The boundary where air masses with different properties meet is called a front, and fronts cause sudden changes in weather.
Basic Knowledge
An air mass is a large body of air with nearly uniform temperature and humidity. Depending on where it forms, air masses include the Siberian air mass (cold and dry), Okhotsk air mass (cold and moist), and Ogasawara air mass (warm and moist).
Types of fronts:
① Warm front (▲▲ on the warm-air side): Warm air gradually rises over cold air. Nimbostratus clouds bring weak rain over a wide area for a long time.
② Cold front (▼▼ on the cold-air side): Cold air pushes warm air upward. Cumulonimbus clouds develop, bringing heavy rain, lightning, and sudden gusts for a short time.
③ Stationary front: A front that does not move because the forces are balanced (the Baiu front and autumn rain front).
④ Occluded front: A state in which a cold front catches up with a warm front.
Temperate cyclones usually accompany warm and cold fronts and move from west to east.
Air mass（A large body of air with uniform temperature and humidity）
Warm front（Warm air rises over cold air. Widespread weak rain. Nimbostratus）
Cold front（Cold air pushes warm air upward. Localized heavy rain. Cumulonimbus）
Stationary front（A front that does not move. Baiu and autumn rain fronts）
Occluded front（A state in which a cold front catches up with a warm front）
Temperate cyclone（A low-pressure system that develops in the mid-latitudes. Characteristically accompanied by fronts）
Further Study (Background and Significance)
When a cold front passes, the temperature suddenly falls and the wind direction changes from southerly to northerly. Heavy rain and lightning occur because warm air is forced to rise rapidly, causing cumulonimbus clouds to develop.
When a warm front passes, rain is gentle and lasts a long time. Clouds develop in the order cirrus → cirrostratus → altostratus → nimbostratus, and the temperature gradually rises.
Temperate cyclones develop through westerly wave motion (the meandering of westerly winds flowing through Earth's mid-latitudes) and are closely related to north–south waves in the jet stream. This theory, called the wave theory, was established in the 20th century.
The accuracy of weather forecasts is about 86% (Japan Meteorological Agency, next-day forecasts), due to the development of observation networks and computer simulations.
- Warm front: ▲▲ (semicircles) → warm-air side
- Cold front: ▼▼ (triangles) → cold-air side
- Warm front = widespread・weak・long-lasting rain (nimbostratus)
- Cold front = localized・heavy・short-lasting rain (cumulonimbus) + lightning
- Baiu front・autumn rain front = stationary fronts
- Low-pressure systems move from west to east
- After a cold front passes, temperature falls and wind direction changes
Points to Note (Easily Confused)
① Do not reverse the symbols and rain characteristics of warm fronts (▲) and cold fronts (▼). ② Do not confuse nimbostratus (warm front) with cumulonimbus (cold front). ③ An air mass and a front are different (a body of air / a boundary). ④ After a cold front passes, the temperature falls.
Practice
- What type of front is the Baiu front?
- Do cumulonimbus clouds develop at a warm front or a cold front?
- Describe the change in temperature after a cold front passes.