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  1. Weather

  2. Weather or Not • Uneven heating of the Earth and the rotation of the Earth causes weather systems • Local weather patterns are determined by winds carrying air masses • Air masses are bodies of air with uniform temperature and humidity conditions extending and travelling for miles in the atmosphere • Think of the atmosphere as a body of water that is constantly flowing/moving • Even if its calm at the surface, wind currents higher up may reach 180mph

  3. Weather or Not • Remember that warm air at the surface becomes less dense (spreads out) • Can create a huge dome of warm air with low density • Can be 1000 miles wide and a mile thick • Cold air moves more slowly and stays closer together (more dense) • Warm low-density air raises and colder air takes it place: convection

  4. Weather or Not • Air masses also have different amounts of moisture content: humidity • Depends primarily on where the air mass formed • Tropical waves move off the coast of Africa during spring, summer, and fall and carried westward by air currents near the equator • Source region of an air mass can be maritime or continental; and polar or tropical

  5. Weather or Not APEX Vidoes

  6. Weather or Not • When a cold front moves through, the heavier, cool air pushes under the lighter, warm air • As the warm air rises it begins to cool • If the air is humid enough, it will condense into clouds and precipitation • After the cold front moves in, the region will have cooler and drier air • During a warm front, the warm air catches up with cold air (moving in same direction) • Warm air rises above the cold • Moisture condenses to form rain or snow • APEX Diagrams

  7. Weather or Not • On a weather map symbols represent different pressures • H is for high-pressure area (cold air) • Cooler, clear skies, and fair weather • L is for low-pressure area (warm air) • Warmer air, clouds may form, rain, and storms H L

  8. Weather or Not • An occluded front forms when cold air mass catches up with warm front • Cold air pushes up the warm air, it spirals counterclockwise, and cools • Warm air isn’t felt (occluded=hidden)

  9. Occluded front • Occluded fronts often occur at the end of a storm, when a cold air mass catches up with a warm front.

  10. Stationary Front • A stationary front occurs when warm and cold air meet but neither are moving (battle is a standoff) • Bring days of gray skies • Upper level winds can get them moving again

  11. Coriolis effect and our weather • Weather between 30ºN and 60ºN (and 30ºS and 60ºS) move from west to east • Weather in other latitudes move from east to west

  12. Cloud Formation • Warm humid air rises and cools to the dewpoint • Water condenses into microscopic liquid or ice crystals • Encounters dust particles (condensation nuclei) condensing into droplets and becoming visible

  13. Cloud Types • Cirrus- high, wispy ice clouds (from Latin meaning “curly”) • Alto- from Latin meaning “middle” • Stratus- low to medium height gray clouds (from Latin meaning “spread out”); overcast sky that may result in drizzle or fog • Cumulus- low, heaped-up fluffy white clouds (from Latin meaning “heap”); form below 6000 feet • Nimbus- from Latin meaning “rain”

  14. Cloud Types

  15. Clouds • High clouds, such as cirrus or cirrocumulus, don't drop precipitation but are an indication that rain or snow is on the way. • Middle clouds, like altostratus, usually are composed mostly of water, not ice • Low clouds can mean anything from fair weather to thunderstorms • Vertical clouds span the entire spectrum and are made of both water droplets and ice crystals • APEX Videos

  16. Thunderstorms • Thunderstorms require a few atmospheric conditons: • Moisture in the lower to middle levels of the atmosphere • A lifting force to cause air to begin to rise; typically heated air near the ground • Unstable air (air that will continue rising once it begins rising)

  17. Thunderstorms • Lightning develops from a buildup of charges as tops of clouds become positively charge and bottoms of clouds become negatively charged • Thunder is caused by lightning heating up the air, which causes it to expand and contract quickly (contracting leads to the sound)

  18. Tornadoes • Required conditions: • Wind shear is the change in speed and direction of wind at different altitudes causing columns of clouds to spin • Sinking air pushes on end of the column toward land, while the other end rises; causes faster spinning • Spinning cloud joins a cloud above it, creating a funnel cloud • Funnel cloud touching the ground creates a tornado

  19. Tropical Cyclone • Low-pressure weather systems with circular wind motion that rotates counterclockwise in the Northern Hemisphere • Typhoon: when thunderstorms converge in the western Pacific or off coast of China • Hurricane: when storms are born off the coast of Africa and travel across the Atlantic Ocean

  20. Tropical Cyclones • Conditions required: • Thunderstorms grow over warm ocean waters with hot, humid air • Air spins and rises; air within the system cools because it releases heat • When wind reaches 74mph, it is called a hurricane

  21. Blizzards • Winter storms that bring low temperatures, high winds, heavy snow, and poor visibility