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Climate

Manaus has a tropical monsoon climate (Am) according to the Köppen climate classification system, just dry enough in its driest month to not be a tropical rainforest climate, with the average annual compensated temperature of 27.4 °C (81.3 °F) and high air humidity, with a rainfall index around 2,300 mm (90.6 in) annually. The seasons are relatively well-defined concerning rain: July to September is relatively dry, and December to May is very rainy. Thunderstorms are frequent every day in the summer, but they can occur at any time of the year. There have been occasional occurrences of hail in the city.

Due to the city's proximity to the equator, the heat is constant in the local climate. There are no cold days in winter, and rarely very intense polar air masses in the South-Central part of Brazil and in the southwest of the Amazon have some effect on the city, as occurred in August 1955. But although they are rare, they influence the climate, causing the temperature to drop to 18 °C (64.4 °F) or below. The proximity to the forest usually avoids extremes of heat and makes the city wet.

According to the National Institute of Meteorology (INMET), the highest temperature registered in the city was 39 °C (102.2 °F), in 2015 and the lowest was 12 °C (53.6 °F) in 1989.

On November 26, 2009, a case of acid rain was recorded in Manaus. Air pollution, caused in large part by the accumulation of smoke from burning, associated with the carbon dioxide emitted by cars, was the cause of this phenomenon. Although the incidence of acid rain is common in some Brazilian capitals where there is a great concentration of cars, in Manaus and other cities of the Amazonas the situation is aggravated by the prolonged period of drought with the smoke from forest fires.