July thru October - Rainy Season - By "rainy", we are talking about evening downpours. It rarely rains for more than an hour at a time. The mornings are usually sunny, but a thunderstorm or two can come towards the late afternoon or at night. This usually starts in late July or August, but by September there is a thunderstorm on most days, often resulting in flooded streets but cooler nighttime temperatures. Because Bangkok is flat, you rarely find any place with more than a foot of water, and it usually drains away ... slowly ... in a few hours. By late September and October, the major rivers are high and there is sometimes permanent flooding along the river, sometimes quite deep. There is a massive investment in pumps to manage the problem, but sandbagging of economically valuable areas and flooding of adjacent areas is not uncommon. This season usually ends abruptly in late October when the winds suddenly change to the north.