page_title: United States rainfall climatology
section_title: Guam
hierarchical_section_title: United States rainfall climatology / Pacific islands / Guam
caption_attribution_description: At the time this image was acquired on December 8, 2002, Super Typhoon Pongsona was at its peak strength — Category 4 — with winds of 120 knots (1 knot = 1.15 mph). The image shows Pongsona over the Northern Mariana Islands in the western Pacific Ocean, just south of the Tropic of Cancer, and about 1500 kilometers north of Papua New Guinea. Pongsona had dissipated four times before reaching Typhoon level, and gained a level approximately every day, up to Category 4. However, the descent was quicker. By late on the 9th, Pongsona began losing power, and by the 11th had dropped to the lowest level, Category 1, with winds of 75 knots. After this, it dissipated for good.
context_page_description: The characteristics of United States rainfall climatology differ significantly across the United States and those under United States sovereignty. Late summer and fall tropical cyclones bring precipitation which falls across the Gulf and Atlantic states. During the winter, and spring, Pacific storm systems bring Hawaii and the western United States most of their precipitation. Low pressure systems moving up the East coast bring cold season precipitation to the Mid-West and Northeast states, as well as Great Salt Lake and the Finger Lakes region. The snow to liquid ratio across the contiguous United States averages 13:1, meaning 13 inches of snow melts down to 1 inch of water.
  During the summer, the North American monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic Ocean bring the promise of afternoon and evening air-mass thunderstorms to the southern tier of the country as well as the Great Plains.
context_section_description: Guam's climate is moderated by east to northeast trade winds through the year.  The average annual rainfall for the island is 86 inches (2,200 mm).  There is a distinct dry season from January to June, and a rainy season from July to December.  Typhoons frequent the island, which can lead to excessive rainfall.  During El Niño years, dry season precipitation averages below normal.  However, the threat of a tropical cyclone is over triple what is normal during El Niño years, so extreme shorter duration rainfall events are possible.
caption: Typhoons, such as Pongsona (2002), can bring excessive rainfall to Guam
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