Summers can get very hot, sometimes above 38 °C (100 °F) during the day, and with humidity decreasing from northeast to southwest. Warm southern winds blow from the plains and intermontane regions of the Western United States during much of July and August, very cool or hot but changeable air masses often occur during spring and in September. Winters are usually bitterly cold, with frequent Arctic air descending from the north.[10] with high temperatures not breaking −17 °C (1 °F) for weeks at a time.