Water and Atmospheric Moisture
Moisture, Clouds, and
Precipitatoin
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Humidity
Global Precipitation
Precipitation Processes
Lifting Mechanisms
Big Question:
What Causes Air to
Precipitate?
Global Precipitation
Dark Blue > 80 inches
Beige < 10 inches
Hydrologic Cycle
Relative Humidity
The ratio between the actual amount of water vapor
held in the atmosphere compared to the amount
required for saturation. It is influenced by
temperature.
Humidity
• Humidity: water vapor content of air
• Capacity primarily a function of temperature
• Relative humidity (RH)
(actual water vapor content)
(max. water vapor capacity of the air)
• RH affected by
– evaporation/condensation
– temperature
x 100
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
Relative Humidity Examples
• Examples
At 22 ºC (72 ºF), one kg of air can hold 50
grams of water vapor.
– RH if holding 25 grams? 50 grams? 60 grams?
– What happens if water is added? Removed?
– What happens to RH if temp. drops? Increases?
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 glass
• ice on your car window
Adiabatic Cooling:
Clouds and Condensation
• 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: A Cloud on the Ground
This Advectional fog forming in San Francisco is due
to Moist Ocean air being moved horizontally over
cooler land surfaces
This fog is a common
Summer condition Along
the California coast As
high temperatures
Farther inland draw
moist ocean Air over
coastal areas
Fog by San Francisco Bay
Yakima River, Washington
Valley Fog:
Distributed by Topography
Valley Fog in
the Valleys
of the
Appalachian
Mountains
Pacific
Ocean
An often
Very dense
Type of
Valley Fog
Called Tule
Fog in the
Central
Valley of
California
Temperature Inversions
When warmer air overlies cooler air, pollutants
and fog are trapped beneath the inversion.
Common Winter Radiation Inversion in Valleys
Temperature Inversions
Common Summer Inversion in Los Angeles
Types of Lifting of Air
Air Lifting processes create clouds & clouds
Are the only means of precipitation on Earth
Four types of lifting are recognized:
1. Convergent Lifting
2. Convectional Lifting
3. Orographic Lifting
4. Frontal Lifting
Convergent Lifting of Air
Convergent Lifting occurs
When Air masses meet &
are forced To rise vertically
This process is best seen at
The Equator where the
Trades Winds Meet & rise
to form towering Clouds
& heavy Precipitation
Convergent Lifting &
Global Precipitation Patterns
Convergent Lifting provides the greatest amount of
Precipitation
On Earth
Near the
Equator
Shown
In dark
Green on
This map
Local Convectional Lifting
Anywhere the
Land is warmer
Air will rise
In this example
A plowed field
Creates warmer
Air temperatures
Than nearby
Green cropland
& Local air rises
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
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
The Orographic Effect
Along the west coast of United States air moving eastward
From the ocean contains moisture – as this air moves over
The coastal mountains, clouds & precipitation occur
Leaving Only dry air on the inland side of the mountains
Wet (Windward) Side
Dry (Leeward) Side
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 Formation
• Requires
– condensation nuclei (solid particles)
– saturation (air at dew point)
• Result is temperature dependent
Result varies geographically
Bergeron Ice-Crystal Process =
Snowflake
Collision-Coalescence
Process = Raindrop
Precipitation Types / Properties
What about hail?
Snowflakes and Temperature
Snow crystal images from an electron microscope
Seasonal and Global Variation
in Lifting Mechanisms and
Precipitation
• Convective – increases when insolation is
most intense and when marine air moves
over land masses
• Orographic – requires elevation change
• Frontal – midlatitudes only
• Convergent – near ITCZ only
Summary
• Humidity
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Relative Humidity
Relationship to Temperature
Dew Point
LCL
• Precipitation (Rain, Snow, Sleet, Hail)
– Ice Crystal Process, Collision-Coalescence
• Lifting Mechanisms
– Convective, Orographic, Frontal, Convergence
• Atmospheric Stability