Air Masses and Fronts

An air mass: weather with a home address

Imagine a huge room where the air has been sitting still for a week. It slowly takes on the character of the room: warm or cold, damp or dry. The atmosphere works the same way. When a giant body of air lingers over one kind of surface for days, it soaks up that surface’s temperature and moisture. Meteorologists call this an air mass, and the place it formed is its source region.

Air masses are enormous, often covering a large part of a continent and reaching several kilometres up. Inside one, conditions are fairly similar from place to place. The real action happens at the edges, where one air mass meets another.

Air masses are named with two ideas: whether they formed over land (continental, so dry) or over water (maritime, so moist), and whether they come from cold or warm latitudes (Arctic, polar or tropical).

The three visitors that shape Lethbridge weather

• Arctic air (continental Arctic). Born over the snow and ice of northern Canada in winter, where the sun barely rises. Bitterly cold, very dry and very dense. This is the air behind our deep-freeze cold snaps.
• Pacific air (maritime polar). Starts out cool and moist over the North Pacific. By the time it has climbed over the coastal ranges and the Rockies, it has dropped much of its moisture on the western slopes and arrives here milder and drier. Strong westerly Pacific flow is the ingredient behind our chinooks.
• Gulf moisture (maritime tropical). Warm, humid air from the Gulf of Mexico. It usually stays well to our south and east, but in late spring and summer a southerly flow can pull some of it north across the Great Plains, adding fuel for thunderstorms.

Fronts: where air masses collide

Different air masses do not mix easily, much like oil and vinegar in a salad dressing. Cold air is denser and heavier, so it hugs the ground; warm air is lighter and gets pushed up and over. The narrow boundary between two air masses is called a front, a name borrowed from the battle lines of the First World War. Fronts are where clouds, precipitation and wind shifts tend to concentrate.

Cold front: the bulldozer

A cold front is the leading edge of advancing cold air. Because cold air is heavy, it plows under the warmer air like a snowplough blade, shoving it upward steeply. That fast, forceful lift can build towering clouds, gusty winds and heavy but fairly short bursts of rain, snow or thunder. Behind it you usually notice a wind shift (often to the north or northwest here), falling temperatures, a rising barometer and, often, clearing skies.

Warm front: the slow glide

A warm front is the leading edge of advancing warm air. Warm air cannot push cold air out of the way, so it glides gently up and over it, like a car slowly climbing a long ramp. The lift is gentle and spread over hundreds of kilometres, so the clouds come in layers. High, wispy cirrus may show up a day or more ahead, followed by a thickening grey sheet (altostratus), then low, dark nimbostratus with long periods of steady rain or snow.

Stationary and occluded fronts

When neither air mass is winning, the boundary stalls and becomes a stationary front. Clouds and precipitation can then linger in the same area for a day or more. An occluded front forms later in a storm’s life, when a faster cold front catches up to a warm front and lifts the warm air right off the ground. On weather maps, cold fronts are drawn in blue with triangles, warm fronts in red with half-circles, stationary fronts alternate the two, and occluded fronts are purple.

Arctic fronts: the big one for Lethbridge

The most dramatic front we see here is the Arctic front, the leading edge of a fresh Arctic air mass. Arctic air is so cold and dense that it behaves almost like a liquid. It is often quite shallow, sometimes only a kilometre or two deep, and it pours across the land the way spilled syrup spreads across a table.

That is where the Rockies come in. Shallow Arctic air usually cannot climb over the mountains, so they act as a wall that steers it south along the foothills. Picture water poured into a tilted gutter: it runs along the edge. This is why a cold wave can race down the east side of the Rockies from northern Alberta toward Montana while British Columbia, on the other side of the wall, stays much milder.

Sometimes the cold air and the chinook wrestle back and forth for days. Places only a short drive apart can sit on opposite sides of the boundary: brilliant sunshine and plus temperatures in one town, ice fog and minus temperatures in the next.

The Alberta clipper

An Alberta clipper is a small, fast-moving low pressure system that forms on the east side of the Rockies, often in Alberta, and then races southeastward toward the U.S. Midwest and Great Lakes. The name comes from the clipper ships of the 1800s, which were famous for their speed.

Because clippers start on the dry side of the mountains, they do not carry much moisture. They usually bring a quick burst of snow, often light, along with strong winds and a fresh shot of cold air behind them. Here at home, a developing clipper may first bring wind and mild air, then a turn to colder, gusty northwest winds once it pulls away. Further downstream, clippers are known for blowing snow and sudden cold.