Transcript Lecture 7: Water in the Atmosphere

AOSC 200
Lesson 4
MOISTURE
•WATER VAPOR CONSTITUTES ONLY A SMALL
FRACTION OF THE ATMOSPHERE.
•VARIES FROM 0 TO 4 PER CENT
•HOWEVER WATER IS PROBABLY THE MOST
IMPORTANT GAS IN THE ATMOSPHERE FOR
UNDERSTANDING ATMOSPHERIC PROCESSES.
•THE SOURCE OF ATMOSPHERIC WATER IS
EVAPORATION
Satellite picture of clouds over North America,
early January, 1998
Yellow and red in this
picture are clouds
which covered Canada.
These clouds produced
freezing rain and
produced up to 4
inches of ice
Extensive damage was
done to transmission
lines.
p. 83
Crumpled steel electrical transmission towers – Canada, Jan. 1998
p. 83
Sequence of events leading to saturation of water vapor in air
Fig. 4-1, p. 84
Observations of vapor pressure as a function of temperature
Fig. 4-3, p. 87
Fig. 4-5, p. 83
HUMIDITY
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HUMIDITY DESCRIBES THE AMOUNT OF WATER VAPOR IN THE
AIR.
HUMIDITY IS DESCRIBED QUANTITATIVELY AS, ABSOLUTE
HUMIDITY, MIXING RATIO AND RELATIVE HUMIDITY
SATURATION IS ACHIEVED WHEN THE NUMBER OF WATER
VAPOR MOLECULES LEAVING A WATER SURFACE IS EQUAL
TO THE NUMBER RETURNING FROM THE ATMOSPHERE TO
THE WATER SURFACE.
SATURATION VAPOR PRESSURE IS THE PRESSURE EXERTED BY
THE WATER VAPOR AT SATURATION.
ABSOLUTE HUMIDITY IS THE MASS OF WATER PER UNIT
VOLUME . UNITS ARE USUALLY GRAMS PER CUBIC METER.
MIXING RATIO IS THE MASS OF WATER VAPOR IN AN UNIT
MASS OF AIR. USUALLY IN GRAMS PER KILOGRAM.
RELATIVE HUMIDITY IS THE ACTUAL AMOUNT OF WATER
VAPOR IN THE AIR OVER THE AMOUNT OF WATER VAPOR
REQUIRED FOR SATURATION.
Fig. 4-7, p. 93
Curvature Effect
• On the surface of a water droplet each water molecule
is pulled down by fewer neighbor molecules. This
curvature effect makes it easier for molecules to
evaporate from a droplet.
• Hence the saturated vapor pressure will be higher.
• It now takes a lower temperature before the water
will condense to form a droplet– super-saturation
• Cloud chamber.
• In practice droplets form around condensation nuclei.
Atmospheric water vapor comes from the earth’s surface, hence its
mixing ratio drops with altitude. The mixing ratio also depends on
the temperature – hence the change with latitude.
Fig. 4-2, p. 86
Assume that the
absolute humidity is
constant over the 24
hour period. In the
early morning the
temperature is low, the
saturation vapor
pressure is small, the
relative humidity is
large.
In the afternoon the
temperature is high,
the saturation vapor
pressure is high, the
relative humidity is
low.
Fig. 4-4, p. 90
RELATIVE HUMIDITY
• RELATIVE HUMIDITY CHANGES AS DAILY
TEMPERATURE CHANGES.
• IT WILL CHANGE FROM ONE LOCATION TO
ANOTHER
• IT CHANGES WHEN AIR MOVES VERTICALLY
IN THE ATMOSPHERE.
• DAILY VARIATION OF TEMPERATURE AND
RELATIVE HUMIDITY.
• HOWEVER THE WATER VAPOR CONTENT OF
THE AIR CAN STAY THE SAME.
• DEW POINT IS THE TEMPERATURE AT WHICH
WATER VAPOR WILL CONDENSE OUT OF THE
ATMOSPHERE AS DEW.
• ON SOME DAYS THE WATER VAPOR GOES
DIRECTLY TO ICE. THIS TEMPERATURE IS
CALLED THE FROST POINT.
Dew and Frost
•Dew is formed when the temperature at the
surface falls below the dew point.
•Frost is one of the few examples of
deposition in nature.
When the temperature of the air around this web was cooled to below
the dew point temperature, dew formed, making the web more visible
Fig. 4-5, p. 91
Temperature (°C)
-10
-10
20
20
Relative
Humidity
25
75
25
75
Mixing Ratio
(g/kg)
0.45
1.35
3.67
11.15
Vapor Pressure
(mb)
0.72
2.16
5.87
17.60
Sat. vapor
pressure (mb)
2.88
2.88
23.47
23.47
Dew point Temp.
(°C)
-26.2 -13.5
-0.5
15.6
Dew point
depression (°C)
16.2
20.5
4.4
3.5
Average surface dew point temperature for January.
Average surface dew point temperature for July
CONDENSATION AND DEPOSITION
• CURVATURE EFFECT – EVEN IF AIR IS
SATURATED OVER A FLAT SURFACE, IT
MAY NOT BE FOR A CURVED SURFACE.
• SUPERSATURATION – RELATIVE
HUMIDITY CAN BE ABOVE 100%
WITHOUT CONDENSATION
• NUCLEATION – DROPLETS USUALLY
FORM AROUND PARTICLES –
CONDENSATION NUCLEI
• CONDENSATION NUCLEI CAN BE
HYDROSCOPIC, HYDROPHOBIC,
ICE NUCLEI.
Heat index table
One’s body temperature
is a balance between the
rate at which it receives
energy and the rate that
it loses energy by
evaporating water from
the skin.
In high relative
humidity less water
evaporates and the body
temperature is higher.
The heat index is the
effective temperature
that the body feels.
Box 4-1, p. 89
Advection
fog
Fig. 4-11, p. 97
Steam Fog
Fig. 4-12, p. 98
FOG FORMATION
• Fog is defined as a cloud with its base at or
near the ground
• Fogs result when air is cooled or by the
addition of water vapor to cause saturation
• Radiation fog – cooling of the earth’s surface
by emitting thermal radiation
• Advection fog – warm and moist air blown over
a cool surface. need turbulence at the surface
• Evaporation/steam fog – air picks up
additional water over water surfaces
• Upslope fog – air is cooled as it flows up a slope
Four mechanisms that cause air to ascend
Fig. 4-13, p. 99
Lifting Mechanisms that form Clouds
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•
•
•
•
Lifting condensation level
Orographic lifting
Frontal lifting
Convection
Convergence
• http://www.youtube.com/watch?v=ur0k7U
Drrvg
• http://www.youtube.com/watch?v=ur0k7U
Drrvg
Major cloud
types arranged
by altitude.
Fig. 4-16, p. 102
Precipitation Growth in Warm Clouds
• Cloud droplets are typically 10 microns in size.
small raindrops are typically 1000 microns
(almost one million droplets)
• How does one go from one to the other?
• Simplest explanation is that the droplet grows
by condensation of water vapor on its surface.
But this would take several days.
• One way for raindrops to form is by a
processes known as collision-coalescence.
• This process requires lots of droplets - high
absolute humidity - seen only in the tropics.
Collision-Coalescence
• Warm cloud because process because it only occurs
in the tropics.
• As the cloud droplet is formed it is pulled down by
gravity but then is moved upward by the rising air
within the cloud. Hence within a cloud one has
droplets moving in all directions and they can collide
and form larger droplets by coalescing.
• Once a drop grows to a size when the force of gravity
exceeds the uplift from the rising air, the drop moves
downward through the cloud picking up other
droplets as it falls.
Collision Coalescence
Collisioncoalescence
process
Fig. 4-31, p. 114
Precipitation Growth in Cold Clouds
• Outside of the tropics there are just not enough
cloud droplets to form rain drops by the collisioncoalescence process.
• A Swedish atmospheric physicist, Dr. Bergeron,
would vacation in the fall in the mountains. He
often took an early morning walk along a path
that led through a pine forest and encountered
uplift fogs.
• He noted that on the days when the temperature
was above 0 C, the fog went all the way to the
ground, whereas if the temperature fell below -10
C, the fog lay above the tops of the trees.
Bergeron walk
Why?
• He further noted when the temperature fell
below -10 C, that the pine needles were
covered with ice.
• The reason is that the vapor pressure of
water above ice is less than the vapor above
water.
Attraction of water
vapor to ice versus
water
Fig. 4-33, p. 116
Saturation vapor pressure over ice and over water
Fig. 4-35, p. 117
Fig. 4-34, p. 116
Accretion and Aggregation
• It should be noted that the ice crystal growth gives
a ‘snow flake’ which eventually will also begin to
fall and then rise in the updraft.
• These ice crystals then begin to aggregate into
larger snowflakes.
• The ice crystal can also collide with a supercooled
water droplet which instantly freezes - accretion.
• As the ice crystals descend at some point the
temperature can rise above 0 C, and the crystal
melts to form a raindrop.
Process of aggregation
Fig. 4-32, p. 115
Aggregation
Fig. 4.27
These picture shows fall-streaks. They consist of ice
particles that have fallen out of a cloud and evaporate
before they reach the ground.
Fig. 4.33
Steps in the formation
of the precipitation
types
Fig. 4.38
Warm Front
Fig. 9.13
Fig. 4-37, p. 118
FORMS OF PRECIPITATION
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RAIN - DROPLETS OF WATER GREATER THAN 0.5 MM IN
DIAMETER. WHEN DROPLETS SMALLER THAN 0.5 MM CALLED
DRIZZLE.
•
MUCH RAIN STARTS OUT ALOFT AS ICE CRYSTALS
•
SNOW - ICE CRYSTALS. IF AIR IS COLD (LOW HUMIDITY), WE
GET LIGHT AND FLUFFY SNOW (POWDER). IF AIR IS WARM
THAN ABOUT -5 CELSIUS, THEN WE GET WET SNOW (GOOD
FOR SNOWBALLS).
•
SLEET - SMALL PARTICLES OF ICE. RAINDROPS ENCOUNTER
FREEZING AIR ON DESCENT. IF FREEZING NOT COMPLETE FREEZING RAIN.
•
HAIL - LAYERS OF ICE FORM AS THE HAILSTORM TRAVELS
YUP NAND DOWN IN A STRONG CONVECTIVE CLOUD
•
RIME - FORMED BY FREEZING OF SUPERCOOLED FOG ON
OBJECTS.
The effects of airflow over a mountain
Fig. 4-42, p. 122
Clouds and Precipitation near Mountains
• As air ascends mountain it cools adiabatically,
clouds form, and precipitation occurs.
• Above this altitude the relative humidity stays at
100%
• At the peak of the mountain the absolute humidity
is determined by the saturation vapor pressure at 12C.
• As the air descends its absolute humidity
remains the same as at the peak
Clouds and Precipitation near Mountains
• As the air descends it is compressed, so it warms
• Hence the saturation vapor pressure will increase,
and the relative humidity will decrease
• The net effect of the air ascending and descending
the mountain is that the air becomes drier and
warmer.
• On the island of Hawaii, the west side of the coast
(westerly winds) has rain forests, the eastern side
has deserts.
Table 4-4, p. 123