in the Grand Canyon region are generally dry, but substantial precipitation occurs twice annually, during seasonal pattern shifts in winter (when Pacific storms usually deliver widespread, moderate rain and high-elevation snow to the region from the west) and in late summer (due to the North American Monsoon, which delivers waves of moisture from the southeast, causing dramatic, localized thunderstorms fueled by the heat of the day). Average annual precipitation on the South Rim is less than , with of snow; the higher North Rim usually receives of moisture, with a typical snowfall of ; and Phantom Ranch, far below the canyon's rims along the Colorado River at gets just of rain, and snow is a rarity. Temperatures vary wildly throughout the year, with summer highs within the Inner Gorge commonly exceeding and winter minimum temperatures sometimes falling below along the canyon's rims. Visitors are often surprised by these potentially extreme conditions, and this, along with the high altitude of the canyon's rims, can lead to unpleasant side effects such as dehydration, sunburn, and hypothermia. Weather conditions can greatly affect hiking and canyon exploration, and visitors should obtain accurate forecasts because of hazards posed by exposure to extreme temperatures, winter storms and late summer monsoons. While the park service posts weather information at gates and visitor centers, this is a rough approximation only, and should not be relied upon for trip planning. For accurate weather in the canyon, hikers should consult the National Weather Service's NOAA weather radio or the official National Weather Service website. The National Weather Service has had a cooperative station on the South Rim since 1903. The record high temperature on the South Rim was on June 26, 1974, and the record low temperature was on January 1, 1919, February 1, 1985, and December 23, 1990. Air quality the Grand Canyon area had some of the cleanest air in the United States. However, air quality in the area can be affected by air pollution from coal-fired power plants, mining, oil and gas, vehicles, and urban and industrial pollution from nearby states, California and Mexico. Events such as forest fires and dust storms in the Southwest can have a considerable impact. Differences in visibility tend to be seasonal: best during the winter and poorest during the summer. Winter cold fronts tend to carry clean, crisp air from the northwest, a less populated area. However they can also carry