In the winter, cold, dry air masses tend to dominate the central and eastern United States as cold winds blow from the northwestern Canadian prairies. In summer, the warm southern winds blow moist, hot air into the eastern and central US, leading to days of high humidity and sultry conditions. In spring and fall, moving low and high pressure areas pull alternating warm and cold air masses across a given area, leading to a lot of variability in day-to-day weather.
