Chapter 12 Section 8 What is a front? Objective: Describe the different kinds of fronts and the weather they cause.

Key Terms front: boundary between air masses of different densities cold front: forward edge of a cold air mass, formed when a cold air mass pushes under a warm air mass warm front: forward edge of a warm air mass, formed when a warm air mass pushes over a cold air mass

Boundaries of Air In the United States air masses usually travel west to east. As they move they meet up with other air masses Individual air masses do not usually mix A front forms between them Can be hundreds of kilometers long Brings changes in weather

EXPLAIN: What do fronts bring?

Cold Fronts Cold air is denser than warm air As a cold front moves through an area, the warm air over a region is pushed upward Gustly winds are formed Cold fronts usually bring rain and cloudy skies Once the cold front passes, the cold air masses move in

2. DESCRIBE: What kind of weather does a cold front bring?

Warm Fronts Warm air: moves more slowly than cold air rises above cold air As the warm air rises, it cools cirrus clouds form high up precipitation may follow

3. DEFINE: What is a warm front?

Stationary Fronts Sometimes cold and warm air masses stay put for a while they do not move they remain stationary brings very little change in the weather

4. DEFINE: What is a stationary front?

Occluded Fronts occluded front: a warm air mass is between two cooler air masses cooler air masses move beneath the warm air mass and push it up they come together in the middle and may mix makes the temperature near the ground cooler The warm air mass is cut off, or occluded, from the ground as the warm air cools and its water vapor condenses, the weather often turns cloudy, rainy, or snowy

5. DESCRIBE: What happens to the weather with an occluded front?

12-8 Classwork What are four kinds of fronts? How does a cold front form? What kinds of clouds form when a warm front approaches? Which kind of front causes very little change in weather? What kind of weather is common with an occluded front?

12-8 Classwork 6. INFER: A city gets a day of rain and on the next day skies clear. The temperature rises. Which kind of front has passed? 7. INFER: If a city has clear skies and warm temperatures for three days, which kind of front might be keeping the weather the same?