 In tropical areas , when the monsoon arrives , high daytime high temperatures drop and overnight low temperatures increase . During the wet season , a combination of heavy rainfall and , in some places such as Hong Kong , an onshore wind , improve air quality . In Brazil , the wet season is correlated with weaker trade winds off the ocean . The pH level of water becomes more balanced due to the charging of local aquifers during the wet season . Water also softens , as the concentration of dissolved materials reduces during the rainy season . Erosion is also increased during rainy periods . Arroyos that are dry at other times of the year fill with runoff , in some cases with water as deep as 10 feet ( 3 @.@ 0 m ) . Leaching of soils during periods of heavy rainfall depletes nutrients . The higher runoff from land masses affects nearby ocean areas , which are more stratified , or less mixed , due to stronger surface currents forced by the heavy rainfall runoff . 
