Rising moist air favours rain near the ITCZ. Descending air suppresses rain in the subtropics; seasonal migration shifts these belts.
A high solar angle concentrates incoming energy on a smaller surface area. Over the year, the tropical Earth-atmosphere system absorbs more solar energy than it loses to space as longwave radiation. This radiative surplus helps drive circulation that transfers heat towards higher latitudes. Global-scale circulation spans the Earth and includes persistent patterns such as the Hadley cells. Synoptic-scale circulation covers regional high- and low-pressure systems over large parts of continents and oceans, including tropical cyclones.
Near the intertropical convergence zone (ITCZ), trade winds converge and moist air rises. As pressure falls with height, the air expands and cools. Condensation forms clouds and releases latent heat, helping sustain deep convection. Frequent intense rainfall is therefore associated with the rising branch of the Hadley circulation.
Air spreads polewards aloft, then descends in the subtropics. Compression warms the descending air and lowers its relative humidity. This inhibits cloud development and helps explain subtropical aridity. Near the surface, air returns towards the ITCZ; Earth's rotation deflects these trade winds westwards.
The heating maximum, ITCZ and Hadley circulation shift towards the summer hemisphere, with a seasonal lag. Places under the rain belt for much of the year are generally wetter. Places reached seasonally have distinct wet and dry periods. The shift is larger over continents than oceans. Topography, ocean currents and monsoon circulations modify this broad model.
Step by step
Heating surplus
Strong tropical heating supplies energy for atmospheric motion.
Ascent and descent
Rising moist air cools and condenses; descending air warms and becomes less humid.
Seasonal movement
The rain belt shifts, changing how long each place experiences rising air.
Worked example: A savanna wet season
A tropical location north of the equator may receive summer rain when the ITCZ moves north. When the main rising-air belt moves south, subsidence and drier winds become more influential. This explains seasonality; it does not mean a cloud-free dry season every year.
Watch out for this
The ITCZ stays exactly over the equator.
Its position changes with the distribution of heating and is not a perfectly continuous, straight belt.
Check your understanding
Why does descending subtropical air often discourage rainfall?
- It warms by compression, reducing relative humidity.
- It expands and cools as pressure rises.
- It removes the Sun's energy from the tropics.