Moisture, Clouds, and Weather

1
Moisture, Clouds, and Weather

• Chapter 19

2
Climate and Weather

• Climate is what you expect.
• Weather is what you get.
• -Robert A. Heinlein

What is the difference between the two?
3
Weather

• Weather The state of the atmosphere
• for a specific place
• at a particular time

• Weather Elements
• 1) Temperature
• 2) Pressure
• 3) Humidity
• 4) Wind
• 5) Visibility
• 6) Clouds
• 7) Significant Weather

4
Moisture in the air

• Water vapor molecules evaporate and enter air,
  mixing among the N2, O2, and other gases.
• Humidity amount of water vapor in the air
• Absolute humidity is mass of vapor in a given
  volume of air
• Relative humidity is amount of water vapor in the
  air relative to the maximum it can hold at a
  given temperature

5
Moisture in the air

• Relative Humidity is expressed as a percentage
• What happens when the air holds all the moisture
  it can?
• It is saturated, the Relative Humidity is
  at 100, and the water vapor condenses
• Relative Humidity changes with temperature

6
Moisture in the air

• As air cools, its relative humidity rises until
  reaching 100 and saturation.
• You are familiar with this! If you have a cold
  drink in the summer, water droplets form on the
  outside of the can as the air right next to the
  glass is cooled.
• The temperature at which saturation occurs is
  called the DEW POINT.

What is frost?
7
Cooling and Condensation

• There are 3 atmospheric processes that cool air
  to its dew point
• Radiation Cooling
• Contact Cooling
• Pressure Changes (adiabatic temperature changes)
• -As air rises, it expands and cools

8
Adiabatic Temperature Changes

• To compress air, you must do work on it (energy
  is added), and it gets warmer.
• If the compressed air expands, it does work
  (energy is lost) and it cools.
• Rising air performs work to expand, so it does
  work and cools adiabatically at a predictable
  rate, depending on its moisture content
• Dry adiabatic lapse rate 10oC per 1000 meters
• Wet adiabatic lapse rate 5-9oC per 1000 meters

Why does rising air expand?
9
Clouds

• Clouds form when rising air cools to its dew
  point, causing water vapor to condense as visible
  water droplets or ice crystals

10
Rising air and Precipitation

• There are 3 mechanisms that cause air to rise
• Orographic Lifting

11
Rising air and Precipitation

• 2. Frontal Wedging

12
Rising air and Precipitation

• 3. Convection-Convergence

13
Clouds

• Clouds are classified into a system that uses
  Latin words to describe the appearance of clouds
  as seen by an observer on the ground.
• There are 3 basic shapse of clouds

14
Clouds

• Cirrus clouds (curl of hair)
• Form 20,000 to 50,000 feet
• Made of ice crystals not water

15
Clouds

• Stratus clouds (layer)
• Horizontally layered (sheet-like)

16
Clouds

• Cumulus (heap)

17
Clouds

• The three basic shapes of clouds can be modified
  or combined, so there are many different types of
  clouds based ont he three basic shapes
• Numbus (rain)
• Altus (high)

18
Clouds
19
Precipitation

• Rain
• Tiny water droplets collide, coalesce, and form
  raindrops
• Rain often starts as ice particles high int he
  atmosphere that melt as they fall
• Sleet
• Warm raindrops that freeze as they fall through a
  cold air mass
• Snow Falling ice crystals that remain frozen
• Hail
• Made of concentric ice layers
• Winds blow falling ice back up high into the
  clouds, allowing hailstones to grow

20
Pressure and Wind

• Warm air rises....why?
• The rising air creates a partial vacuum, so air
  flows in horizontally to replace it...this is
  WIND
• Winds at the Earths surface always blow away from
  high pressure areas towards low pressure
• Differential heating of the Earths surface is the
  ultimate cause of wind

21
Pressure and Wind

• Winds are responses to differences in pressure
• Wind speed is determined by the magnitude of the
  pressure difference, or the PRESSURE GRADIENT
• How do we quantitatively show pressure on a map?

22
Pressure and Wind

• Isobars lines of equal pressure

23
Jet Streams

• Pressure gradients create winds at high and low
  elevations
• Surface winds are slowed by friction, whereas
  winds at higher elevations are not
• Therefore, winds at high elevations have much
  higher velocities than surface winds, and these
  winds are called jet streams
• Jet streams are narrow bands of high-altitude
  wind

24
Air Masses

• Air Mass - extremely large body of air whose
  temperature and humidity are fairly similar in
  any horizontal direction at any given altitude
• In English a big blob of air that pretty much
  is the same throughout

25
Air Masses

• Four Types (characterized by source region)
• cP - continental polar (e.g., Canada)
• cT - continental tropical (e.g., Brazil, Mexico)
• mP - marine polar (e.g., Arctic Ocean)
• mT - marine tropical (e.g., Caribbean)

26
(No Transcript)
27
Fronts

• Front - the transition zone between two air
  masses of different densities

28
Fronts

• Four Types (characterized by what types of air
  are on either side, or what happened)
• Cold Front
• Warm Front
• Stationary Front
• Occluded Front

29
Cold Front

• When a cold air mass replaces a warm air mass
• Drawn as a blue line with triangles/shark teeth
• Triangles point in the direction the front is
  moving
• Therefore, colder air is behind the front and
  warmer air is in front of the front

30
Cold front

• Can be associated with thunderstorms, severe
  weather
• Cold air is heavier than warm, so it pushes
  conditionally unstable air upwards

31
Warm Front

• When a warm air mass replaces a cold air mass
• Drawn as a red line with half-circles
• Half-circles point in the direction the front is
  moving
• Therefore, warmer air is behind the front and
  colder air is in front of the front

32
Warm front

• Overrunning - warm air is less dense than cold,
  so it gradually creeps up slope of cold air
• Produces widespread precipitation, nothing too
  violent

33
How To Find Fronts

• Sharp temperature change over a short distance
• Changes in moisture content (i.e., dew point)
• Change in wind direction
• Pressure dip/pressure change
• Clouds, precipitation

34
Stationary Front

• Two different air masses, adjacent to each other,
  not advancing or retreating
• Drawn with alternating blue/triangle and
  red/half-circle segments on opposite sides
• Behind triangles is the cold air mass behind the
  half-circles is the warm air mass

35
Occluded Front

• When a cold air mass overtakes and mixes with a
  warm air mass
• Drawn with alternating purple triangles and
  half-circle segments on the same side

36
Occluded Front

• Two Types
• Warm-Occlusion
• Cold-Occlusion

37
Upper Air Support The Big Picture
38
Cyclogenesis(or How To Make A Cyclone In 4 Easy
Steps!!!)

• Mid-Latitude Cyclone - a cyclonic storm that
  most often forms in middle and high latitudes
• Cyclone derives from that it turns cyclonically
  (CCW in the N. Hemisphere, CW in the S.
  Hemisphere)

39
Cyclogenesis(or How To Make A Cyclone In 4 Easy
Steps!!!)
40
Cyclogenesis(or How To Make A Cyclone In 4 Easy
Steps!!!)

• Get a stationary front
• Get a low in there to make it start turning
• Get the cold front to wrap around (cold fronts
  move faster than warm fronts... UH OH!)
• Cold front overtakes warm front (i.e., occluded
  front)

41
Thunderstorms

• All thunderstorms require instability (potential)
  and lift. The lift is the mechanism that releases
  the instability.
• Lift is produced by things such as....
• Wind convergence
• Convection
• Oorgraphic lifting
• Frontal boundaries

42
Thunderstorms

• Three stages of a thunderstorm
• Rising air condenses and forms cumulus clouds
• Heat released by condensation warms the air in
  the cloud, producing violent convection and
  updrafts that keep water droplets and ice from
  falling
• Eventually droplets and ice (hail) become too
  heavy and fall

43

• Wind Shear effect generated by thunderstorms as
  warm air and cold air simultaneously rises and
  fall
• May be a factor in tornado formation

44
Thunderstorms

• Lightning two possible hypothesis for origin of
  lightning
• charge separation caused by friction between ice
  and wind
• Cosmic rays charge particles in atmosphere

45
Tornadoes

• Most violent atmospheric disturbance
• Extreme pressure difference ? extremely high
  winds
• Visible because of water vapor, debris

46
Tornadoes

• Winds may exceed several hundred km (miles) per
  hour, although most are much weaker (close to 100
  MPH)
• Small (tenths of a km (mile) to a km (mile) wide)
• Short lived (minute to few hours on ground, move
  48km/h (30 MPH)
•  Associated with severe thunderstorms, formation
  process quite uncertain
•  Often on back end (SW) of storm

47
Quiz

• What causes wind at the Earths surface?
• Name one type of cloud.
• What is a front?
• Bonus In what layer of the atmosphere does
  weather occur?

48
Break