Weather
Weather or NotUneven heating of the Earth and the rotation
of the Earth causes weather systemsLocal 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
Weather or NotRemember 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
thickCold air moves more slowly and stays
closer together (more dense)Warm low-density air raises and colder
air takes it place: convection
Weather or NotAir 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
Weather or Not
(c) continental (m) marine
(T) Tropical (cT) warm, dry (mT) warm, wet
(P) Polar (cP) cool, dry (mP) cool, wet
APEX Vidoes
Weather or NotWhen 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 airDuring 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
Weather or NotOn 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
HL
Weather or NotAn 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)
Occluded frontOccluded fronts often occur at the end
of a storm, when a cold air mass catches up with a warm front.
Stationary FrontA 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
Coriolis effect and our weatherWeather between 30ºN and 60ºN (and
30ºS and 60ºS) move from west to east◦Weather in other latitudes move
from east to west
Cloud FormationWarm 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
Cloud TypesCirrus- 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”
Cloud Types
CloudsHigh 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
ThunderstormsThunderstorms 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)
ThunderstormsLightning 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)
TornadoesRequired 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
Tropical CycloneLow-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
Tropical CyclonesConditions 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
BlizzardsWinter storms that bring low
temperatures, high winds, heavy snow, and poor visibility
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