Wet season The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year. The wet season is a time when air quality improves,  freshwater quality improves,   and vegetation grows significantly. Tropical cyclones , a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction southern hemisphere or counter clockwise northern hemisphere. Mean surface temperature anomalies during the period to with respect to the average temperatures from to See also: Global warming and Urban heat island The fine particulate matter produced by car exhaust and other human sources of pollution forms cloud condensation nuclei , leads to the production of clouds and increases the likelihood of rain.