Moist air masses from the North Pacific are often drained by coastal mountains before they reach Boise. Thus, clear, sunny weather rules the summers. July is the hottest month, with a sleepable low of 59 degrees. Boise's record high is 111 degrees set in July of 1960. On average, it doesn't freeze in Boise between early May and early October.
Early autumn is bright and idyllic.
Late autumn is often gray and damp. In winter, the usual northwest airflow alternates with warm wet storms from Baja, and with occasional Arctic blasts straight down out of Canada. Cold snaps with low temperatures of 10 degrees or lower often last longer that summer heat waves, but there is rarely much wind to add to the chill. January is the coldest month, with a normal high of 37 degrees and a low of 23 degrees. Boise's record low is -25 degrees, set in December of 1990.