An air mass is a large uniform volume of air that has consistent properties of density, temperature, and humidity, compared to the air surrounding this air mass which may have different properties.
Why do I care? The type of air mass at a location will determine the type of weather conditions the site experiences.
Figure A. Different types of air masses.
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 “pushed” 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.
How does this relate to forestry?
In a given area, the frequency of particular air masses will help determine the climate of that place, which will affect the types of trees that can be grown successfully there. A hot, dry air mass can cause drought, which can stress and kill trees, and increase the risk of devastating wildfires. At the boundaries between air masses, the clash of masses of air with different characteristics can lead to dynamic weather like hail, tornadoes, high winds or ice storms, which can cause windfall or broken limbs.
Want to learn more?
Links to National Science Education Standards:
7th grade science: 7.E.1.3 : Explain the relationship between the movement of air masses, high and low pressure systems, and frontal boundaries to storms (including thunderstorms, hurricanes, and tornadoes) and other weather conditions that may result.
Earth Science: EEn.2.5.2 : Explain the formation of typical air masses and the weather systems that result from air mass interactions.