8.10 Weather Patterns

1. 8.10 Weather Patterns Objectives: Identify two types of pressure systems. Identify and describe what causes weather patterns. Explain why it is useful to understand weather patterns.

2. Pressure Systems • Cool air masses have high pressure, or more weight. • Because, molecules are closer together. • Warm air masses have low pressure. • Because, molecules are further apart.

3. A high-pressure system is a large body of circulating air with high pressure at its center and lower pressure outside of the system. • Dense air sinks, bringing clear skies and fair weather.

4. Low-pressure system is a large body of circulating air with low pressure at its center and higher pressure outside of the system. • Air inside system rises and cools • Water vapor condenses forming clouds and sometimes precipitation.

5. Pressure Systems • High-Pressure System • Air spirals away • Air sinks at center • Clear skies and fair weather • Low-Pressure System • Air spirals to center • Air rises at center • Clouds and sometimes precipitation Forms as a result of pressure differences

6. Air Masses • Large bodies of air that have uniform temperature, humidity, and pressure are called air masses. • Forms when air mass stays over an area for several days. • System takes on the temperature and moisture characteristics of the surface below it. • Can extend over large areas, e.g. 1000 km.

7. Air Mass Classification

8. Air Mass Classification (continued) • Classified by temperature and moisture characteristics. • Those that form over land are referred to as continental. • Over oceans, maritime. • Equatorial regions, tropical • Cold regions, polar • Near the poles, arctic or antarctic.

9. Fronts • Weather front is boundary between two air masses. • Collisions between air masses causes changes in • Temperature • Humidity • Cloud types • Wind • Precipitation • These changes can be severe. • Types of fronts: cold front, warm front, stationary front, and occluded front.

11. Cold Front Warm Front • Cold air pushes under warm air. • Warm air rises, vapor condenses forming clouds. • Temperatures drop as much as 10 oC • Wind gusty and changes direction • Showers and thunderstorms along front, some severe. • Warm, less dense air moves toward and above cold air mass. • Vapor in warm air condenses creating a wide blanket of clouds. • Warmer temperatures and change in wind direction. • Steady rain or snow for several hours, even days.

13. Stationary Front Occluded Front • When an approaching front stalls with warm air on one side and cold on the other. • Cloudy skies and light rain. • Can last several days • Cold fronts move faster than warm fronts. • Forms when cold front catches warm front. • Usually brings precipitation.

14. Severe Weather Thunderstorms

15. Thunderstorms (continued) • Warm temperatures, moisture, and rising air from a low-pressure system. • Three stages: • Cumulus stage: warm air and moisture rise forming cumulonimbus clouds, 10 km high. • Mature stage: continued lifting of warm, moist air with downdrafts of colder air. Heavy winds, rain and lightning. • Dissipation stage: up drafts stop, winds die down, lightning stops, and precipitation weakens.

16. Tornadoes

17. Tornadoes (continued) • A violent whirling column of air in contact with the ground. • Several hundred meters to 1500 m in diameter. • Whirling winds can reach more than 400 km/h (250 mi/h). • Form when thunderstorm updrafts begin to rotate. • Most frequent in central U.S., from NE to TX, where cold air from Canada collides with warm moist air from the Gulf of Mexico.

18. Hurricanes

19. Hurricanes (continued) • Intense tropical storms with winds exceeding 119 km/h (75 mi/h), and average 480 km in diameter (150 thousand times bigger than a tornado). • Formation is described on previous slide. • Damage caused by strong winds and flooding. • Looses strength when moving over land or colder water. • Called typhoon in Asia, and cyclone in Australia.

20. Winter Storms

21. Winter Storms (continued) • Snow and ice can be dangerous. • Ice storms can coat trees, ground and buildings with ice. • Blizzard is a violent winter storm characterized by freezing temperatures, strong winds, and blowing snow. • Hazards: • Reduced visibility • Frostbite • Hypothermia

22. Severe Weather Safety • U.S. Weather Service issues watches and warnings • A watch means that severe weather is possible. • A warning means that severe weather is already occurring. • Know how to protect yourself during dangerous weather.