Why Do We Have Weather

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Why Do We Have Weather?
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An Introduction

• Take good notes! There will be a quiz at the end
  of this presentation.

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WHY DO WE HAVE THE WEATHER ?

• Just think about it!
• Why is there wind? Why does it blow from one
  direction one day and another the next?
• Why is it rainy one day and dry the next?
• How come its cold in the winter?
• How can we have hail in the summer?
• What causes snow and freezing rain?

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Lets take a look at the weather picture and why
we have weather!
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If we were to pick one term to help explain why
we have weather, what do you think would be a
good word?

• You might pick heat or sun.but another good
  choice would be
• Convection

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Convection is the transfer of heat,usually in
gases or liquids.
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After the atmosphere is warmed by radiation and
conduction, the heat is transferred throughout
the atmosphere by convection.

• Since warmed air has more space between the
  molecules, its less dense and rises
• Cooled air is more dense and tends to sink
• In general, air near the equator tends to rise
  and air near the poles tends to sink

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Take a look at this!
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Notice the band of clouds around the equator ?

• This is the ITCZ or inter tropical convergence
  zone

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Why do you think there is this band of clouds
near the equator?
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Did you figure it out?

• Warm, moist air in the topics rises
• Cold air can hold less moisture than warm air
• As the moist air rises, it condenses and forms
  clouds!
• More on this later

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Consequences of Rotation the Coriolis effect
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The Weather Highways

• The rotation of the earth creates the Coriolis
  effect.
• The Coriolis effect causes the air and water to
  be deflected to the right north of the equator.
• This creates global weather highways

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The Westerlies

• Because of our latitude, most of our weather
  comes from the west
• Looking at the weather map, what type of weather
  might we expect?
• What type of weather might we expect in a few
  days?

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Lets break for a short review

• 1.Transfer of heat in liquids or gases_____
• 2. _____ air is dense and tends to sink.
• 3. Band of clouds found around the equator______
• 4. Cold air holds _____ moisture than warm air
• 5. The Coriolis effect causes the air and water
  to be deflected to the _____ of the equator

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How did you do?

• 1. CONVECTION
• 2. COLD
• 3. ITCZ
• 4. LESS
• 5. RIGHT

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Now What?

• Ok, so we know that the weather moves around on
  these highways and that warm air rises and cold
  air sinks.
• But why is it sunny one day, and rainy the next?

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Lets take another look at the weather map

• Notice that there are Hs and Ls on the map
• There are also blue lines with spikes and red
  lines with half circles
• Lets take a closer look!

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High Pressure Areas

• When cooler air sinks and is warmed, the air can
  hold more moisture
• This usually means sunny skies
• Winds tend to move clockwise around a high

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Low Pressure Areas

• When warm air rises and is cooled, the air can
  not hold as much moisture
• Often, these areas are associated with
  precipitation and stormy weather
• Winds tend to move counter clockwise around the
  low

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So, if you see a big H on the weather map over
the area you live, you can expect fair weather
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When you see a big L in your area, there will
probably be stormy weather
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These highs and lows move or less along the jet
stream and bring us our weather changes
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Fronts and Air Masses

• An air mass is a large body of air whose
  temperature and moisture are fairly similar at a
  given altitude
• Fronts are boundaries separating different air
  masses
• There are four different air masses that affect
  the United States

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The Air Masses

• cP( continental polar) cold, dry stable
• cT( continental tropical) hot, dry, stable air
  aloft, unstable at the surface
• mP( maritime polar) cool, moist, unstable
• mT( maritime tropical) warm, moist, unstable

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This map shows the air mass source regions and
there paths
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Ok, now we see the difference in the air masses

• Lets look at the different fronts and their
  impact on weather
• Can you see the four different types of fronts on
  the map?

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Warm Fronts

• A warm front is warm air displacing cool air
  diagram
• Shallow leading edge warm air must overrun cold
  air
• These are usually slow moving

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Cold Fronts

• Cold air advances into region of warm air
• Intensity of precipitation greater, but short
  lived
• Clearing conditions after front passes
• Usually approaches from W or NW

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Stationary Fronts

• Surface positions of the front do not move
• Often a region of clouds

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Occluded Fronts

• Cold front overtakes warm front
• Often found close to the low pressure center

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Ready for a little quiz?Youll need a sheet of
paper

• Write your answers as we go
• Here we go!

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1.

• Winds in a low pressure system move _____ around
  the low

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2.

• What type of front can be found close to point D ?

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3.

• Which of these fronts would you expect to have
  greater precipitation, but be short lived as the
  front passes?

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4.

• Give the name of the air mass that would have the
  following characteristics
• cool, moist, unstable

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5.That important weather word that refers to the
transfer of heat
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6.In general, air near the equator tend to_____
( rise or fall )
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7.

• It causes air and water to be deflected to the
  right north of the equator

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8.Which of the weather highways usuallycontrols
our weather
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9.

• Warm air holds ( more or less ) moisture than
  cold air

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10.

• If there is a big H on the weather map where you
  live, would you expect fair or stormy weather

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How did you do?

• Lets check the answers!

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Answers

• 1. Counterclockwise 8.
  Westerlies
• 2. Cold 9.
  More
• 3. Cold 10.
  Fair
• 4. Maritime polar (mP)
• 5. Convection
• 6. Rise
• 7. Coriolis