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Chapter 4
Atmospheric Moisture
and Precipitation
Precipitation
• Precipitation-the falling of liquid or solid water
from the atmosphere to reach the Earth’s land or
ocean surfaces
• How does is form?
– What is the name of the nucleus required for water
droplets to form?
– KEY=Upward Movement (what happens?)
• 2 types of upward movement:
– 1. Orographic Lifting
– 2. Buoyed Air (surface)
Phases or States of Substances
• Three phases: solid, liquid, and gas
• In terms of water: ice, water, water vapor
• Importance of latent heat
– KEY: A change in state requires an exchange
of latent heat
– Changes in latent heat lead to changes of state
– Absorption of latent heat  warming
– Release, loss of latent heat  cooling
Changing States of Water
(See Figure 4.1 in textbook)
Water vapor
Freezing
Ice
Water
Melting
The Hydrosphere
• Portion of the Earth system that contains all three
phases of water
• Mostly ocean saltwater: ~ 97.2%
• Fresh water: ~ 2.8%
– 2.15% of the 2.8% is in glaciers/ice
– .63% is groundwater
• 3 categories of fresh water:
– 1. Subsurface
– 2. Groundwater
– 3. Surface
The Hydrosphere
(See Fig. 4.2 in textbook)
Small, but significant
The Hydrologic Cycle
• Hydrologic Cycle-the complete process whereby
Earth’s free water moves through the gaseous,
liquid, and solid states
• 2 main processes:
– 1. Evaporation from oceans/land surfaces
– 2. Water vapor condensing and falling back as precip.
• On land it can evaporate and return as water vapor, be
absorbed by the soil, or runoff into rivers and streams
• In atmosphere, water can condense or deposit and
then fall as precipitation (snow, rain, sleet, etc.)
• Frozen precipitation takes longer to re-enter the
system (snow, ice, glaciers, permafrost)
The Hydrologic Cycle
Global Water Balance
• Like with what we saw with the balance of
incoming (solar/shortwave) and outgoing
(terrestrial/longwave) radiation, there must
be a balance in the hydrologic cycle
• The total evaporation from land and ocean
must equal the total precipitation to land
and ocean
• Whole cycle and balance very complicated,
and as an example...
Humidity
• Humidity-a measure indicating how much water
vapor is present in the air
– Given as a Dew Point or RH measure
• RULE=Warmer air can hold more water vapor
than colder air
• Saturation occurs when air is “holding” the
maximum amount of water vapor
– What happens when the air is saturated?
Measures of Water Vapor Content
• 3 main measures of water vapor:
– 1. Specific humidity
– 2. Dewpoint temperature
– 3. Relative humidity
Specific Humidity
• Actual amount of water
vapor held by a parcel of
air
• Mass of water vapor in a
mass of air (g of water
vapor per kg of air)
• Important because
describes amount of water
vapor is available for
precipitation
• Max SH varies greatly
with temp (Fig. 4.4)
Dewpoint Temperature
• The temperature of an air
mass at which the air
holds its full capacity of
water vapor
• If continued cooling
occurs  condensation
(dew, fog) or deposition
(frost)
• Given in °C or °F
• Scale of comfortability
• Important for forecasting
clouds (fog) and
precipitation
Relative Humidity
• RH-a measure, expressed as a %age, that indicates
the amount of water vapor currently present
compared to the maximum amount that the air can
hold at that specific temperature
• When RH = 100%, the air is saturated…meaning?
• Important for weather forecasting and fire
applications
• What’s the problem with RH?
• Measured with a sling psychrometer
Relative Humidity
• It can change slow or
fairly rapidly.
• Method of change #1:
increase or decrease in
water vapor (slow)
• Method of change #2:
increase or decrease in
air temperature
• In general, highest RH
in morning, lowest RH
in afternoon – why?
The Adiabatic Process
• A change in temperature that results from a
change in pressure (no heat is added or removed
by outside influences) (compression or
expansion)
• If air is forced upward, it will expand and cool;
if air is descending, it will compress and warm.
• Air will cool differently if it above or below the
dew point mark
The Adiabatic Process
• Three lapse rates of importance
– Dry adiabatic (“dry air”) (DAR) lapse rate
• Rate of temperature change when a parcel of air is
not saturated
• Approximately 10°C/1000m
– Wet/adiabatic (“wet air”) (PAR) lapse rate
• Rate of temperature change when a parcel of air is
saturated (above lifting condensation level)
• Ranges from 4 – 9°C/1000m
• We will use 6°C/1000m
– ELR rate?
– LCL?
Lifting Condensation Level (LCL)
• LCL (m) = Air Temp (°C) – Dew Point (°C)
DAR
X 1000 meters
– Example: Air temperature = 30°C, Dew Point
temperature = 10°C…LCL?
• 2,000 meters
The Adiabatic Process
ELR, DAR, and PAR
• ELR=ΔTemperature/ ΔAltitude
• Example: Find the ELR given a surface
temperature of 32°C and a temperature at 5,000m
of -8°C
– Answer: 8°C/1,000m
• Air Stability
– ELR<6°C=Stable
– ELR>10°C=Unstable
– ELR between 6 and 10°C=Conditionally stable
• Examples
Clouds
• Dense concentrations of liquid water droplets, ice
particles, or a combination of liquid and ice that is
suspended in the air
• Required Components—water vapor and cloud
condensation nuclei (dust, pollen, sea salt)
• Temperature determines whether liquid droplet or
ice particle will form
• 4 families (High, Middle, Low, and Vertical)
• 2 Classifications (Stratiform and Cumuliform)
Cloud Nomenclature
• Named using Latin bases
– By height: alto- (mid-level), cirro- (high)
– By form: strato- (layered, blanket-like, greater
horizontal extent), cumulo- (globular, puffy,
great vertical extent)
– By process: nimbo- (rain cloud, rain-bearing,
shower)
• Obtain names by combining the above
terms
Basic Cloud Families and Types
Fog
• A cloud (generally stratiform) that is close to or in
contact with the ground
• Some types and their formation:
– Radiation fog: forms at night when temp of air at the
ground cools to dewpoint and condensation occurs (CA
Central Valley)
– Advection fog: forms when warm moist air moves over
a cooler surface (CA coast)
– Upslope: moist air moves up a slope, cools to dewpoint,
and condensation forms (northeastern US in fall)
– Evaporation (mixing) fog: cold, dry air moves over a
warm body of water (river valleys in northeastern US)
Precipitation
• Liquid or solid water that falls from the
atmosphere (more specifically from clouds)
and reaches the surface of the Earth
• Contrast with virga, which is liquid or solid
water falling from clouds that does not
reach the surface
• Together precipitation and virga are called
hydrometeors (hydro- “water;” meteor“something falling from/through the
atmosphere”)
Importance of Precipitation
•
•
•
•
•
Global energy flow – movement of heat
Life – supply of water
Landforms/landscapes – shapes Earth
Aesthetics – falling snow
Allergies – removes pollen, dust, etc. from
air
Forms of Precipitation
• Rain: liquid water falling from a cloud
• Snow: ice particles falling from a cloud
• Sleet: falls as liquid water, then passes through a
thick layer of freezing/sub-freezing air and
becomes an ice grain/pellet
• Freezing rain: falls as liquid water and freezes
upon contact with ground b/c ground and very thin
layer of air is < 0°C
• Hail: masses of ice that fall from a thunderstorm;
form when water freezes in a storm cloud, and
motions cause ice to circulate and gain mass
Intensity of Precipitation
• Some different names for precipitation,
depending on the intensity at which it falls:
– Drizzle, flurries, freezing drizzle: very light,
continuous or intermittent
– Rain, snow, freezing rain: light to moderate
intensity, usually continuous
– Shower/storm: heavy intensity, usually shortlived
– Hurricane, blizzard: extreme intensity
precipitation combined with strong winds,
long-lived/continuous
Measurement of Precipitation
• Rain
– Cylinder rain gauge, tipping bucket
– Measure to nearest 0.01”
– Below 0.01” is considered a trace
• Snow
–
–
–
–
Use ruler at different locations on flat surface
Measure to nearest 0.1”
Melt to obtain water equivalent
“Standard” equivalent is 10” of snow to 1” of
rain, but varies from 30:1 in light, powdery
snow to 2:1 in “old” snow
Four Precipitation Processes
• Cyclonic/frontal
– Air is forced upward due to air
temperature/density differences
along fronts
– Results in light, steady precip or
intense precip, depending on
temp differences
– Not too common in Arizona or
the southwestern US,
occasionally see during winter
and with remnants of hurricane
or tropical storm
– Cold fronts act as an anvil
pushing warm air up from the
surface (Midwest)
Four Precipitation Processes
• Orographic
– Air is forced over a
mountain, hill, or other
topographic barrier
– Results in light, steady
precip or intense precip,
depending on strength of
uplift
– Common in Arizona
during the monsoon
Rainshadow
• Dry area that exists on
leeward side of
topographic barrier
• So which type of
precipitation process?
• Why does this happen?
• Examples:
– Coastal Ranges
– Sierra Nevada
Four Precipitation Processes
• Convectional
Figure 4.20
– Air is forced upward
due to heating of the
surface/lower levels of
the atmosphere
– Generally results in
intense precipitation
– Common in Arizona
during the monsoon
Four Precipitation Processes
• Convergence
– Air is forced upward as
opposing wind “collides”
– Results in light, steady
precip or intense precip,
depending on strength of
“collision”
– Fairly common in Arizona
during monsoon
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