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Why Do We Have
Weather? Ch. 19
An Introduction
Take good notes! There will be a
quiz at the end of this
presentation.
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 it’s cold in the winter?
 How can we have hail in the summer?
 What causes snow and freezing rain?
Let’s take a look at the weather
picture and why we have weather!
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
Convection is the transfer of heat,
usually in gases or liquids.
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, it’s 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
Take a look at this!
Notice the band of clouds
around the equator ?
This is the ITCZ or inter
tropical convergence zone
Why do you think there is this
band of clouds near the equator?
Did you figure it out?
 Warm, moist air in the tropics rises
 Cold air can hold less moisture than warm
air
 As the moist air rises, it condenses and
forms clouds!
More on this later
Consequences of Rotation: the Coriolis
effect
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
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?
Let’s break for a short quiz
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
How did you do?
1. CONVECTION
2. COLD
3. ITCZ
4. LESS
5. RIGHT
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?
Let’s take another look at the weather
map
 Notice that there are
H’s and L’s on the map
 There are also blue
lines with spikes and
red lines with half
circles
 Let’s take a closer
look!
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
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
So, if you see a big H on the
weather map over the area you
live, you can expect fair weather
When you see a big L in your area,
there will probably be stormy
weather
These highs and lows move or
less along the jet stream and
bring us our weather changes
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
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
This map shows the air mass
source regions and there paths
Ok, now we see the difference in
the air masses
 Let’s look at the different fronts and their impact on
weather
 Can you see the four different types of fronts on the
map?
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
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
Stationary Fronts
 Surface
positions of the
front do not
move
 Often a region
of clouds
Occluded Fronts
 Cold front
overtakes warm
front
 Often found close
to the low
pressure center
Ready for a little quiz?
You’ll need a sheet of paper
 Write your answers as we go
Here we go!
1.
 Winds in a low
pressure system
move _____
around the low
L
2.
 What type of
front can be
found close to
point D ?
3.
 Which of these
fronts would you
expect to have
greater
precipitation,
but be short
lived as the
front passes?
4.
 Give the name of
the air mass
that would have
the following
characteristics:
 cool, moist,
unstable
5.
That important weather word that
refers to the transfer of heat
6.
In general, air near the equator
tend to_____ ( rise or fall )
7.
It causes air and water to be
deflected to the right north
of the equator
8.
Which of the weather highways usually
controls our weather
9.
Warm air holds ( more or less )
moisture than cold air
10.
 If there is a big
H on the
weather map
where you live,
would you expect
fair or stormy
weather
H
How did you do?
Let’s check the answers!
Answers
1. Counterclockwise
2. Cold
3. Cold
4. Maritime polar (mP)
5. Convection
6. Rise
7. Coriolis
8. Westerlies
9. More
10. Fair