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Weather
Weather
Weather
• State of the atmosphere at a specific time
and place
• Includes:
– Air Pressure (inches of Mercury or millibars)
– Wind (mph or knots)
– Temperature (Fahrenheit or Celsius)
– Moisture (Dew Point or Humidity)
Weather
• Role of the Sun
– The heat evaporates the water into the
atmosphere
– This forms clouds
– Which returns water to the Earth as
Precipitation
– Heats the air
Weather
• Temperature
– Is measure of the movement of molecules
– The higher the temperature, the faster the
molecules move
– The more direct radiation we receive from the
sun, the warmer it feels
– The more indirect radiation we receive from
the sun, the cooler it is
Temperature
Wind
• Wind
– Air moving in a specific direction
– As the sun heats the air:
• It expands
• Becomes less dense
• Rises
• Has Lower Atmospheric Pressure
Wind
• Cold Air
– More Dense
– Sinks
– High Atmospheric Pressure
• Air moves from High pressure to Low
pressure
– Results in wind
What Causes Wind?
High Pressure vs. Low Pressure
Characteristic
High Pressure
Low Pressure
Pressure
Sinking air
Rising Air
Air Flow
Towards Center
Wind Direction
Away from
Center
Clockwise
Weather
Fair
Counter
Clockwise
Clouds and
Precip.
Humidity
Humidity
• The amount of water vapor in the air
• Relative Humidity
– Amount of water vapor in the air compared to
what it can hold at a specific temperature
• Warmer air can hold more water vapor than
cold air
• Dew Point
– The temperature at which air is saturated and
condensation forms
Relative Humidity
Clouds
Cloud Formation
• Clouds form as warm air is forced upward
and cools
• Water vapor condenses to form tiny ice
crystals
• The shape and height of clouds vary with
temperature, pressure and the water vapor
in the atmosphere
• Nimbus, clouds that are dark and full of
water that sunlight can’t penetrate
Rain Shadow
Cloud Nomenclature
Shape
• Stratus
– Smooth, even sheets
– Low Altitudes
• Cumulus
– Puffy, white clouds
– Flat bases common
• Cirrus
– High, thin, feathery
– Made of ice crystals
Height
• Cirro
– High Clouds
• Alto
– Middle elevation
• Strato
– Low clouds
Clouds Types
Cirrus: Thin and Wispy
Cirrostratus: Sheet Like,
Transparent
Altocumulus: Parallel Bands or
Round Masses
Nimbostratus: Dark Low Level
Stratocumulus: Low Lumpy Layers
Cumulus; Fair Weather Cotton
Balls
Cumulonimbus: Thunderstorms
Weather Prediction
Cloud Type
Weather
Cumulus
-Fair when small
-Rain when large
Stratus
-Rain will be coming
Cirrus
- Fair Weather
Cloud Types
Weather Patterns
• Weather patterns change continuously
because air and moisture move in the
atmosphere
• Air Mass
– High and Low Pressure Weather Systems
– Use a barometer to measure air pressure
Air Masses
• Air Mass
– Large body of air with properties like the part
of Earth’s surface over which it formed
• Low Pressure Air Mass
– Stormy weather
• High Pressure
– Fair weather
Fronts
A front is a boundary
between two different
air masses
• Clouds and
precipitation, and
storms occur at the
frontal boundaries
• Cold Front
– Colder air advances
under warm air
• Warm Front
– Warm air advances over
colder air
Fronts
• Occluded front
– Involves three air masses of different
temperatures
• Stationary Front
– Air masses and their boundaries stop
advancing
Cold Front
Warm Front
Occluded Front
Severe Weather
• Thunderstorms
– Occur inside warm, moist air masses and at
fronts
– Warm air is forced up rapidly, where it cools
and condenses
– Strong updrafts of warm air and sinking, rain
cooled air causes strong winds
Cumulus Stage
Mature
Dissipating Stage
Severe Weather
• Lightning
– Movement of air inside a storm cloud that
causes parts of the cloud to become
oppositely charged
– Current flows between the regions of opposite
electrical charge, forming a lightning bolt
Severe Weather
• Thunder
– Lightning superheats the air, causing it to
expand rapidly then contract forming sound
waves
Sever Weather
• Tornados
– A violent, whirling wind that moves in a narrow
path over land
• Hurricane
– A large, swirling, low pressure system that forms
over tropical oceans
• Blizzard
– A winter storm with strong winds, cold
temperatures, and low visibility, that lasts more
then three hours
Severe Weather Safety
• Watch
– Means conditions are favorable
• Warning
– Means severe weather conditions already
exist
– http://www.fas.org/irp/imint/docs/rst/Sect14/
Sect14_1c.html
Weather Forecasts
• Meteorologists
– study and predict the weather
• The National Weather Service makes
weather maps
– Station Models show weather conditions at a
specific location
– Isotherms are contour lines of equal
temperatures
– Isobars are contour lines of equal pressure
– Weather fronts move east to west
Station Model