The climate of Guam is characterized as tropical marine. The weather is generally hot and very humid with little seasonal temperature variation. The mean high temperature is 81 °F (27 °C) and mean low is 76 °F (24 °C). The coolest months are January and February, when temperatures fall to mid to low 70s °F (20s °C) at night. Daily maximums and minimums vary no more than 10 °F (6 °C). Relative humidity ranges from 65-80% during daylight hours to 85-100% at night.

Guam has two distinct seasons, one wet and one dry. Most of the average annual rainfall of 96 inches (2,180 mm) falls during the wet season from July through November. The remaining months constitute the dry season. The mean annual rainfall shows significant geographic variation. It ranges from 80 inches (2,200 mm) in central and coastal lowlands (and in northern Guam) to 110 inches (2,790 mm) over the southern Guam uplands. Variations from year to year may be quite large. For instance, the maximum recorded rainfall at Inarajan Agriculture Station was 151.96 inches in 1980, while the minimum was 43.93 inches in 1998.

A subtropical high-pressure area lying north of the island throughout much of the year results in a dominant airflow pattern characterized by trade winds prevailing from the northeast. Frequent storms, common in the summer and fall, disrupt this pattern and ocassionally intensify to typhoon status (Lander, 1994). An average of three tropical storms and one typhoon pass within 180 nautical miles (330 km) of Guam each year. Typhoons occur year-round, but the highest risk is in October and November. The most intense typhoon to pass over Guam recently was Super Typhoon Pongsona, with sustained winds of 125 miles per hour. It hit Guam on December 8, 2002 and caused massive destruction. Because of such risks, many homes and businesses on Guam are made of reinforced concrete and have typhoon shutters.