24.5 Weather Patterns
Reading Focus
Key Concepts
How do air masses form? What are the four types of fronts?- What types of weather are associated with cyclones and anticyclones?
- How do thunderstorms and tornadoes form?
Vocabulary
air mass
front
cold front
warm front
stationary front
occluded front
cyclone
anticyclone
thunderstorm
lightning
thunder
tornado
hurricane
Reading Strategy
Outlining Before you read, make an outline of this section. Use the green headings as the main topics and the blue headings as subtopics. As you read, add supporting details.
Weather Patterns
Air Masses
Fronts
Cold Fronts
________________
________________
________________
“We've been basking in this high-pressure system for a week,” says the weather reporter on a local TV station, pointing to a large weather map of the country. “But here comes trouble in the form of a cold front moving in from the northwest.”
What does it mean to be “basking in a high-pressure system”? Why is a cold front “trouble”? What, in fact, is a “cold front”? In this section, you'll find out the answers to these questions, as well as the meanings of many terms you've heard weather reporters use.
Air Masses
The weather that you experience from day to day is the result of the movement and interactions of air masses in Earth's atmosphere. An air mass is a large body of air that has fairly uni- form physical properties, such as temperature and moisture content, at any given altitude. Air masses can cover large portions of continents. An air mass forms when a large body of air becomes fairly stationary over a region of Earth's surface or as air moves over a large, uniform region like an ocean. The air is strongly influenced by the properties of the region.
Figure 19 A cold front moving in brings stormy weather. The arrow shows the direction that the front is moving.
Table of Contents