Forecasting Weather

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Forecasting Weather

• http//www.usatoday.com/weather/basics/wworks0.htm

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What are Fronts?

• A boundary between two air masses is called a
  __Front_________
• The shape of the _Front_______ depends on the
  density of the air masses and their speed

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When a front forms

• Pressure falls when a front approaches (low
  pressure edges of air masses move in)
• Changes in wind direction occur. High pressure
  air moves Clockwise, low pressure air moves
  Counterclockwise
• Temperature changes occur
• Precipitation occurs and can be in the form of
  strong storms

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• Low pressure is associated with the formation of
  clouds and precipitation.
• Warm air rises and when it reaches a certain
  elevation it cools and become saturated.
• Clouds form and when they are saturated, it
  precipitates

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There are four main types of fronts
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Warm Front

• Warmer air invades a cold air mass and is lifted
  over and replaces cooler air.
• Cirrus and stratus clouds are associated with
  warm fronts
• Gentle rain for days, slow clearing, and rising
  temperatures follow

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Cold Front

• Cooler air invades a warm air mass and moves
  under the warm air and replaces it
• Cumulus clouds and thunderstorms produce HEAVY
  RAIN for a short period of time
• Cooler temperatures and
• clear and fair weather invade the area once the
  storms have passed

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Stationary Front

• Occurs when pressure differences cause a warm
  front or cold front to stop moving
• It may remain in the SAME area for a few days.
• Weather conditions include light winds and
  precipitation across the entire frontal region.

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Occluded Front

• Results from two cool air masses merging and
  forcing warmer air between them to rise
• Strong winds and heavy precipitation may occur

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How do weather systems move?

• Weather systems move across North America from
  West to East.

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Weather forecasting

• Making predictions about future weather based on
  weather data.

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Weather Maps

• Provide data from stations all over the earth
• Help meteorologists forecast weather
• Data included
• Temperature, air pressure, change in air pressure
  during the last three hours, wind speed,
  direction, dew point, visibility, cloud cover,
  cloud types and precipitation.

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Isobars

• Isobars - pressure lines drawn on weather maps to
  connect places having equal air pressure.
• Isobars that are close together indicate a large
  pressure difference over a small area

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Isotherms

• Isotherms are lines drawn to connect places
  having equal temperatures

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High and Low Pressure

• Pressure is indicated in the middle of the
  isobars
• H stands for a high pressure center
• L stands for a low pressure center.
• Winds develop and move from high to low pressure
  areas.

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Technology and Weather Forecasting

• Satellites photograph sections of the earths
  surface and show cloud coverage

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Air Mass

• A large body of air that has the same properties
  as the surface over which it develops
• An air mass over the GULF of MEXICO would be warm
  and moist
• An air mass over CANADA would be cool and dry

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Air masses

• Cool and dry --gt high pressure, low humidity
  CANADA
• Warm and moist --gt low pressure, high humidity
  GULF of MEXICO

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Thunderstorms

• Occur inside warm moist air masses and fronts
• Result when warm moist air rises quickly and
  cools and condenses rapidly
• Heavy raindrops fall, dragging air with them,
  creating strong winds
• Produced by cumulonimbus clouds

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Lightning

• Atoms of air which are caught in the strong winds
  lose and gain electrons, creating cloud regions
  which are positively or negatively charged
• Currents flow between these regions resulting in
  lightning

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Thunder

• Rapid heating and cooling of air due to lightning
  (rapid expanding and contracting)

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Tornado

• Forms along fronts
• Wind at different heights blows in different
  directions and at different speeds
• This along with a strong updraft produces a
  rotation

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Hurricanes

• A low pressure area develops where the ocean
  water is very warm
• Trade winds blowing in opposite directions meet
  causing a swirl of air that is rotating
• As moist air rises over the low pressure system,
  it cools and condenses and begins to descend
• This pattern is repeated over and over

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Clouds - condensed water vapor

• Recipe for a Cloud
• Water vapor
• Condensation nuclei (dust, smoke, ice, salt)
• Relative humidity of 100 - saturation point or
  dew point

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Types of Clouds Stratus

• fair weather or precipitation
• Layers of sheets of clouds, can be dull gray
  blanket of clouds
• Low altitude clouds

See pages 425-427
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Types of Clouds Cumulus

• White puffy clouds with flat bases
• Formed when moist air rises
• Fair weather or thunderstorms

See pages 425-427
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Types of Clouds Cirrus

• Wispy
• High altitude
• Contain ice crystals
• Fair weather

See pages 425-427