Guam has a tropical rainforest climate (Köppen "Af"), though its driest month of March almost averages dry enough to qualify as a tropical monsoon climate (Köppen "Am"). The weather is generally hot and humid throughout the year with little seasonal temperature variation. Hence, Guam is known to have equable temperatures year-round. Trade winds are fairly constant throughout the year, but there is often a weak westerly monsoon influence in summer. Guam has two distinct seasons: Wet and dry season. The dry season runs from January through May and June being the transitional period. The wet season runs from July through November with an average annual rainfall between 1981 and 2010 of around . The wettest month on record at Guam Airport has been August 1997 with and the driest was February 2015 with . The wettest calendar year has been 1976 with and the driest was in 1998 with . The most rainfall in a single day occurred on October 15, 1953, when fell.