Precipitation differs with altitude, and most of Idaho's precipitation comes from the Pacific Ocean. The mountainous region of northern Idaho receives the most precipitation in the form of rain and snow annually, but in the summer, southeast Idaho receives rain from the south. The Clearwater, Boise and Payette river basins receive up to 40 or 50 inches of precipitation per year, but the
Northwestern and Southwestern valleys receive only about 10 inches per year. Snowfall varies widely as well. The greatest long-term snow depth recorded is 182 inches in Mullan Pass between 1942 and 1956. Average snowfall throughout the state ranges from 20 inches in Lewiston to 52 inches in Coeur d'Alene.
Idaho's growing season, the period when there is no ground freezing, is diverse, ranging from about 200 days in Lewiston to a very short period in the mountains. The Snake and Weiser river basins experience the longest growing periods, with the period becoming shorter as you move upstream. The melting snows of the spring and early summer furnish irrigation water during the growing season.
Temperatures also vary with altitude and latitude. The highest recorded temperature in Idaho is 118 degrees Fahrenheit in Orofino, and the lowest recorded temperature is -60 degrees Fahrenheit in Island Park Dam. The highest annual temperatures are found in the flat, southern areas of the state, with an average temperature of about 55 degrees Fahrenheit, and the lowest annual temperatures are found in the northern, mountainous regions, with an average temperature of about 35 degrees Fahrenheit. From the southwest part of the state around Boise to the southeast side of the state, the most continental temperatures are found, and a yearly range could be from -40 degrees Fahrenheit to 110 degrees Fahrenheit. Humidity generally remains low throughout the year.
Warm southern air and moisture bring thunderstorms to southern Idaho in the summer, but largely Idaho is devoid of violent weather activity. Idaho experiences no hurricanes, though small windstorms are common, and tornadic activity is rare. Floods, however, are a threat to many parts of the state, mainly in the spring when winter snow melts and runs down the mountains.
