Transcript Water and Atmospheric Moisture

Water and Atmospheric Moisture
Hydrologic Cycle
Moisture, Clouds, and
Precipitation
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Humidity
Global Precipitation
Lifting Mechanisms
Precipitation Processes
Big Question:
What Causes Air to
Precipitate?
Global Precipitation
U.S. Current Relative Humidity
[click on map]
ABSOLUTE HUMIDITY:
Absolute humidity (expressed as grams of water vapor per cubic
meter volume of air) is a measure of the actual amount of water
vapor (moisture) in the air, regardless of the air's temperature. The
higher the amount (weight) of water vapor per kilogram, the higher
the absolute humidity.
RELATIVE HUMIDITY:
Relative humidity (RH) (expressed as a percent) also measures water
vapor, but RELATIVE to the temperature of the air.
In other words, it is a measure of the actual amount of water vapor in
the air compared to the total amount of vapor that can exist in the air
at its current temperature.
WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD
AIR, so with the same amount of absolute/specific humidity, cooler
air will have a HIGHER relative humidity, and warmer air a
LOWER relative humidity.
Humidity
• Capacity of air is primarily a function of
temperature
• Relative Humidity (RH) =
(actual water vapor content)
(max. water vapor capacity of the air)
x 100
• Heated air becomes lower in RH because
denominator gets larger
• Cooled air becomes higher in RH
Saturation vs
Air Temperature
The actual amount of
Water air can hold changes
With air temperature
Air at 104 F can hold 3 times
As much water as 68 F air !
(47 grams vs only 15 grams)
Air at 68 F can hold 4 times
As much water as air at 0 F
(15 grams vs only 4 grams)
47 grams
15 grams
32 F
104 F
68 F
4 grams
Saturation and Dew Point
• Saturated v.
unsaturated air
• Dew-point
temperature
– temperature to which
air must be cooled to
reach saturation
(100% RH)
• water on outside of
drinking glass
• ice on your car
window
• dew and fog
Adiabatic Cooling: Clouds and Lifting
Condensation Level (LCL)
• LCL / Cloud base = dew point altitude
Relative Humidity and Temp.
RH fluctuates over a day or season.
Measuring Relative Humidity
Sling psychrometer
Hair hygrometer
After Saturation Occurs the Air
Must Release Extra Water as Fluid
Water forms on the outside of a cold glass as the cold
Air surrounding the glass chills the air to the Dew
Point Temperature
The resulting water
is not from the glass,
the water is from
condensation of
moisture in the air
around the glass
In Nature Extra Moisture is
Transformed to Water Droplets
Cold air next to the rain-soaked cliff is chilled
To The Dew Point Temperature & creates a
Misty Cloud
along a Rocky
Mountain slope
Air near the
Slope is 100%
Saturated
Fog by the Golden Gate
Temperature Inversions
Common Summer Inversion in Los Angeles
Which of the four types is this?
Clarence River, Australia
Fog: A Cloud on the Ground
This April fog occurred in the San Fernando Valley after a
clear, cold April night. It evaporated by noon.
Fog in Glendale
Temperature Inversions
When warmer air overlies cooler air, pollutants
and fog are trapped beneath the inversion.
Common Winter Radiation Inversion in Valleys
Which of the four types is this?
Which of the four types is this?
Appalachian
Mountains
An often
very dense
type of
valley fog
called Tule
Fog in the
Central
Valley of
California
Pacific
Ocean
Atmospheric Lifting
Mechanisms
Air Lifting processes create clouds & precipitation
Are the only means of precipitation on Earth
Four types of lifting are recognized:
1. Convectional Lifting
2. Convergence
3. Orographic Lifting
4. Frontal Lifting
Global Precipitation Patterns
Convergence
Convergence occurs
when large air masses
meet & are forced to
rise vertically by
crowding of molecules.
This process is best seen
at the ITCZ where the
Trades Winds meet &
rise to form towering
clouds & heavy
precipitation
Convectional Lifting
Anywhere air is warmer
than its surrounding air, it
will rise.
In this example an island
heats more than the
surrounding water and
causes a massive cumulus
cloud to form.
Convectional Lifting Over Florida
Warmer temperatures over the peninsula of
Florida, which is land, cause air to rise
compared to the cooler oceans nearby
Rising air in this
Shuttle Picture is
Shown by a
Cloud pattern
which generally
follows the shape
of the southern
Florida peninsula
Convectional Lifting in the Desert
Extremely high afternoon temperatures in late
summer often leads to thunderstorms
throughout the world’s arid regions.
Mojave Desert
The Grand Canyon in August
Orographic Lifting of Air
When air moving
Horizontally
Encounters a
Mountain it must
Rise over the crest
As it rises, it cools
To create clouds,
And most often
precipitation
Moisture
Lost
Moist
Air
Run off
Dry
Air
NO Run off
Frontal Lifting of Air
Although not a mountain range, masses of moving air
Create the same effect – Unlike mountains air masses
Can provide lifting in many different locations
Fronts can lift air
Which is stable,
Creating clouds
& large amounts
Of precipitation
As rain, snow,
Sleet or hail
Precipitation Types / Properties
Snowflakes and Temperature
Snow crystal images from an electron microscope
Seasonal and Global Variation
in Lifting Mechanisms and
Precipitation
• Convergence – increases when subsolar point
(ITCZ) is in the region.
• Convection – maximized when insolation and
temperature are most intense and when
marine air moves over warm land masses;
common also in deserts, with their intense
summer heating
• Orographic – requires forced upslope rising of
air (mountains)
• Frontal – midlatitudes only, where cold and
warm air meet and collide
Summary
• Humidity
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Relative Humidity
Relationship to Temperature
Dew Point
LCL/Cloud Base
• Precipitation (Rain, Snow, Sleet)
– When air is substantially cooled below the dew
point, large droplets or ice crystals form and
may fall if large enough.
• Lifting Mechanisms
– Convective, Orographic, Frontal, Convergence