Air masses and fronts

An air mass is a huge body of air, often thousands of kilometers wide, with similar temperature and moisture everywhere. It picks up those properties from the land or sea it sits over: a cold, dry air mass forms over frozen Canada, and a warm, humid one forms over the Gulf of Mexico. Air masses travel, slowly changing the weather of everywhere they cross.

When two air masses meet, they do not mix like paint. They stay separate, and the boundary between them is a front. Forecasters draw fronts as lines on weather maps. Because the two sides differ in temperature and moisture, fronts are the storm factories of the middle latitudes: the atmosphere's strongest weather happens right along these edges. A line on the map is really a whole weather system in motion.

Why does warm air go up? Cold air is denser, so it slides under the warm air and lifts it. Rising air cools, its moisture condenses, and clouds form. Along a cold front, a bulging wedge of cold air pushes in quickly and shoves warm air up sharply. Towering clouds bring hard, short downpours and gusty winds, and cooler, clearer weather follows the front's passage.

A warm front is gentler. Warm air glides slowly up a retreating cold air mass over a long distance, so its clouds spread into grey sheets and the rain is steady and lighter. As the warm front passes, the air itself turns milder. Watch the sky and you can sometimes read the sequence overhead: high thin clouds first, then lower, darker layers, just before the front's rain arrives.