Why Do We Have Weather?

1
Why Do We Have Weather?
2
An Introduction

• What is weather?
• Weather Patterns
• Weather Forecasts

3
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?

4
Lets take a look at the weather picture and why
we have weather!
5
What is Weather?

• State of the atmosphere at a specific time and
  place
• Includes such conditions as air pressure, wind,
  temperature, and moisture in the air
• Temperature is a measure of air molecule movement
• Suns energy causes air molecules to move
  rapidly temperatures are high and it feels warm
• When less of the Suns energy reaches air
  molecules, they move less rapidly and it feels
  cold

6
Wind

• Air moving in a specific direction
• As the sun heats air, it expands, becomes less
  dense, rises, and has low atmospheric pressure
• Cooler air is denser and sinks, causing high
  atmospheric pressure
• Air moves from high pressure areas to low
  pressure areas, causing wind

7
Humidity

• The amount of water vapor in the air
• Warmer air can hold more water vapor, tending to
  make it more humid
• Relative humidity the amount of water vapor in
  the air compared to what it can hold at a
  specific temperature
• When air cools, it cant hold as much water
  vapor, so the water vapor condenses to liquid or
  forms ice crystals
• Dew point the temperature at which air is
  saturated and condensation forms

8
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

9
Convection is the transfer of heat,usually in
gases or liquids.
10
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

11
Take a look at this!
12
Notice the band of clouds around the equator ?

• This is the ITCZ or inter tropical convergence
  zone

13
Why do you think there is this band of clouds
near the equator?
14
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!

15
Clouds

• Clouds form as warm air is forced upward and
  cools. Then water vapor condenses in tiny
  droplets that remain suspended in the air
• Shape and height of clouds vary with temperature,
  pressure, and water vapor in atmosphere

16
Clouds

• Shape
• Stratus-smooth, even sheets or layers at low
  altitudes
• Cumulus-puffy, white clouds, often with flat
  bases
• Cirrus-high, thin, white feathery clouds made of
  ice crystals
• Height
• Cirro high clouds
• Alto middle-elevation clouds
• Strato low clouds
• Nimbus clouds are dark and so full of water that
  sunlight cant penetrate them
• Precipitation water falling from clouds
• When water droplets in clouds combine and grow
  large enough, precipitation falls to Earth
• Air temperature determines whether the droplets
  form rain, snow, sleep, or hail

17
Cirrus
18
Cumulus
19
Stratus
20
(No Transcript)
21
Consequences of Rotation the Coriolis effect
22
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

23
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?

24
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

25
How did you do?

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

26
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?

27
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!

28
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

29
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

30
So, if you see a big H on the weather map over
the area you live, you can expect fair weather
31
When you see a big L in your area, there will
probably be stormy weather
32
These highs and lows move or less along the jet
stream and bring us our weather changes
33
Fronts and Air Masses

• Because air and moisture move in the atmosphere,
  weather is constantly changing
• Air pressure measured by barometer
• An air mass is a large body of air whose
  temperature and moisture are fairly similar at a
  given altitude properties like the part of
  Earths surface over which it formed
• Fronts are boundaries separating different air
  masses clouds, precipitation, and storms occur
  at frontal boundaries
• There are four different air masses that affect
  the United States

34
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

35
This map shows the air mass source regions and
there paths
36
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?

37
Warm Fronts

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

38
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

39
Stationary Fronts

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

40
Occluded Fronts

• Cold front overtakes warm front
• Involves three air masses of different
  temperatures
• Often found
• close to the low pressure center

41
Ready for a little quiz?

• Here we go!

42
1.

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

43
2.

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

44
3.

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

45
4.

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

46
5.That important weather word that refers to the
transfer of heat
47
6.In general, air near the equator tend to_____
( rise or fall )
48
7.

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

49
8.Which of the weather highways usuallycontrols
our weather
50
9.

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

51
10.

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

52
How did you do?

• Lets check the answers!

53
Answers

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

54
Severe Weather

• Thunderstorms occur inside warm, moist air masses
  and at fronts
• Warm, moist air is forced rapidly upward, wehre
  it cools and condenses
• Strong updrafts of warm air and sinking,
  rain-cooled air cause strong winds

55
Lightning

• Movement of air inside a storm cloud causes parts
  of the cloud to become oppositely charged
• Current flows between the regions of opposite
  electrical charge, forming a lightning bolt
• Thunder lightning superheats the air, causing
  it to expand rapidly and then contract, forming
  sound waves

56
Tornado

• Violent, whirling wind that moves in a narrow
  path over land

57
Hurricane

• Large, swirling, low-pressure system that forms
  over tropical oceans

58
Blizzard

• A winter storm with strong winds, cold
  temperatures, and low visibility, that lasts more
  than three hours ?

59
Severe Weather Safety

• A National Weather Service WATCH means conditions
  are favorable for severe weather to develop
• A warning means that severe weather conditions
  already exist
• Meteorologists study and predict weather
• National Weather Service makes weather maps

60
Reading a weather map

• ISOBAR connects areas of equal pressure BAR
  comes from BARometric pressure

61
Reading a weather map...

• Isotherm Connects areas of equal temperature
  therm means temperature

62
Weather Station (not on the TV)Weather
conditions at specific location
Current Conditions!!