Weather 101

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Weather 101

• Bryan McAvoy
• National Weather Service
• Greenville-Spartanburg, SC

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Atmospheric General Circulation
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Start With the Atmosphere

• Most of the mass of the atmosphere is
  concentrated close to the ground.
  

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What Controls the Seasons?

• Season are largely driven by the Earths 23.5
  degree tilt on its axis.
  
• The Earth is also about 3 percent closer to the
  sun in January, but this has less effect.

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• The higher the solar altitude (the higher the
  sun), the more energy is received at a point.
  
• Also, when the sun is more directly overhead,
  its energy has to travel through less of the
  atmosphere.
  
• Average incoming solar radiation is equal to
  about 14 100 watt bulbs per square meter.
  

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Uneven Heating Causes Our Weather

• More energy is received at the Earths equator
  than at the poles.
  
• The globe receives a surplus of heat from the
  equator up to about 30 degrees of latitude.
  
• Thereafter, the Earth loses more heat than it
  gets from the sun.
  
• All weather is driven by the Earths effort to
  balance out the surplus heat at the equator with
  the deficit at the poles.

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How Does the Earth Balance Out Heat?

• First, assume the earth is NOT rotating.
• Air rises at the equator owing to strong
  heating.
  
• Air travels toward the poles. The air cools is it
  travels northward.
  
• Once it reaches the poles, the air sinks and
  starts to travel back south.
  

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The Coriolis Effect (aka Monkey Wrench)

• The Earths rotation causes that poleward moving
  air to deflect to the right (eastward in our
  case) and equatorward to the left.

• This is called the Coriolis Effect.
• Air makes it to about 30 degrees north and starts
  to travel due east.
  
• This area of westerly wind that covers most of
  the US is called the westerlies

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The Three Cell Structure of the Atmosphere

• The earths rotation causes the global
  circulation to break down into three cells.
  
• There is strong rising at the equator, and
  sinking at 30 degrees north, then strong rising
  again at 60 degrees north, and sinking at the
  poles.
• Dont worry, this is all going somewhere!

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The Atmospheres Plan B

• The problem with the 3 cell structure is that the
  atmosphere is still not in balance. Heat builds
  toward the equator and cold air builds in the
  polar regions. This gives rise to areas of high
  and low pressure.
• They do the dirty work redistributing heat.

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Global July Pressure Pattern

• Rising motion is associated with low pressure,
  and sinking with highs
  
• Not surprisingly, there are large, semipermanent
  areas of high and low pressure that develop where
  large scale sinking and rising motion exist in
  the atmosphere.
• Highs also form over continents in the winter.
  Siberia, for example.

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Can you name these features?
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High and Low Pressure

• High pressure, or an anticyclone, is defined as
  an airmass having similar temperature and
  moisture characteristics. For example the Bermuda
  High is warm and moist, the Siberian cold and
  dry.
• Fronts and low pressure form where two different
  airmasses (aka highs) come together.
  
• The wind flows from high to low pressure. This is
  called the gradient wind. The stronger the high
  and low, the tighter the gradient, and the
  stronger the wind. Think of it as a steep vs a
  shallow hill.

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The Gradient Wind

• There is a balance between the pressure gradient
  force, that would have the wind blow from high to
  low pressure, and the Coriolis effect that
  deflects the wind back toward the high.
• We call this the gradient wind. Typically, at
  ground level the winds travel from high to low
  pressure, but at an angle to the isobars.
  

Air flows clockwise around a high, and counter
clockwise around a low
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Quiz

• See the map on your desk. In color!
• There is also a map on Page II-11
• Indicate which way the wind blows on the weather
  map.

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Moisture, Precipitation and Fronts
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Rising Air Causes Much of Our Weather

• Start with a parcel of air at the surface of the
  earth.
  
• It beings to rise, being just a little less
  dense than the surrounding air.
  
• As it rises it expands (less pressure) and it
  cools.
  
• Eventually the air parcel cools to its dewpoint
  (Which doesnt change).
  
• At this point clouds from.
• If the lift is strong enough or if the atmosphere
  is unstable then stratiform rain or thunderstorms
  form.

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Question

• How do you make air rise?

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Low Pressure and Fronts Cause Lift

• Lift, or rising motion in the atmosphere, occurs
  with low pressure.
  
• Remember the 3 cell model? Air comes together
  high in the atmosphere and sinks, forming areas
  of high pressure.
  
• Air is forced outward from the high at the
  surface, where it convergence along fronts.
  
• The lowest pressure on a front is call a low

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Other Kinds of Lift

• Orographic lift. Not as kinky as it sounds.
• Mountains force air to rise
• In the Summer, outflow from thunderstorms causes
  lift.

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Cold Fronts

• Equatorward moving mass of colder, drier air
  rapidly displaces warmer more humid air.
  
• Frontal slope is steep and lift is usually
  strong along the front
  
• Often results in thunderstorms or heavy
  stratiform rain.

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Cold Front Animation
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Warm Fronts

• Poleward moving warm, moist air overruns and
  gradually displaces cooler, drier air.
  
• Very shallow front with weaker lift but more
  widespread precipitation.
  
• Cold Air Damming precipitation is a form of warm
  frontal lift.

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Warm Front Animation
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The Polar Front

• Dividing line between cold airmasses over Canada
  and milder air to the south.
  
• Represents dividing line between surges of warm
  and cold air in the mid-latitudes.
  
• Can be a warm front, cold front or stationary.
• Polar front can be found in the US even in July,
  but in a heavily modified form.
  
• The front can be indentified by its close
  proximity to the polar Jet Stream

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The Jet Stream and Polar Front Attached at the
hip?
Polar Front
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What is the Jet Stream?

• The jet stream is a long, narrow band of strong
  winds in the mid to upper troposphere.
  
• The jet stream forms along a cold front aloft.
  
  
• It is the upward extension of the Polar Front
• Strong lift and wind shear are associated with
  the jet stream

Jet core of 150 knots off the GA coast. Height is
around 30,000 feet
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A High Amplitude Jet Stream Spells Trouble

• You will hear us talk about two orientations of
  the jet stream high and low amplitude.
  
• A low amplitude jet is oriented west to east. We
  call this fast flow.
  
• A high amplitude jet oriented north to south
  along lines of longitude.
  
• This pattern usually brings intense cold, heat,
  severe storms and flooding.

Example of a high amplitude jet
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Development of a Wave Cyclone (low pressure on a
front)
High Pressure
High Pressure
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A kink develops on the polar front as the low
begins to develop. Surges of cold and warm air
form warm and cold fronts.
Rainfall expands ahead of warm front
Cold air advances
Warm Sector
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Low Deepens. More Isobars. Tighter Pressure
Gradient.
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Low Occludes. Continues to deepen.
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Satellite Depiction of a Wave Cyclone

• Baroclinic leaf

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Scratch pad. Dont view in slide show!
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Examples of Airmasses in the Southeast

• Heavy rain and Cold Air Damming

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Tropical Airmasses are Often Responsible for
Flooding

• There are several different airmass source
  regions.
  
• The Maritime Tropical airmasses are the ones
  often associated with flooding rain.

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Example of a Maritime Tropical Airmass

• 70 Degree dewpoint does a good job of denoting a
  summertime tropical airmass.
  
• T.S. Cristobal off Cape Hatteras.
• Note the lower dewpoints on the west side of the
  storm.

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Cold Air Damming
Cold air, reflected here as high pressure, gets
trapped between the mountains and the ocean. Less
dense warm air lifts over the cold air, resulting
in mixed precipitation.
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Weather Type Changes With Distance
snow
sleet
freezing rain
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Cold Air Damming Vertical Profile
Very Cold Air
Snow
Snow
Snow
Warm Air Moves Up the Slope and Cools, forming
precipitation
Rain
Snow
Rain

Cold Air
Ice
Sleet
Snow
North
South
Depth of cold air decreases as it moves southward
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Snow, Sleet and Freezing Rain Atmospheric Profiles
Strong warm nose. Snow melts completely, but
doesnt refreeze. Freezing rain at surface
Sounding is completely below freezing. Snow falls
at surface
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Sleet Profile
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The End!

• If you have any comments or suggestions, let me
  know.

bryan.mcavoy_at_noaa.gov
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Cold Air Damming
Very Cold Air
Snow
Snow
Snow
Rain
Snow
Rain
Warm Air Moves Up the Slope and Cools

Cold Air
Snow
Ice
Sleet
North
South
Depth of cold air decreases as it moves southward
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Cold Air Damming
Very Cold Air
Snow
Snow
Snow
Rain
Snow
Rain
Warm Air Moves Up the Slope and Cools

Cold Air
Snow
Ice
Sleet
North
South
Depth of cold air decreases as it moves southward
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Cold Air Damming
Very Cold Air
Snow
Snow
Snow
Rain
Snow
Rain
Warm Air Moves Up the Slope and Cools

Cold Air
Snow
Ice
Sleet
North
South
Depth of cold air decreases as it moves southward
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