During the 20th century, precipitation increased by about 0.5-1 percent per decade over most middle and high latitudes of the Northern Hemisphere's continents. Rainfall over the sub-tropical Northern declined about 0.3 percent per decade, while no significant change occurred over the tropics.
In North America, precipitation has increased significantly. Precipitation in the United States has increased by an average of 5-10 percent in the last century. Along the northern tier states and in Southern Canada, rainfall has increased 10-15 percent. Much of the increase in rainfall has been taking place between September and November. Rainfall is also tending to be more concentrated in heavy downpours, according to studies by the National Oceanic and Atmospheric Administration NOAA. At the beginning of the 20th century, only 9 percent of the nation experienced a storm each year in which more than two inches of precipitation fell in a 24-hour period. In recent decades, such a severe storm has occurred each year over close to 11 percent of the nation.
