When air sits for a long time over cold, dry land in winter or over a hot, humid ocean in summer, it becomes a huge body of air that takes on that region's temperature and humidity: an air mass.
When two air masses with different properties meet, they do not mix easily and a boundary surface forms; the line where that surface meets the ground is a front.
Choosing the warm front in the diagram shows warm air gliding up a gentle slope over the cold air, making wide clouds and light rain; choosing the cold front shows cold air cutting steeply under the warm air, making towering clouds and short, heavy showers.
Which air is doing the pushing decides the slope, the clouds, the rain and the temperature after the front has passed.
What Is an Air Mass?
🌬️ A Huge Body of Air with Similar Properties
①An air mass is a huge body of air spread over a wide area with similar temperature and humidity throughout
②Air that stays in one place takes on its character: dry over continents, humid over oceans, cold at high latitudes, warm at low latitudes
③Air masses affecting Korea, for example: Siberian (winter, cold and dry), Okhotsk Sea (early summer, cool and humid), North Pacific (summer, hot and humid)
Types of Fronts
Warm Front
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Cross-sections of a warm front and a cold front showing how the air moves and where clouds and rain form
Comparing fronts
Warm Front vs Cold Front
Feature
Warm front
Cold front
Movement
Warm air pushes toward the cold air
Cold air pushes toward the warm air
Slope of the front
Gentle
Steep
Clouds
Stratus type (spread wide)
Cumulus type (built tall)
Rain
Light rain over a wide area for a long time
Heavy showers over a narrow area, briefly
After it passes
Warmer, rain stops
Colder, skies clear
Summary
Fronts
Front = the line where the boundary between two different air masses meets the ground
There are warm fronts, cold fronts, occluded fronts (formed when a cold front catches up with a warm front) and stationary fronts (where two air masses are evenly matched and stay put)
🎯 Common Test Points
①An air mass is a body of air with similar temperature and humidity over a wide area
②A front is where the boundary between two different air masses meets the ground
③Warm front: gentle slope, stratus clouds, light rain over a wide area
④Cold front: steep slope, cumulus clouds, heavy showers over a narrow area
⑤A faster cold front that catches up with a warm front forms an occluded front; the early-summer rainy-season front in East Asia is a stationary front
Examples and Unit Review
Example 1
How do warm and cold fronts differ in air movement, clouds and rain?
1
At a warm front the warm air rises gently over the cold air, so stratus clouds spread widely and light rain falls over a wide area for a long time.
2
At a cold front the cold air cuts sharply under the warm air and forces it up quickly, so cumulus clouds tower and heavy showers fall over a narrow area.
Switching between the two fronts in the diagram shows that the gentler the slope, the wider the clouds spread sideways.
Example 2
How do temperature and weather change after a cold front passes, and how does an occluded front form?
1
After a cold front passes, cold air moves in, the temperature drops, and the skies clear once the showers end.
2
A cold front moves faster than a warm front, so when it catches up with the warm front ahead and the two merge, an occluded front forms.
▸ It gets colder and the skies clear. An occluded front forms when a faster cold front catches up with a warm front and merges with it
A mid-latitude low usually has a cold front to the west and a warm front to the east; over time it occludes and weakens.
Unit Test
Which statement about air masses and fronts is correct?
①After a warm front passes, the temperature drops
②A cold front brings light rain over a wide area for a long time
③A front is the boundary where two air masses of different properties meet
④Stratus clouds develop at a cold front
⑤An occluded front forms when a warm front catches up with a cold front
▸ ③ A front is the boundary where two air masses of different properties meet
1
After a warm front passes the temperature rises (① wrong), and wide light rain is a feature of a warm front (② wrong).
2
Cumulus clouds develop at a cold front (④ wrong), and an occluded front forms when a cold front catches up with a warm front (⑤ wrong); a front is the boundary where two air masses meet (③ correct).