A front is the line where the boundary surface (frontal surface) between air masses with different characteristics intersects the ground. Weather changes greatly after passage depending on the type of front.
Basic Knowledge
Fronts are classified into four types.
① Warm front: A front where warm air slowly rises over cold air. Nimbostratus, cirrus, and cirrostratus clouds develop widely in front of the front (to the east), and gentle rain continues for a long time. After passage, temperatures rise, winds come from the south, and the weather improves.
② Cold front: A front where cold air rapidly wedges beneath warm air. Cumulonimbus clouds develop near the front, and heavy rain falls for a short time. After passage, temperatures drop rapidly, northwesterly winds blow, and the weather quickly improves.
③ Stationary front: A front that remains almost in the same place because the strengths of cold and warm air are balanced. The Baiu front and Shuyu front are examples, and rainy weather continues for a long period.
④ Occluded front: A front formed when a cold front catches up with a warm front. It forms during the final stage of the development of a mid-latitude cyclone.
Frontal surface (The boundary surface between air masses with different characteristics)
Warm front (Warm air rises. Gentle, long-lasting rain. Symbol: semicircles)
Cold front (Cold air wedges beneath warm air. Heavy, short-lasting rain. Symbol: triangles)
Stationary front (A front that hardly moves. Baiu front and Shuyu front)
Occluded front (A front formed when a cold front catches up with a warm front. Dissipation stage of a cyclone)
Air mass (A large mass of air that has acquired uniform temperature and humidity over a wide region)
Further Study (Background and Significance)
A mid-latitude cyclone forms when a warm front and a cold front develop together around the same low-pressure system. A cold front extends southwest from the center of the cyclone, while a warm front extends southeast. The warm sector (region of warm air) lies between the two fronts.
Because a cold front moves faster than a warm front, it eventually catches up with the warm front and forms an occluded front. At this point, the cyclone has passed its peak development and begins to weaken.
The Baiu front remains stationary in June and July, when the Okhotsk air mass from the north (cool and humid) and the Ogasawara air mass from the south (warm and humid) have nearly equal strength. Temperatures differ greatly depending on whether one is north or south of the front, and near the end of the rainy season, the front may become active and cause torrential rain.
- Warm front = warm air rises = gentle, long-lasting rain = temperature rises after passage
- Cold front = cold air wedges beneath warm air = heavy, short-lasting rain = temperature drops rapidly after passage
- Stationary front = Baiu front and Shuyu front
- Occluded front = weakening stage of a cyclone
- A mid-latitude cyclone has a cold front to the southwest and a warm front to the southeast
- A cold front moves faster than a warm front
- Front symbols: warm = semicircles, cold = triangles, stationary = alternating symbols on both sides
Points to Note (Common Confusions)
① Do not reverse the rainfall characteristics of a warm front (slow and long-lasting) and a cold front (rapid and short-lasting). ② Remember the temperature changes after passage: warm = rises, cold = falls. ③ A stationary front has not disappeared; it simply moves very little. ④ An occluded front does not mean that the front is “closed”; it is the state in which a cold front has caught up with a warm front.
Practice
- What happens to the temperature after a cold front passes?
- Which of the four types of fronts is the Baiu front?
- Name a representative cloud that develops near a warm front.