Climate Zones and the ITCZ

Average thirty years of weather and a map of the world sorts itself into a handful of climates, stacked by latitude and stirred by the seasons. This lesson draws that map (the five great zones and the in-between belts) and then descends into the tropics, where the daily convection machine, the squall lines, the ITCZ and the easterly waves make the weather that intercontinental routes must cross.

The engine, recalled

The frame for everything here is the wind module's three-cell circulation: air rises at the equator, sinks near 30° in the subtropical highs, rises again on the polar front near 60°, and sinks over the poles, with the tropical easterlies (the trade winds) on the equatorward side of the highs and the westerlies with their travelling depressions on the poleward side. All of it lives below the tropopause: the lower stratosphere above carries only weak, broadly zonal flow, with none of the weather-making vertical circulations. That tropospheric circulation, and the pressure-belt ladder of the previous module, are what carve the world into climate zones: each belt of rising air is a belt of rain, each belt of sinking air a belt of drought.

The five climates of the world

The classification the examination uses has five major zones, each defined by its temperature and rainfall character:

Tropical rain climate — hot in every month (the coolest month still above 18 °C), with heavy convective rain: the equatorial belt (the Amazon, the Congo, Indonesia).
Dry climate — evaporation exceeds precipitation: the deserts under the subtropical highs near 20-30° (Sahara, Arabia, Kalahari, central Australia), plus the deep continental interiors.
Mid-latitude climate (warm temperate rain climate) — mild winters and rain in all seasons, delivered by the travelling polar-front depressions: western and central Europe, the eastern USA, eastern China.
Subarctic climate (cold snow forest climate) — long, severe winters, short summers, months of snow cover: the boreal forest belt of Siberia, Canada and Scandinavia.
Snow climate (polar climate) — the warmest month below 10 °C: the tundra and ice caps (the Arctic, Antarctica, Greenland).

The in-between climates

The zone boundaries are not fences, because the whole belt system migrates with the sun, poleward in summer, equatorward in winter, as the previous module established. A place near a boundary therefore lives under one regime in one season and its neighbour in the other, and that alternation is itself a climate. The two classics:

Mediterranean climate — dry summers under the poleward-shifted subtropical high, wet winters when the westerlies and their depressions move back south.

Savannah — equatorward of the deserts, with a wet season when the ITCZ migrates overhead and a dry season when the subtropical high returns.

Tropical convection: the daily engine

Between the transitional belts lies the zone this lesson's second half belongs to. Tropical showers and thunderstorms need intense surface heating, a deep and very moist layer, conditional instability, and a convergence trigger to release it; the tropics supply all four daily. The typical numbers: surface temperatures around 25 to 30 °C, dew points around 20 to 25 °C, and a zero-degree isotherm near 15,000 to 16,000 ft (about 4,500 to 5,000 m), so airframe icing begins far higher than in mid-latitudes. And because the tropical tropopause stands at about 16 to 17 km (55,000 ft and more), the convection has room to build cumulonimbus far taller than anything in the mid-latitudes, as the atmosphere module's tropopause ladder showed.

Squall lines and cloud clusters

Tropical convection rarely stays as single cells. It organises upward in scale:

Cloud clusters — broad masses of cumulonimbus sharing one cirrus canopy, hundreds of kilometres across.

Mesoscale convective complex — the largest scale: a huge, long-lived, organised cluster system that can persist through the night on the rain-cooled outflow of its own storms.

Tropical squall line — the most violent arrangement: cumulonimbus organised into a fast-moving line, hundreds of kilometres long, led by a gust front with squalls, heavy rain and severe turbulence. The West African lines that sweep westward across the continent are the textbook example, and the flight-hazards module returns to what one does to an aeroplane.

The ITCZ: where the trades collide

The Intertropical Convergence Zone (ITCZ) is the boundary where the northeast trades of one hemisphere meet the southeast trades of the other. Converging air can only rise, so the zone is a broken, shifting belt of cloud clusters and cumulonimbus ringing the planet: the rising branch of the whole circulation, and the rain-maker of the tropical rain climate. Yet it is no front: both trades deliver warm, moist tropical air of essentially the same character, so unlike air masses never meet there — what collides at the ITCZ is flow, not temperature.

The zone follows the overhead sun with a lag, and swings much farther over land than over sea: land heats and cools fast, oceans barely move.

January — it lies near or just south of the equator, dipping to roughly 5-15°S over the southern continents.
July — it runs some 5-15°N over the oceans but is dragged to roughly 25-30°N over Asia: the extreme excursion that the next lesson's monsoon story explains.

Flying the ITCZ

In the zone itself the trade winds die away into the doldrums: calms and light, variable winds under a sky that alternates between quiet gaps and walls of cloud cluster and cumulonimbus reaching the tropical tropopause, with heavy showers and thunderstorms — activity that varies enormously from day to day and place to place. For a crew crossing it (the South Atlantic tracks between Europe and South America do so on every flight) the hazards are embedded cumulonimbus with severe turbulence, lightning and torrential rain, icing above the high freezing level, and tops beyond any cruising level: the zone cannot be overflown, so it is picked through with weather radar, expecting deviations and carrying the fuel for them.

Easterly waves

The tropical easterlies themselves are not perfectly steady: every few days an easterly wave (a westward-travelling trough in the trade-wind flow, moving at about 10 to 15 kt) ripples along the belt. Convection organises around it: cloud clusters, showers and thunderstorms concentrate near and east of the trough axis, so the wave's passage turns ordinary trade-wind weather into a day of organised storms and back again. The waves matter twice over: they modulate day-to-day tropical weather, and, as the previous module's storm recipe noted, an easterly wave is a classic initial disturbance from which a tropical revolving storm can grow.