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 (c) continental (m) marine (T) Tropical
(c) continental
(m) marine
(T) Tropical
(cT) warm, dry
(mT) warm, wet
(P) Polar
(cP) cool, dry
(mP) cool, wet
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 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