Figuring Out Fronts
Colliding air masses are known as fronts. Like one car rear-ending another, the incoming front—typically from the west in the northern hemisphere—rams into the outgoing front, pushing it eastward. The faster the new front, the more violent the collision and the stormier the resulting weather. There are three types:
1. Warm fronts A warm air mass arrives and rises slowly above the cold air ahead and gradually cools to its dew point. Signs Low barometric pressure, high humidity, low cloud ceiling Result Fairly calm winds (max speed of 20 mph) at the front’s leading edge; steady rain for days
2. Cold fronts Fast-moving, unstable cold air pushes under the warm air ahead, forcing it up quickly and cooling it. Heavy rain might result. Signs High barometric pressure, high cloud ceiling, good visibility unless precipitation is present Result Fair weather that can change quickly; strong winds, generally from the north or west; and severe but brief thunderstorms or snow squalls
3. Occluded fronts A battle royal of three air masses. A fast-moving cold front overtakes a warm front, lifting (occluding) the warm air mass. The incoming cold front then collides with the departing cold air mass. If the incoming cold front is warmer than the departing one (a situation dubbed a warm occluded front, WOF), the new cold front climbs over the exiting one while trapping the warm front high in the middle. If the incoming cold front is colder than the departing one, it wedges under it (aka, a cold occluded front, COF). Signs Wind direction changes, usually so it blows from the north-northwest; falling, then rising barometric pressure Result Storms possible; light to heavy rain followed by dry weather after the front exits. With WOFs, cold temps get milder; with COFs, cold temps get even colder.