Air Masses, Fronts and Frontal Weather
Air masses
Air masses take the character of their source region: polar or tropical for temperature, maritime or continental for moisture. Know the four main types and their typical weather. Polar maritime is cold, unstable and showery; tropical maritime is warm, moist and stable with low stratus; polar continental is cold and dry; tropical continental is hot and dry. Modification occurs as they travel, and exam questions frequently ask which air mass brings a particular weather type.
Fronts and their sequence
A warm front approaches with a predictable cloud sequence: cirrus, cirrostratus, altostratus, nimbostratus, then stratus, with continuous rain, falling pressure, poor visibility and a gradual temperature rise. A cold front is steeper and faster: cumulonimbus, heavy showery rain, squalls, a sharp wind veer, a rapid temperature fall and often a line squall. A stationary front brings prolonged unsettled weather. An occluded front forms when a cold front catches a warm front and lifts the warm air; it brings mixed weather and often a wide band of cloud and rain. The occlusion is a favourite trap: candidates forget the warm sector is lifted away from the surface.
Traps and worked example
Remember that the warm sector between fronts has warm, moist, often stable air with drizzle and low cloud. Frontal fog may form ahead of a warm front as rain evaporates into cooler air. Pressure falls ahead of a front and rises behind it.
Worked example: an observer notes falling pressure, a wind veering from southeast to southwest, a temperature rise and continuous rain. This is consistent with the passage of a warm front, with the wind veer and temperature rise marking the frontal passage.