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Grade 10-11 (age 15-17)

high school Weather Phenomena

When two air masses with different properties meet, a frontal surface forms between them, and near the front, the line where that surface meets the ground, clouds form, rain falls, and the wind and temperature change. At a warm front, warm air rides gently up over cold air and light rain falls over a wide area for a long time; at a cold front, cold air drives sharply under warm air and showers pour down over a narrow area. A mid-latitude cyclone carries these two fronts with it as it moves from west to east with the westerlies, and when the faster cold front catches up with the warm front, it forms an occluded front and the low gradually weakens. In the diagram, switching the type of front compares, in cross-section, how steep the frontal surface is and where the clouds and rain are.

What is a front?

🌦️ When two air masses meet
①When two air masses with different temperature and humidity meet, they do not mix easily, and a boundary forms between them
②That boundary is the frontal surface, and the line where it meets the ground is the front
③Light warm air always rises over heavy cold air, so the frontal surface leans over the cold air
④Warm air cools as it rises and forms clouds, so near a front it rains and the wind and temperature change

Fronts in cross-section

Warm front
💡 What to look for in the diagram
①Warm front: the frontal surface is gentle, so warm air rises slowly over a wide area, stratiform clouds spread far ahead of the front, and light rain falls for a long time
②Cold front: the frontal surface is steep, so warm air is pushed up quickly over a narrow area, cumuliform clouds tower near the front, and showers pour down briefly
③Occluded front: the faster cold front catches up with the warm front, the warm air is lifted off the ground, and the low weakens
④Front symbols point the way the front is moving: red semicircles for a warm front, blue triangles for a cold front, and purple semicircles and triangles for an occluded front

Warm and cold fronts compared

Front comparison

Warm front vs. cold front
FeatureWarm frontCold front
Slope of the frontal surfacegentle (rises about 1 km over 100 to 300 km)steep (rises about 1 km over 50 to 100 km)
Cloudsstratiform (spread out sideways)cumuliform (tower upward)
Rainlight rain over a wide area for a long timebrief showers over a narrow area
Speedslowfast
Temperature after it passesrisesfalls

Mid-latitude cyclones and weather changes

How a mid-latitude cyclone moves
moves west → east with the westerlies / warm front to the southeast, cold front to the southwest
From the center of the low, the warm front extends to the southeast and the cold front to the southwest, with warm air between them.
Weather at one place as a cyclone passes
warm front approaches (cloudy, light rain) → warm front passes (warmer, clearing) → cold front passes (showers, gusts, cooler)
When the warm front passes, the wind shifts from southeast to southwest; when the cold front passes, from southwest to northwest.
📡 Reading a weather map
①The closer together the isobars, the larger the pressure difference and the stronger the wind
②Around a Northern Hemisphere low, the wind turns counterclockwise and blows in toward the center, the air rises, and it is cloudy or rainy
③Around a Northern Hemisphere high, the wind turns clockwise and blows outward, the air sinks, and it is mostly clear

Worked Examples and Exam Practice

Example 1
How do the temperature and weather change right after a cold front passes?
1
At a cold front, cold air drives sharply under warm air, so cumuliform clouds form near the front and showers fall over a narrow area.
2
Once the front has passed, the area is covered by cold air, so the temperature drops and the wind shifts from southwest to northwest.
▸ The showers end, the temperature drops, and a northwest wind blows.
When a warm front passes, the opposite happens: warm air covers the area and the temperature rises. Just ask which air mass the area ends up in after the front passes.
Example 2
Which way does the surface wind turn as it blows in toward the center of a Northern Hemisphere mid-latitude cyclone?
1
In the Northern Hemisphere the Coriolis effect turns the wind to the right, so wind blowing in toward the center of a low turns counterclockwise.
2
Around a high, the wind does the opposite, turning clockwise as it blows outward.
▸ counterclockwise (clockwise around a high)
In the Southern Hemisphere the Coriolis effect acts to the left, so winds turn clockwise around a low and counterclockwise around a high.
School-exam style
Which statement about fronts and mid-latitude cyclones is correct?
①A warm front brings showers to a narrow area
②The temperature falls after a cold front passes
③Mid-latitude cyclones move from east to west
④Around a Northern Hemisphere high, the wind turns counterclockwise
⑤An occluded front forms when a warm front catches up with a cold front
▸ ② The temperature falls after a cold front passes
1
Cold air settles in behind a cold front, so the temperature falls once the front passes.
2
A warm front brings light rain over a wide area (①), mid-latitude cyclones move from west to east with the westerlies (③), the wind turns clockwise around a Northern Hemisphere high (④), and an occluded front forms when a cold front catches up with a warm front (⑤).

Summary

Fronts and weather in brief
front = boundary between air masses with different properties / warm front (gentle, wide rain) / cold front (steep, showers)
Mid-latitude cyclones and their fronts passing from west to east are the main thread of Korea's changing weather.
🎯 Exam Points
①A warm front has a gentle slope and stratiform clouds that bring light rain over a wide area, and the temperature rises after it passes
②A cold front has a steep slope and cumuliform clouds that bring showers over a narrow area, and the temperature falls after it passes
③An occluded front forms when a cold front catches up with a warm front, at the stage when the low is weakening
④A mid-latitude cyclone moves from west to east with the westerlies, with a warm front to the southeast of its center and a cold front to the southwest
⑤In the Northern Hemisphere, winds turn counterclockwise around a low and clockwise around a high
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