to October. During El Niño events, precipitation averages about 10 percent above normal, while La Niña events lead to precipitation amounts which average close to 10 percent below normal. Pago Pago harbor in American Samoa has the highest annual rainfall of any harbor in the world. This is due to the nearby Rainmaker Mountain. Atlantic islands Puerto Rico There is a pronounced rainy season from April to November across the commonwealth, encompassing the annual hurricane season. Due to the Commonwealth's topography, rainfall varies greatly across the archipelago. Pico del Este averages of rainfall yearly while Magueyes Island averages only a year. Despite known changes in tropical cyclone activity due to changes in the El Niño/Southern Oscillation (ENSO), there is no known relationship between rainfall in Puerto Rico and the ENSO cycle. However, when values of the North Atlantic oscillation are high during the winter, precipitation is lower than average for Puerto Rico. There have not been any documented cases of snow falling within Puerto Rico, though occasionally it is brought in from elsewhere as a publicity stunt. United States Virgin Islands The climate of the United States Virgin Islands has sustained easterly trade winds through the year. There is a rainy season which lasts from September to November, when hurricanes are more prone to visit the island chain. The average rainfall through the island chain ranges from at Annually to at East Hill. Changes due to global warming Increasing temperatures tend to increase evaporation which leads to more precipitation. As average global temperatures have risen, average global precipitation has also increased. Precipitation has generally increased over land north of 30°N from 1900 to 2005, but declined over the tropics since the 1970s. Eastern portions of North America have become wetter. There has been an increase in the number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics. Over the contiguous United States, total annual precipitation increased at an average rate of 6.1 percent per century since 1900, with the greatest increases within the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (−9.25 percent). From this excess precipitation, crop losses are expected to increase by US$3 billion (2002 dollars) annually over the next 30 years. See