The planet's heat engine
Global circulation is the long term movement that carries the tropical radiation surplus toward the energy deficit polar regions. It is a model for the average pattern, not for today's surface chart.
Start with Oxford's idealisation: an all sea Earth with the thermal and geographic equators together. Unequal heating still makes air rise in the hot equatorial belt and sink further poleward. The Earth's rotation then turns the flow and breaks the single cell picture into the Hadley, Ferrel and Polar cells in each hemisphere.
In the Hadley cell, moist air converges and rises at the ITCZ, giving extensive Cu and Cb, thunderstorms and tropical rain. Aloft it moves poleward, and near 30 degrees it cools and sinks into the subtropical highs. That descent suppresses cloud and creates the dry subtropical belts. Some surface outflow returns to the ITCZ as the trade winds, and some turns poleward as the westerlies. The polar cell pushes cold air out towards mid latitudes beneath warmer air, forming the polar front.
Joshi adds the working vocabulary: the equatorial doldrums, the ITCZ, the NE and SE trades, the horse latitudes, the roaring forties, the polar easterlies and the zonal westerlies. Mid latitude disturbances are normally steered west to east by the upper westerlies, while tropical disturbances are steered westward by the tropical easterlies and can recurve once they reach the transition zone.
Climate zones move with the Sun
A climate classification describes the typical temperature and precipitation of a place over many years. It is a planning clue, never a replacement for a forecast.
Oxford's basic belts are warm and wet equatorial, warm and arid subtropical, cool and wet temperate, and cold and dry polar, with a large continental land area in the north. The five broad Koeppen classes refine that: A tropical rain, B dry, C warm temperate, D snowy forest or sub arctic, and E snow or polar. The tropical transitional belt, about 10 to 20 degrees of latitude, has dry trade wind winters and a wet summer. The temperate transitional belt, about 35 to 40 degrees, has a warm dry summer and a cool wet winter.
The axis tilt is 23 and a half degrees. The Sun is overhead at the Equator at the March and September equinoxes, at 23 and a half degrees N at the June solstice and at 23 and a half degrees S at the December solstice. So the thermal equator, the ITCZ and the climatic belts all migrate into the summer hemisphere. Near the Equator rain falls in most months, often peaking near the equinoxes; in the transitional tropics the summer brings tropical rain while the winter has drier trade wind weather.
This seasonal shift also explains the wind belts. SE trades that cross the geographic equator can veer into a southwesterly monsoon flow, and NE trades crossing the other way can back into a northwesterly flow. The three cell diagram is a starting point for reading real seasonal weather, not a fixed map.
Land, sea and terrain reshape the pattern
Latitude sets the baseline, but land warms and cools faster than sea. Terrain, ocean currents and season distort every ideal climate chart.
Over continents, summer heating lowers surface pressure and can break down a subtropical high, while winter cooling builds pressure. January Asia is exceptionally cold because it is far from the sea, with long nights and steady radiation loss, and the Himalayas then block most of that cold air from India and Pakistan. In North America the Rockies limit the warm Pacific influence while an open northern route lets Arctic air spill south. The warm North Atlantic current and the westerlies keep western Europe mild. The more oceanic Southern Hemisphere stays closer to the ideal latitude pattern.
Annual and daily temperature ranges are largest over continental interiors and smallest over oceans and coasts. The freezing level also changes with latitude, season and land heating: near the Equator it is high, elsewhere it varies widely, especially over land. Relative humidity follows a similar latitude and season pattern. Treat these as planning clues for density altitude, icing and cloud depth, not as fixed flight levels.
Joshi applies this to India. The Himalayas, the Thar Desert and the surrounding oceans give tropical conditions in the south and temperate or alpine conditions in the north. India receives roughly three quarters of its annual rain from June to September, but totals range from very arid western Rajasthan to extremely wet Meghalaya. The IMD working seasons are winter in January and February, pre monsoon from March to May, the southwest monsoon from June to September, and the post or northeast monsoon in October and November. The pre monsoon heat weakens the pressure gradient and brings dust haze, dust storms, Norwesters and thunderstorms.
Surface winds, the ITCZ and monsoon weather
A wind name is shorthand for a circulation and its weather. Pair the belt with its vertical motion, its season, its sea track and the terrain before you predict cloud or visibility.
The temperate latitude westerlies lie between the subtropical highs and the temperate lows, broadly 40 to 60 degrees, and are at their most persistent and stormy over the open Southern Ocean, the roaring forties. The trade winds converge toward the thermal equator, NE trades to the north and SE trades to the south. The eastern tropical oceans are often fairer, while convergence near the ITCZ makes cloud, showers and instability more likely.
The ITCZ sits on the thermal equator, not necessarily the geographic Equator. It is usually only tens to a few hundred miles wide, has no true frontal surface, and can hold extensive Cu, Cb and thunderstorms, or, in stable air, As and Ns with continuous rain. Tops can exceed 50,000 ft and severe turbulence and icing can begin near the high freezing level. The heaviest average rainfall is equatorial, and intense tropical thunderstorms are frequent over the East Indies.
For India, the southwest monsoon arrives in two branches, Arabian Sea and Bay of Bengal. The monsoon trough normally runs across the Gangetic plains toward the head Bay: south of it the low level flow is broadly west or southwest, north of it it is east or southeast, and its position governs the rain. When the trough shifts north to the Himalayan foothills a break monsoon leaves the plains drier and moves the heavy rain to the hills. In active periods expect embedded Cb, heavy rain, low cloud and poor visibility.
Seasonal monsoons and upper wind clues
A monsoon is a seasonal wind reversal, driven by continental high pressure in winter or a thermal low in summer, working with the trade winds and shaped by terrain.
Oxford separates three flows. The Asian northeast monsoon from the Siberian high is cool and fairly dry over Bangladesh, Myanmar and Thailand, but its sea track makes southeast India, Sri Lanka and the east coast of Malaysia wet and thundery. The southwest monsoon is SE trade air that crosses the Equator and veers toward the summer Baluchistan low; a long warm sea track gives it heavy Cu and Cb, thunderstorms, turbulence and low cloud bases over India and nearby coasts. India's post monsoon period is the retreating southwest monsoon, with surface winds turning northeast, and cyclones an important late season coastal hazard.
Know the broad route wind rule as a check, not a substitute for forecast winds. From about October to May the winds above 3 km are generally westerly and strengthen with height. From June to September easterlies prevail above about 6 km, while the lower flow is west or southwest south of the monsoon trough and southeast north of it.
Upper winds follow the temperature gradient. Subtropical jets lie near 200 hPa around 25 to 40 degrees in winter and further poleward in summer. The polar front jet follows the moving polar front. The tropical easterly jet appears in the Northern Hemisphere summer, roughly 10 to 20 degrees N. With the southwest monsoon onset the westerly subtropical jet jumps north of the Himalayas and the high level winds over India turn easterly.
Waves: pattern, steering and flight meaning
A wave is a recurring bend in the flow. It matters because it decides where air, moisture, storms and jet stream hazards travel.
Oxford's easterly waves are low pressure troughs that form over West Africa between about 5 and 20 degrees N and move west toward the Caribbean and sometimes beyond. They are most common in summer and autumn, bring weather like a weak tropical storm, and can develop further. Westerly waves are the linked warm and cold front bands of polar frontal depressions moving west to east.
Joshi's standing waves are the time averaged result of opposing components, shaped by mountains, continents, oceans and sea surface temperature contrasts. Their north and south flow moves heat and moisture and alters storm tracks. Rossby or planetary waves are giant ridge and trough meanders in the high level flow and jet streams, with wavelengths of thousands of kilometres, and their position strongly influences the weather pattern. Do not confuse these with the small terrain generated mountain waves.