2. Water and Weather

3. Chapter Six: Weather and Climate 6.1 Introduction to Weather 6.2 Weather Patterns 6.3 Climates and Biomes

4. Investigation 6B How does Doppler radar work? Storms

5. 6.2 Meteorology A meteorologist is a person who uses scientific principles to explain, understand, observe, or forecast Earth’s weather.

6. 6.2 Meteorology Many meteorologists have college degrees in physics, chemistry, or mathematics.

7. 6.2 Meteorology Meteorologists use satellite and computer technology to inform people about the weather.

8. 6.2 Water in the atmosphere Higher temperatures cause liquid water molecules to move faster. These water molecules become water vapor in the atmosphere.

9. 6.2 Cloud formation Different conditions cause different clouds. Cumuliform clouds include: –cirrocumulus –altocumulus –cumulus –cumulonimbus

10. 6.2 Cloud formation Stratiform clouds form when a large mass of stable air gradually rises, expands, and cools. Stratiform clouds include: –cirrostratus –altostratus –stratus –nimbostratus

11. 6.2 Cloud formation Sometimes a cloud formation combines aspects of both cumuliform and stratiform clouds. We call these clouds stratocumulus clouds.

12. 6.2 Cloud formation Cirrus clouds are thin lines of ice crystals high in the sky, above 6,000 meters. They are just a thin streak of white across a blue sky.

13. 6.2 Rain Rain is the result of a cooling air mass. Cooling an air mass is like wringing out a wet sponge. Tiny droplets form a cloud or fog. Larger droplets fall as rain.

14. 6.2 Snow Snow usually forms when both ice crystals and water droplets are present in the sky. The water droplets attach to ice crystals and freeze. When the ice crystals are large enough, they will fall to the ground as snow.

15. 6.2 Air masses and fronts An air mass is a large body of air with consistent temperature and moisture characteristics throughout. Two air masses that affect the United States are the continental polar air mass and the maritime tropical air mass. Changing conditions and global winds cause these air masses to move.

16. 6.2 Fronts A cold front occurs when cold air moves in and replaces warm air.

17. 6.2 Fronts A warm front occurs when warm air moves in and replaces cold air.

18. 6.2 Fronts On a weather map, a cold front is shown using a line marked with triangles. The triangles point in the direction the front is moving. A warm front is shown using a line marked with semicircles.

19. 6.2 Low- and high-pressure areas When a cold front moves into a region and warm air is forced upward, an area of low pressure is created near Earth’s surface at the boundary between the two air masses. A center of high pressure tends to be found where a stable cold air mass has settled in a region.

21. 6.2 Thunderstorms Thunderstorms occur because of convection in the atmosphere. The downdraft and updraft form a type of convection cell called a storm cell within the cloud.

22. 6.2 Lightning Lightning is a bright spark of light that occurs within a storm cloud, between a cloud and Earth’s surface, or between two storm clouds.

23. 6.2 Hurricanes A hurricane is a tropical cyclone with wind speeds of at least 74 miles (119 km) per hour. The Saffir-Simpson Hurricane Scale is one scale used for rating hurricanes.

24. 6.2 Tornadoes A tornado, like a hurricane, is a system of rotating winds around a low-pressure center. As the rotating wind pattern narrows and lengthens, it forms a funnel cloud.

25. 6.2 El Nino Southern Oscillations Storm patterns across the globe can happen in cycles. Usually the trade winds blow warm water from east to west across the Pacific Ocean. Every so often the trade winds weaken and the warm water reverses direction.

26. 6.2 El Nino Southern Oscillations Along with warm water comes greater thunderstorm activity across the Pacific. This change in wind flow, air pressure, and thunderstorm activity is known as the El Niño Southern Oscillation.