Describe how large air masses form over different regions and explain what happens to weather when two air masses meet at a front.
An air mass is a huge body of air, sometimes thousands of miles across, that sits over one region long enough to take on that region's temperature and humidity. Air sitting over the Gulf of Mexico becomes warm and humid; air sitting over northern Canada becomes cold and dry. When wind eventually pushes that air mass elsewhere, it carries its original temperature and moisture with it.
A front is the boundary where two different air masses meet, and fronts are where most noticeable weather changes happen. At a cold front, a cold air mass plows underneath a warmer one, shoving it up quickly, which often causes short, intense storms followed by clearing, cooler air. At a warm front, a warm air mass slides up and over a retreating cold one much more gradually, often bringing a longer stretch of light rain before it warms up.
Mastery looks like: They can explain what an air mass is, why cold air sinks under warm air at a front, and predict the general weather difference between a cold front and a warm front.
Common stumbles: Assuming all fronts bring the same kind of weather; forgetting that density, cold air being heavier, is the reason cold air wins the shoving match at a front.