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Weather
Water in the Atmosphere
Water Cycle—know it
Humidity
 Measure of the amount of water vapor in the air
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Amount of humidity depends on temp
 Warm air can hold more water than cold air
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Relative Humidity- % of water
vapor in air compared to how much
it can hold at a certain temp
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Measured with a psychrometer
 Two thermometers
 Wet bulb-covered in moist cloth
 Dry bulb
 Psychrometer is “slung”, wet bulb temperature drops due to cooling caused by evaporation
 High relative humidity = slow evaporation, little change in wet bulb temp
 Low relative humidity = fast evaporation, large change in wet bulb temp
 Consult chart to find dew point and relative humidity
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Dew Point- the temperature at which condensation occurs
Weather
 How to use sling psychrometer
Weather
Clouds
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Clouds form when water vapor in air condenses to form liquid water or ice
crystals
 Dew point- temperature at which condensation begins
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Two conditions required for condensation to occur and clouds to form
 Cooling of the air- below freezing = ice , above freezing = water
 Presence of particles- water needs a surface on to condense, such as salt, dust or smoke,
grass, window panes
 Dew – liq. Water that condenses from air onto cooler surface
 Frost – ice deposited on a surface that is below freezing
 Types of clouds- based on shape and altitude
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Cirrus – wispy, feathery, high altitudes, made of ice crystals, indicate fair weather
Cumulus – fluffy, round, low level, short = fair weather, towering = storm
 Cirrocumulus-cotton ball, storm on its way
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Stratus – flat layers, low level, cover all or most of the sky, dull, grey color
Alto- means “high” but are medium level
Nimbo – produces rain or snow
 FOG- clouds that form near the ground, caused by ground cooling at
night after a humid day
Weather
Weather
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Precipitation
Precipitation- any form of water that falls from clouds and reaches Earth’s surface; ex. Rain, sleet, freezing rain, snow,
and hail
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Warm climate- precip. almost always = rain
Colder climate – precip. might be snow or ice
Rain- drops of water, most common type of
precipitation, 0.5mm
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Drizzle – smaller drops
Mist- smaller than drizzle
Rain Gauge- open ended tube that collects rain, ruler or
gauge read to determine how much rain has fallen
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Freezing Rain- rain drops that fall as liquid but freeze
upon contact with a surface,
air temp = above 0oC, ground temp = below ooC
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Snow – water vapor in a cloud is directly converted to ice
air temp = below 0oC, ground temp = below ooC
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Hail – round pellet of ice larger than 5mm, layers of ice inside due to
being carried up and down with in cumulonimbus clouds
air temp = at or above 0oC, ground temp = above, at or below ooC
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Sleet – start as falling rain drops but freeze on way down,
smaller than 5mm
air temp = at or below 0oC, ground temp = above, at or below ooC
Weather
Air Masses
Air mass- huge body of air in the lower atmosphere that has similar
temperature, humidity and air pressure at any given temperature
Four major types
 Maritime tropical
 Continental tropical
 Maritime polar
 Continental polar
 Polar = north of 500N
or south of 50oS
Weather
Air masses moved by:
 Prevailing westerlies-push air masses from west to
east
 Jetstream-found within prevailing westerlies,
bands of high speed winds ~10 km above earth’s
surface
Fronts:
 Fronts- the boundary between where air masses
meet
 Storms and changeable weather often develop along
fronts
Types of Fronts:
 Cold Fronts-
Weather
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Arrive quickly
Abrupt weather changes
Forms when a faster moving cold
air mass pushes under a slower
moving warm air mass, forcing the
warm air to rise
 Denser cold air slides under the lighter warm air, warm air pushed upward
 Thunderheads can form as the moisture in the warm air mass rises, cools, and
condenses. As the front moves through, cool, fair weather is likely to follow
 Warm fronts
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Arrive slowly
Forms when a moist, fast moving,
warm air mass slides up and over a
slower cold air mass
 As the warm air mass rises, it condenses
into a broad area of clouds. If the air
mass contains moisture, it can bring gentle rain or light snow, followed by
warmer, more humid, milder weather
 Occluded fronts
Weather
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Warm air mass caught
between two cooler air
masses
 Denser cool air masses move
under the less dense arm air
mass and pushes the warm
air upward
 Warm air mass is cut off, may cool causing water to condense producing clouds
and maybe rain
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Temperature near ground becomes cooler as warm air mass is occluded or “cut
off”
 Can bring strong winds and heavy precipitation
 Stationary
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Warm and cold air mass meet
Neither moves
Water vapor in warm air mass condenses
into rain, snow, fog or clouds
 If front stalls, there may be many days of
clouds and precipitation
Weather
Cyclones and Anticyclones
 Circled L on weather map stands for “low” & indicates area of low
pressure
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Cyclone-Winds spiral in toward center, counterclockwise (when viewed from
above)
 Warm air at center rises, air pressure decreases
 Air cools, condenses forming clouds and precipitation.
 Circled H on weather map stands for “high” & indicates area of high
pressure
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Anticyclone- winds spiral outward from center, clockwise (when viewed from
above)
 Cool air moves downward from higher in troposphere, cool air warms so relative
humidity drops
 Dry clear weather
Weather
Storms
Storm- violent disturbance in the atmosphere
 Sudden changes in pressure yielding rapid air movements
Winter Storms
 All year round most precipitation starts in clouds as snow, if air is colder than 0oC all the way
to the ground, precip will fall as snow
 Lake effect snow1.) land near lakes cools faster than water
2.) cold dry air mass moves across lake picking up water vapor and heat
3.) air mass reaches other side of lake, rises and cools forming clouds and precip—snow
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Effect of snow
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Buffalo and Rochester, NY
Blockage of roads
People trapped in homes
Damage of crops
Burst pipes
Safety
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Find shelter from wind
Cover exposed parts of body
Keep car engine running only if
exhaust pip is clear
Weather
Storms
Thunderstorms
 Small storm, precipitation, thunder and lightning
 Can form over land or water
 Thunderhead-cumulonimbus cloud that creates
thunderstorm
 Form on hot humid afternoons and evenings
or when warm air is forced up during cold front
 Heavy rain and sometimes hail can be expected
 Within thunderhead, there are strong updrafts
and downdrafts (strong upward and downward winds)
 Most occur during spring and summer in southern states or on
western plains
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Lightning-sudden spark or electrical discharge due to the
positive and negative electrical charges built up between different
parts of clouds or between clouds and the ground
Thunder- caused by the super heating of the air by lightning
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Damage:
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Air expands explosively, thunder = sound of explosion, you see lightning
before you hear thunder because speed of light is much greater than the
speed of sound
Heavy rains cause flooding, lightning can cause fires or damage trees
Safety
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Avoid places where lightning may strike and objects that conduct electricity
Go inside, if stuck outside, find a low area away from trees, fences and poles, stay away from water
Avoid touching telephones, electrical appliances or plumbing fixtures
Weather
Hurricanes
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Starts as Tropical storm (disturbance)
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Lasts longer than other storms (a week or more)
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Powered by warm ocean water
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600 km across
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Winds 119km/h or more
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Formation
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Begins as a low pressure area over warm water
Warm, humid air rises and forms clouds
Winds spiral inward toward the area of low pressure
Winds strongest in narrow band around the storms center
Eyewall= ring of clouds around center which encloses the quiet eye
Winds get stronger as eye approaches, weather changes quickly when eye arrives to calm conditions, after eye
passes storm resumes but wind blows opposite direction
Damage
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High waves
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Storm surge-”dome” of water that sweeps across the coast where hurricane lands-washes away beaches, destroying
buildings, eroding land
Safety
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Watch = hurricane conditions possible w/in 36 hours vs. warning hurricane conditions expected w/in 24 hours
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Evacuate- move away temporarily
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If you hear a hurricane warning and are told to evacuate, leave the area immediately
Weather
Tornadoes
 Rapidly whirling, funnel-shaped cloud that reaches down from a thunderstorm to
touch Earth’s surface
 Water spout- tornado that occurs over a lake or ocean
 Few hundred meters across
 Wind speeds as high as 500km/h
 Develop in thick cumulonimbus clouds, same as thunderstorms
 Often occur in spring and early summer when ground is warm
 Great Plain where most tornadoes occur
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Warm humid air mass moves north from Gulf of Mexico into great plains and a cold dry air
mass moves south from Canada-cold air mass causes warm air to rise rapidly
 Squall line- line of thunderstorms, can produce 10 or more tornadoes
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Damage
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Strong winds cause damage and flying debris
Fujita scale- ranking of tornado based on damage
they cause
Safety
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Move to basement or storm shelter
Stay away from windows
Watch = possible vs. warning =tornado has been
seen
Weather
Meteorologist- scientists who study and predict weather
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Make observations and collect/analyze data using maps, charts, computers and other tech to prepare
weather reports
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Tech and tools = balloons, satellites, radar, surface instruments
Maps
Isobars-lines joining places on the map
that have the same air pressure
Isotherms- line joining places that
have the same temperature
**these work the same way that
contour lines worked on the topographic
map
Maps in newspapers are simplified versions of maps produced by National Weather Service
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Standard symbols on weather maps show
fronts, areas of high/low pressure, types of
precipitation and temperature (oF)
Limits of Forecasts
Butterfly effect- a small change in the weather today can mean a larger change in the
weather a week later
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Name based on scientist suggestion that even the flapping of a butterfly’s wings causes a tiny
disturbance in the atmosphere which may lead to a larger disturbance (storm)
Weather
Newspaper Weather Map
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The symbols on this map show fronts, high- and low-pressure areas, the high and low
temperature readings for different cities, and precipitation. The color bands indicate different
temperature ranges.
Weather
Station modelsThe tables below show what various weather symbols represent.
Weather