Atmospheric Moisture and Clouds

1
Atmospheric Moisture and Clouds
2
Changes in States of Water

• Evaporation-liquid to a gas
• Melting-solid to a liquid
• Freezing-liquid to a solid
• Sublimation-solid to a gas (does not pass through
  the liquid stage)
• Deposition-gas to a solid (does not pass through
  the liquid stage)

3
Specific and Relative Humidity

• Humidity is the amount of water vapor in air.
• Specific humidity is the weight of water vapor
  per weight of a chosen mass of air, including the
  water vapor
• Relative humidity-the ratio of the airs actual
  water vapor content to it potential water vapor
  capacity at a given temperature

4
Relative Humidity

• This is the one that we most talk about and the
  one we hear on the news.
• Relative humidity grams of water
  vapor______
• potential grams of water vapor
• After getting the answer, we
• then change it to a percent by
• multiplying by 100.

5
Temperature and Relative Humidity

• WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR
• If the mass of water vapor stays the same
• When temperature goes ?, then relative humidity
  goes ?
• When temperature goes ?, then relative humidity
  goes ?
• Temperature and relative humidity are inversely
  related. When one goes up the other is going
  down and vice versa.

6
What happens when relative humidity reaches 100?

• This is when the air is saturated and can hold no
  more water vapor.
• When this happens air has reached its DEW POINT
  (temperature at which saturation occurs)
• If the air gets any cooler, the vapor will
  condense to form dew, fog, or clouds.

7
Measuring Relative Humidity

• We dont usually measure the mass of water vapor
  in the air and calculate relative humidity.
• We usually use a hygrometer. The type of
  hygrometer we used in class is called a sling
  psychrometer.

8
Sling Psychrometer
Consists of a dry bulb and a wet bulb
thermometer. When air passes over the wet bulb
water evaporates from the wet bulb. This means
that energy from the bulb is absorbed by the
water, making the wet bulb temperature go
down. The difference in the wet bulb and dry bulb
temperature is directly proportional to the
dryness of the air and allows us to use this
difference to determine relative humidity using a
chart.
http//www.analyticalsci.com/science/Sling20Psych
rometer.htm, January 18, 2008
9
Relative Humidity Chart
10
What is the relative humidity if the temperatures
of the bulbs are the same?

• No water has evaporated so that tells us that
  that air is saturated.
• In other words, the relative humidity is 100.

11
What happens when relative humidity reaches 100?

• This is when the air is saturated and can hold no
  more water vapor.
• When this happens air has reached its DEW POINT
  (temperature at which saturation occurs)
• If the air gets any cooler, the vapor will
  condense to form dew, fog, or clouds.

12
We already know

• WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR
• If the mass of water vapor stays the same
• When temperature goes ?, then relative humidity
  goes ?
• When temperature goes ?, then relative humidity
  goes ?

13
What happens to warm air?
It Rises
14
When warm air rises, pressure goes down. What
happens to the temperature?

• It Goes DownGets Cooler

15
What happens to relative humidity when
temperature goes down?
Relative humidity goes up and air can become
saturated.
16
What happens when air is saturated with water
vapor and the temperature drops?

• Vapor condenses
• (changes to liquid)

17
What forms when water starts to change to liquid?

• Clouds
• Fog
• Dew
• Eventually precipitation

18
What else has to be present for clouds to form?

• Particles on which the
• water vapor can condense

19
So why is it so dry in places like Death Valley?
http//en.wikipedia.org/wiki/Death_Valley,
January 17, 2008
20
Orographic Liftingor Rain Shadows
21
Three basic cloud typesbased on structure
Cirrus-high, white, thin delicate veil like
sheets or wispy feathery appearance Cumulus-consis
t of globular individual cloud masses usually
have a flat base and have the appearance of
rising domes sometimes said to have a
cauliflower appearance or structure Stratus-low
clouds that have a sheet or layered structure
cover most or all of the sky

• Photo Source Australia Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1997/1207mb09.jpg, November 27, 2001

22
Cloud Families Based on Height or Altitude
High Clouds-above 6000 meters Middle
Clouds-2000-6000 meters Low Clouds-below 2000
meters

• Photo Source Australia Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1997/1207mb09.jpg, November 27, 2001

23
High Clouds

• Cirrus
• Cirrocumulus
• Cirrostratus

Photo Source Australia Severe
Weather http//www.australiasevereweather.com/phot
ography/photos/1997/1207mb09.jpg, November 27,
2001
24
Cirrus Clouds
High, white, thin made of ice crystals-not
precipitation producing clouds

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1996/1115jd03.jpg, November 27, 2001

25
Cirrocumulus Clouds

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1996/1203jd01.jpg, November 27, 2001

26
Cirrostratus Clouds
http//www.windows.ucar.edu/tour/link/earth/Atmos
phere/images/cirrostratus1_big_jpg_image.html,
January 17, 2007
27
Middle Clouds

• Altostratus
• Altocumulus

Both are associated with infrequent or light snow.
28
Altostratus Clouds
High stratus clouds that can cover large areas-A
single stratus cloud system can cover a whole
state.

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1991/0623mb02.jpg, November 27, 2001

29
Altocumulus Clouds
Larger and denser than cirrocumulus

• Photo Source Australia Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1995/0504jd02.jpg, November 27, 2001

30
Low Clouds

• Stratus
• Stratocumulus
• Cumulus
• Nimbostratus
• Cumulonimbus

31
Stratus Clouds
Are associated with fog and cover much of the
sky. They may produce light precipitation.
Photo Source Australia Severe
Weather http//www.australiasevereweather.com/phot
ography/photos/1999/0131mb01.jpg, November 21,
2001
32
Stratocumulus Clouds

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1995/0529jd01.jpg, November 27, 2001

33
Cumulus Clouds
Usually associated with fair weather.
Photo Source Australian Severe
Weather http//www.australiasevereweather.com/phot
ography/photos/1992/0123mb01.jpg, November 27,
2001
34
Nimbostratus Clouds
Produce continuous widespread rain or snow.

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1999/1026jd03.jpg, November 27, 2001

35
Cumulonimbus Clouds
Form as a result of upward movement in a cumulus
cloud. These clouds produce rain showers or
thunderstorms.

• Photo Source Australian Severe Weather
• http//www.australiasevereweather.com/photography/
  photos/1992/0212mb02.jpg, November 27, 2001