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

2.  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. 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.  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 FRONTWARM FRONT  Cold air pushes under warm air.  Warm air rises, vapor condenses forming clouds.  Temperatures drop as much as 10 o C  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 FRONTOCCLUDED 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.  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.