Air is not the same everywhere around the globe. It can vary in temperature and humidity depending on where the air came from as it blows around. When air sits over a location for a long time, it takes on the characteristics of the underlying surface. We call these large "bubbles" of air, air masses. An air mass that forms over the Arctic in winter would be cold and dry, while an air mass that formed over a tropical ocean would be warm and moist.
Air masses are â€śpushedâ€ť around the globe by wind and follow the pattern of the troughs and ridges in the flow of air around the earth. 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.