Some tropical places get two rainy seasons, and rainforests get none
A rainforest never has a wet season, because its rain arrives evenly all year. Yet parts of the tropics enjoy two, as the band of storms called the Intertropical Convergence Zone sweeps over them twice annually. Northern Guyana gets one downpour season in early spring and another in early winter.
A wet season is the stretch of the year, at least a month long, when a region receives most of its rain. Under the Koppen climate scheme, a tropical month counts as wet if it averages 60 millimetres of rain or more. Tourist boards sometimes rebrand it the green season. Where the rains come with a shift in the prevailing wind, it is called the monsoon.
Timing follows the sun. In savanna and monsoon regions, daytime heat builds thunderstorms in already humid air, so downpours cluster in the late afternoon and early evening, and much of a day's rain falls in the first minutes of a storm. Mediterranean climates flip the pattern, with wet winters and dry summers, from California to Morocco and Iran; Israel's Negev gets its rain from October to May. The Sonoran Desert sits on the border between regimes and receives both kinds of rainy season.
The rains transform landscapes. Air and river water grow cleaner, aquifers recharge and water softens, and vegetation surges, bringing harvests late in the season. Dry gullies called arroyos can fill ten feet deep. There are costs: soils lose nutrients to leaching, erosion accelerates, rivers burst their banks, and mountainsides can slide. Fires in the preceding dry season can make sandy or loamy soils repel water, worsening floods. Malaria and dengue rise where heavy rain coincides with heat, and food can run short because crops planted for the rains, such as cassava, millet or rice, have not yet ripened.
Local cultures name the season in their own ways. In Mexico it is storm season, and in northern Australia First Nations peoples call the December to March wet Gudjewg, a word widely linked to violent storms, flooding and lush growth, though its exact meaning is debated. When the monsoon arrives, tropical days cool and nights warm, shrinking the gap between them.
Source: Wet season
