into these regions. Without that low-pressure convergence zone, cyclones fail to form.

A cyclone is a low-pressure system, so surface winds spiral inward and converge. The converging air has nowhere to go but up, so air rises at the centre of a cyclone. The opposite system is the anticyclone: high pressure at the centre, descending air, and winds spiralling outward. Keep the pair distinct, since descending central air belongs to anticyclones, not cyclones.

Only an intense, mature tropical cyclone develops a calm, clear centre called the eye (15–250 km across); weaker storms lack one. Within the eye itself the air gently sinks, and sinking air warms by compression. The eye is therefore warmer than its surroundings, not cooler. The eye wall surrounds it with the storm's strongest winds, up to about 250 km/h.

Two other short, violent local storms are worth knowing. Thunderstorms form from intense convection and bring cumulonimbus cloud, thunder and lightning. Tornadoes are narrow, extremely violent whirling columns of air, called waterspouts when they occur over the sea.