Weather Forecasting

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Weather Forecasting

• Predicting Changes in our Atmosphere

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What Causes Changes in the Weather ?

• Air Masses / Air Movement
• Fronts
• Pressure Systems

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Weather Forecasting

• Because much of modern society depends on weather
  conditions, knowing current weather patterns and
  methods used to determine the future state of the
  atmosphere is important.
• This attempt to predict the future weather is
  called weather forecasting.
• Forecasting is based on probability of occurrence.

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Weather Maps

• Station models are used to record the atmospheric
  variables recorded at different locations.
• These variables are
• amount of cloud cover
• barometric pressure
• temperature
• dewpoint
• wind direction
• wind speed

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Using Isolines

• Isolines are lines connecting points of equal
  value.
• Weather maps use different types of isolines
• Isobars show barometric pressure.
• Isotherms show temperatures.

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Synoptic Weather Maps

• Synoptic weather maps provide a synopsis, or
  summary of weather conditions all over the United
  States at the indicated hour.

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What is an Air Mass?

• Air mass a large body of air in the
  troposphere having similar pressure, wind and
  temperature characteristics throughout.

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Air Mass Characteristics

• Air masses have characteristics that are
  determined by the part of the Earths surface
  that the air mass forms over.
• These areas are called source regions.
• Examples Forming over land dry air mass
• Forming over water moist air mass
• Forming over equator warm air mass
• Forming over the poles cold air mass.

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Source Regions
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• Air masses move and swirl over the surface of the
  Earth in different directions and at different
  speeds and often bump into one another.
• Rain, thunderstorms, snow, tornadoes and all
  weather-related events can result when air masses
  meet.

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What is a Weather Front?

• Front the boundary formed as two air masses
  meet and collide.

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Types of Fronts

• Warm Front forms when a warm air mass slides
  over a departing cold air mass. As warm air
  rises the temperature drops and rain or
  precipitation often forms.
• Cold Front forms as a cold air mass overtakes
  and moves underneath a warm air mass. The warm
  air is forced upward rapidly and a cold front
  forms. Cold fronts usually bring violent weather.

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Types of Fronts (cont.)

• Stationary Front occurs when a cold air mass
  meets a warm air mass and neither air mass moves.
  Precipitation sometimes forms between these two
  fronts.
• Occluded Front forms when two cold air masses
  meet and force a warm air mass between them to
  rise completely off the ground. The weather in
  an occluded front is difficult to predict because
  there are three air masses involved.

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What is a Pressure System?

• Pressure as particles move and collide they
  exert pressure
• Differences in air pressure have a great affect
  on the weather.
• When particles are more densely packed together
  they exert high pressure,
• High air pressure usually means clear, fair
  weather.
• When they are less densely packed they exert less
  (low) pressure.
• Low air pressure usually means clouds and rainy
  weather.

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High Pressure System

• Cold air is dense and it sinks.
• As the air sinks it warms up.
• Warm air holds more water vapor.
• The total amount of water vapor remains the same.
• As the air warms the relative humidity decreases
  as droplets in the clouds evaporate.
• A high pressure system usually means fair weather
  as the moisture in the air is evaporated so few
  clouds form.

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Low Pressure System

• Warm air has low density and it rises (forced
  upwards by surrounding denser air)
• As the air rises it cools
• As the air cools the relative humidity increases
  eventually reaching the dew point
• At the dew point condensation takes place and
  clouds form
• Low pressure systems usually form along fronts
  where warm air and cold air meet
• Low pressure systems cause most of the weather in
  the US.
• A low pressure system leads to precipitation in
  the form of rain, snow, hail, sleet.

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Moving Fronts

• When we witness a change in the weather from day
  to day it is due to the movement of air masses.
  It is the movement and collision of air masses
  that causes the weather to change.

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What is Severe Weather?

• Severe weather are events that fall outside of
  the ordinary weather patterns.
• Severe weather can include the following
• Thunderstorms
• Lightning
• Tornadoes
• Hurricanes
• Floods
• Droughts
• Blizzards
• Fog
• Ice Storms

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Thunderstorms

• Thunderstorms are formed by rapid upward movement
  of warm moist air.
• Thunderstorms can occur within a warm moist air
  mass.
• Thunderstorms most often occur at cold fronts.
• Thunderstorms account for the greatest number of
  weather-related fatalities in the US due to the
  flooding, lightning and tornadoes that can be
  associated with them.

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How Thunderstorms Form

• As warm moist air is forced upward it cools and
  the water vapor condenses forming cumulus clouds.
  (These clouds can be very tall up to 10km high)
• As the drips fall they collide and combine to
  form larger drops
• The falling drops create a downward movement of
  air that spreads out as it hits the surface
  causing winds
• If the winds accompanying a thunderstorm are
  greater than 54mph then forcasters classify it as
  a severe thunderstorm (these usually contain
  hail).

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Lightning
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Hurricanes

• Hurricanes are the largest storms that occur on
  Earth.
• Hurricanes are very large, swirling, low pressure
  systems that from over tropical waters (water
  temperature must be at least 80 degrees F).
• Hurricanes can last for days due to the constant
  supply of energy from the warm waters
• To be classified as a hurricane there must be
  sustained winds of greater than 74mph.
• Greatest winds recorded are 236mph Category 5
  Mitch in Nov 1998
• 1900 Galveston TX was most deadly hurricane ever
  with 8000 deaths.

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Eye of the Hurricane

• The eye of a hurricane is the relatively calm
  area in the middle of the storm.
• The eye has the lowest pressure of the storm and
  the eye wall is where the strongest winds occur.
• A storm surge is a dome of water that can raise
  the water level higher than high tide where the
  storm lands.

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How Hurricanes Form

• Hurricanes form over warm tropical oceans where
  two opposing winds meet and begin to swirl
• A low pressure forms in the middle of the
  swirling winds and begins rotating
• Warm moist air is forced upwards in the center
• The dropping pressure in the center pulls more
  air toward the center creating increasing winds
  and lower pressure. This cycle of increasing
  strength continues as long as the storm remains
  over warm water.
• Hurricanes weaken as they hit land because there
  is no longer a supply of energy (from the warm
  water) available.

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Life Cycle of a Hurricane

• Life-cycle of a hurricane form first as a low
  pressure system, then grow into a tropical
  depression (lt31mph winds), then grow into a
  tropical storm (32-74mph winds), then grow into a
  hurricane (gt74mph winds).
• Out of the 10 tropical storms per year average, 6
  will develop into hurricanes and 2 will strike
  the US.

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Key Hurricane Alerts

• Tropical Storm Watch tropical storm conditions
  are possible in your area w/in the next 36 hours.
• Tropical Storm Warning tropical storm conditions
  are expected in your area w/in the next 24 hours.
• Hurricane Watch hurricane conditions are
  possible in your area w/in 36 hours.
• Hurricane Warning hurricane conditions are
  expected in your area in 24 hours or less.

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Hurricane StrengthThe Saffir Simpson Scale

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Hurricane Origins
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Tornadoes

• Tornadoes are violent funnel-shaped storms with
  whirling winds that move in narrow paths over
  land.
• Tornadoes form from severe thunderstorms
• Tornado producing thunderstorms involve the rapid
  upward movement of warm moist air
• The upward moving air begins to rotate (why the
  rotation starts is still not understood)
• As the speed of the rotation increases more warm
  moist air is drawn into the low pressure at the
  center
• A funnel shaped cloud extends from the bottom of
  the cloud sometimes touching ground
• As it touches ground it picks up dirt and debris
  that give it the characteristic dark gray, black
  color.
• Tornadoes are one of the most destructive types
  of storms.

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Strength of TornadoesFujita Scale
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Tornado Alley
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Floods
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Droughts
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Blizzards
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Fog
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Ice Storms
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What is Climate?

• Climate is the state of the atmosphere of a given
  region averaged over time.
• Climate is how hot/cold, wet/dry a place is in
  general.
• Climate is what it is usually like in a place.
• Example Floridas climate is usually sunny and
  hot.

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The Hydrologic Cycle

• The hydrologic cycle, or water cycle, is the
  basis for the Earths weather.

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