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AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 7
Clouds, Precipitation, and
Weather Radar
© AMS
1
Case-in-Point
 Contrails may be affecting weather and
climate
– They are formed when hot, humid air from the jet’s
exhaust mixes with cold, drier air at high altitudes
 Exhaust turbulence causes mixing
– May dissipate within minutes or hours, or they may
spread laterally, formally cirrus clouds that last for
a day
– Effect on local radiation budget
 During day, reflect solar radiation and
cool Earth’s surface
 At night, absorb/emit infrared radiation from
below and enhance greenhouse effect by
warming Earth’s surface
– May stimulate local precipitation by providing
ice crystal nuclei for lower clouds
– Effect on climate may increase in the future with
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the increase in global air traffic
2
Driving Question
 How do clouds and precipitation form?
– Clouds are products of condensation, or in the
case of ice clouds, deposition, within the
atmosphere
– Clouds are essential to the global water cycle
 Without clouds, there would be no precipitation
 Yet most clouds do not yield precipitation
– This chapter covers:
 Development and classification of clouds and fog
 Formation and types of precipitation
 Weather radar
– A valuable tool for monitoring air motion within weather
systems
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 Methods for measuring precipitation
3
Cloud Formation
 Water vapor is invisible
 Clouds are visible products of condensation
or deposition of water vapor within the
atmosphere
 Clouds are increasingly likely to form as air
nears saturation
 Clouds do not form in laboratory clean air
unless it is supersaturated due to the
curvature effect
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Cloud Formation
 The curvature effect
– The curvature of a water surface affects the ability of a
water molecule to evaporate from the surface
– The smaller the droplet, the greater the water vapor
concentration needed for droplet growth
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 Smaller droplets have greater curvature than large droplets
 They also have the fewest surrounding molecules, so they are
more weakly bonded
 Water vapor more readily escapes a curved surface than a flat
surface
– There is more surface area on a curved surface
– Saturation vapor pressure is greater surrounding a small
droplet vs. a large one
– For droplets with a radius of 0.001 micrometer, saturation
vapor pressure is up to 3.4 times greater than for larger
surface → this equates to 340% relative humidity
– For droplets with a radius > 1.0 micrometer, RH only slightly
higher than 100% is required
– What then makes cloud formation possible?
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Cloud Formation
 Role of Nuclei
– Outside the laboratory, clouds form readily as the
relative humidity nears 100%
– Suspended in the air are solid or liquid particles, known
as nuclei, which provide a relatively large surface area
for condensation or deposition to occur
 Nuclei have radii > 1 micrometer
 When condensation starts on the surface of the nuclei, they
become comparably sized cloud droplets (or ice crystals) and
additional growth is more likely
– Nuclei are essential for cloud formation
 They are abundant within the atmosphere
 Sources include volcanic eruptions, wind
erosion of soil, forest fires, and ocean spray
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6
Cloud Formation
January 2008
July 2008
Sea salt serves as hygroscopic
nuclei for cloud development
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7
Cloud Formation
 Role of Nuclei, continued
– Types of nuclei:
 Cloud condensation nuclei (CCN) promote
condensation at temperatures both above and below
the freezing point of water
– Cloud droplets can remain liquid even at temperatures well
below 0 °C (32 °F) - supercooled droplets
 Ice-forming nuclei (IN) promote ice crystal formation at
temperatures well below freezing
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– Freezing nuclei – surfaces on which water first condenses,
then freezes
 Active at temperatures below about -9 °C (16 °F)
– Deposition nuclei – surfaces on which ice deposits directly
from vapor
 Not fully active until temperature is below -20 °C (-4 °F)
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Cloud Formation
 Role of Nuclei, continued
– Types of nuclei, continued:
 CCN are much more common than IN
 Hygroscopic nuclei – special category of CCN
– They have a chemical attraction for water molecules
– Clouds form more readily in their presence, for example,
magnesium chloride in ocean spray can promote
condensation at relative humidity as low as 70%
– Many sources exist in urban-industrial areas
– Cities also spur clouds and precipitation development by
contributing water vapor (raising the relative humidity) and
adding heat (adding to the buoyancy of air)
 Rainfall has been shown to be greater downwind from an
urban-industrial area
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Cloud Formation
 Supercooled water
– Supercooling = fresh water cooled below its
freezing point remains liquid
– Within very small droplets, kinetic energy is
sufficient to prevent growth of ice embryos to
the critical size needed for continued growth
and freezing
 Hence, water droplets can remain liquid even at
temperatures well below freezing
 As droplet temperature falls, the probability of an ice
embryo growing to critical size increases because
average kinetic molecular activity decreases
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Cloud Formation
 Supercooled water, continued
– Homogeneous nucleation
 The formation of ice embryos of critical size due to the chance
aggregation of water molecules
 If no foreign particles acting as IN are present, a cloud droplet
can cool to as low as -39 °C (-38.2 ° F) without freezing
(Schaefer point)
 Additional cooling leads to homogeneous nucleation
– Heterogeneous nucleation
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 If a supercooled droplet contains (or comes in contact with)
foreign particles that are IN, the cloud droplet will freeze at a
temperature below freezing, but well above the Schaefer point
 Water molecules collect on the IN and form an embryo that is
close enough to critical size that additional growth causes the
droplet to freeze
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Cloud Classification
 Clouds are classified based on:
– General appearance
 Cirriform – wispy, fibrous
 Stratiform – layered
 Cumuliform – heaped, or puffy
– Altitude of cloud base
 High, middle, or low
– Temperature
 Warm cloud – 0 °C (32 ° F)
 Cold cloud – at or below 0 °C (32 ° F)
– Composition
 Corresponds to temperature
 See Table 7.1 (next slide)
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12
Cloud Classification
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13
Cloud
Classification
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High Clouds
 Bases above 5000 m (16,000 ft) where average
temperatures are typically below -25 °C (-13 °F)
 Clouds are composed almost exclusively of ice
crystals and have fibrous appearance
 Names include the prefix cirro
cirrus
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cirrostratus
cirrocumulus
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Middle Clouds
 Bases between 2000 and 5000 m (6600 and 16,000 ft)
where average temperatures are typically between 0 °C
and -25 °C (32 °F and -13 °F)
 Clouds are composed of supercooled water droplets or a
mixture of droplets and ice crystals
 Names include the prefix alto
altostratus
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altocumulus
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Low Clouds
 Bases from Earth’s surface (fog) to 2000 m (6600 ft) where
average temperatures are typically above -5 °C (23 °F)
 Clouds are composed of mostly water droplets
 Include stratocumulus, stratus, and nimbostratus
 Usually only drizzle falls from stratus, but significant
amounts of rain or snow may fall from thicker nimbostratus
stratocumulus
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stratus
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Clouds Having Significant Vertical Development
cumulus
cumulus clouds on a
visible satellite
image
Satellite showing
clouds streets
cumulus congestus
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cumulonimbus
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Sky Watching
 Cumuliform and stratiform clouds are often
observed in the sky together
 Clouds at different levels may move in different
directions at different speeds
– Caused by wind shear – change in wind direction or
speed with distance
– Strong vertical shear in horizontal wind speed or
direction may cause cloud streets to form
 Cloud holes may be formed by aircraft
– Turbulence may cause expansional cooling of
supercooled droplets
 Droplets freeze to ice crystals that fall out of the cloud
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Unusual Clouds
 Stationary disk-shaped clouds
(altocumlus lenticularis) are
generated when a prominent
mountain range disturbs large
scale winds
 Mountain-wave clouds –
lenticular clouds situated over
the mountain
 Lee-wave clouds - lenticular
clouds formed in the wave
crests downwind of the
mountain
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20
Unusual Clouds
 In a stable atmosphere, waves and wave-type clouds can
develop along the interface between two layers of air
moving horizontally at different speeds
 In the figure, a layer of relatively warm air moving
horizontal more rapidly overlies a colder, denser air layer
 Vertical shear in the horizontal wind creates KelvinHelmholtz waves along the boundary
 Billow clouds are formed in the wave crests
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Unusual Clouds
 Noctilucent clouds form in the upper mesosphere above an
altitude of about 80 km (50 mi)
– Seldom observed, and then only at high latitudes
– Key to formation is exceptionally low temperatures; ice clouds can
form at extremely low vapor pressures
– Water vapor can be from volcanic eruptions or chemical reactions
involving methane
 Nacreous clouds occur in the stratosphere at altitudes of
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20 to 30 km (12 to 19 mi) and form on sulfuric acid nuclei
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Fog
 Fog is simply a cloud
in contact with the
ground
– Restricts visibility to
1000 m (3250 ft) or less
 Otherwise, the
suspension is called
mist (light drizzle)
 Types:
– Radiation – radiational
cooling causes humid
air near the ground to
saturate
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23
NOAA
visible
satellite
image
showing
fog in river
valleys
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24
Fog
 Types:
– Advection fog
 The advecting air
passes over a
relatively cool surface
causing condensation
to occur as the air
cools
 This happens in the
summer over the
surface of the Great
Lakes
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25
Fog
 Types of fog:
– Steam fog
 Also called Arctic sea smoke
 Develops in late fall or winter when extrememly cold
and dry air flows over a large unfrozen body of water
 Evaporation and sensible heating cause the lower
portion of the air mass to become more humid and
warmer than the air above
 The air is destabilized, and the consequent mixing of
mild, humid air with cold, dry air causes saturation
and fog formation
 It looks like steam or smoke coming off the water
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26
Fog
 Types of fog:
– Upslope fog –
fog develops on
mountainsides
or hillsides as
the ascending
humid air
reaches
saturation
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27
Precipitation Processes
 Precipitation is water in solid or
liquid form that falls from clouds to
Earth’s surface under the influence
of gravity
 Terminal velocity
– Terminal velocity is defined as the constant
downward-directed speed of a droplet (or other
particle within a fluid) due to a balance between
gravity acting downward and air (or fluid)
resistance acting upward
– Helps explain why tiny water droplets and ice
crystals composing clouds will not fall as
precipitation unless they are sufficiently large
 The two basic methods for cloud particle growth
are collision-coalescence (warm clouds) and the
Bergeron-Findeisen process (cold clouds)
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Precipitation Processes
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The terminal velocity of a particle falling through
the air increases with the size of the particle
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Precipitation Processes
 Warm-Cloud Precipitation (collision-coalescence
process)
– Warm cloud = cloud at temperatures > 0 °C
– Droplets may grow by colliding and coalescing with one
another
– Takes place in a cloud with a mixture of droplet sizes,
ideally with some having diameters of at least 20
micrometers
 Laboratory simulations demonstrate that colliding droplets will not
coalesce unless they are of significantly different sizes
– Eventually, the droplets become large enough that their
terminal velocity overcomes updrafts
 Then, precipitation will occur
 A key factor is collision efficiency
– This is the fraction of all droplets in the path of the falling larger
droplet that come into contact with the larger droplet
– Collision efficiency increases with the size of the larger droplet
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Warm-Cloud Precipitation
 The relatively large droplet
falls through a cloud of much
smaller droplets
 The larger drop falls faster
and collides with the smaller
droplets in its path
 The larger drop grows via
coalescence
 This is the collisioncoalescence process
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31
Precipitation Processes
 Cold-Cloud Precipitation (Bergeron-Findeisen
process)
– Cold cloud = clouds or portions of clouds at temperatures
< 0 °C
– Most middle and high latitude clouds form precipitation in
an environment of supercooled water and ice crystals
– At sub-freezing temperatures, water molecules more
readily vaporize from a liquid water surface than an ice
surface because water molecules have stronger bonds in
the solid phase
– At the same sub-freezing temperature, the saturation
vapor pressure is greater over water than over ice (see
next slide showing Table 6.3)
– A vapor pressure that is saturated for water droplets is
actually supersaturated for ice crystals, hence the water
vapor deposits on the ice and the crystals grow at the
expense of surrounding water droplets
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Cold-Cloud Precipitation
 Bergeron-Findeisen process,
cont.
– Terminal velocity increases with
increased ice crystal size
– Larger crystals overtake, collide,
and agglomerate with smaller
crystals and water droplets
– May fall out of cloud base when
large enough, and may reach
Earth’s surface as snow or rain
depending on ambient air
temperatures below the cloud
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Precipitation Processes
 Once a falling raindrop or snowflake leaves the base of a
cloud, it enters unsaturated air and begins to vaporize
 Amount that vaporizes increases with cloud base height
and decreased relative humidity of ambient air
 Virga is streaks of water and ice crystals falling from a
cloud that mostly vaporize before reaching Earth’s surface
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Forms of Precipitation
 Precipitation – water in solid or liquid form that
falls from clouds to the Earth’s surface
 Occurs in many forms
– Liquid (rain, drizzle)
– Freezing (freezing rain and freezing drizzle)
– Frozen (snow, snow pellets, snow grains, ice pellets,
hail)
 Liquid precipitation
– Rain (diameters of 0.5 – 6 mm or 0.02 to 0.2 in.)
 Flattened spheres (not teardrop shaped)
 Unstable at larger diameters and break apart
 Fall mostly form nimbostratus and cumulonimbus clouds
– Drizzle (diameters of 0.2 – 0.5 mm or 0.01 to 0.1 in.)
 Originates mostly in stratus cloud and has limited growth by
collision-coalescence
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Forms of Precipitation
 Frozen precipitation
– Snow (aggregate diameters
can reach 5 – 10 cm or 2 –
4 in.)
 An agglomeration of ice
crystals in the form of
flakes
 Vary in size depending
on water vapor
concentration and the
temperature in the
portion of the cloud
where they grow
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36
Forms of Precipitation
 Frozen precipitation, continued
– Snow pellets (diameters of 2 – 5 mm or 0.08 – 0.2 in.)
 Soft conical or spherical white ice particles
 Form when supercooled cloud droplets collide with an ice
crystal and freeze
– Snow grains (diameters < 1 mm or 0.04 in.)
 Flat or elongated opaque white ice particles
 Form in similar way to drizzle, but freeze prior to reaching the
ground
– Ice pellets (sleet) (diameters < 5 mm or 0.02 in.)
 Spherical or irregularly shaped transparent or translucent ice
particles
 Form when snowflakes partially or completely melt below cloud
base and then refreeze into ice particles before striking the
ground
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37
Freezing,
Frozen, and
Liquid
Precipitation
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Forms of Precipitation
 Frozen precipitation,
continued
– Freezing rain (freezing
drizzle)
 Supercooled liquid
precipitation that freezes
(totally or partially) on
contact with subfreezing
objects
 Forms a coating of ice
(glaze) on exposed
surfaces
Sounding favorable to freezing rain formation
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Forms of Precipitation
 Frozen precipitation,
continued
– Hail
 Characterized by concentric,
onion-like layers of ice
 Develops within severe
thunderstorms characterized by
vigorous updrafts, abundant
supercooled water droplet
supply, and great vertical cloud
development
 Updrafts lift ice pellets into
higher portion of cloud, they
grow by collecting supercooled
droplets and eventually may exit
the cloud base
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Acid Deposition
 Rain and snow are naturally slightly acidic
– They dissolve some atmospheric CO2, producing weak
carbonic acid H2CO3
 When air is polluted with oxides of sulfur and
oxides of nitrogen, acid rain may form
– These gases interact with moisture in the atmosphere to
form tiny droplets of sulfuric acid H2SO4 and nitric acid
HNO3
– These acids dissolve in precipitation and may increase
its acidity by as much as 200 times
– Without precipitation, sulfuric acid droplets convert to
acidic aerosols that reduce visibility and may cause
human health problems – acidic aerosols may settle to
ground as dry deposition
– Acid deposition is the combination of acid precipitation
© AMS and dry deposition
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Acid Deposition
 An acid is a hydrogencontaining compound that
releases hydrogen ions
(H+) when it dissolves in
water
 An alkaline substance
releases hydroxyl ions
(OH-) when dissolved in
water
 Acidity and alkalinity are
expressed in terms of pH,
a measure of the hydrogen
ion concentration
 Rain with a pH < 5.6 is
considered acid rain
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42
Weather Radar:
Locating Precipitation
 Radar is an acronym (radio detection and ranging)
 Emits microwave signals and receives reflected signals from targets as
it continually scans a large volume of the lower atmosphere
 Can detect potentially dangerous weather, such as a tornado’s
circulation within a parent thunderstorm
 Monitors rainfall rates and cumulative rainfall totals
 Widespread use began after WWII
 Mid-1950s, Congress allocated funds to purchase long-range radar
units for meteorological purposes following several tornado and
hurricane disasters
 1959, coastal hurricane radar stations established (WSR-57)
 Late 1960s – common to television weather reporting
 1974 – major upgrade (WSR-74)
 1990s – implementation of Doppler radar network (WSR 88-D)
– Operates in a reflectivity or velocity mode
 Reflectivity – detects location, movement, and intensity of areas of precipitation
 Velocity – detect motion directly toward or away from the radar unit
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Weather Radar:
Locating Precipitation
 Reflectivity mode
– WSR 88-D emits short pulses of microwave energy with wavelengths of 10 to
11.1 cm
– At these wavelengths, radar pulses are scattered readily by rain, snow, or hail,
but not significantly by cloud droplets
– Falling precipitation reflects some of radar signal back to a receiving unit, where
it is processed and displayed on a computer screen as a radar echo
– Some radar echoes may be caused by fixed objects on ground (ground clutter) –
typically this is electronically subtracted from display
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Weather Radar:
Locating Precipitation
A radar reflectivity product in which echo
intensity is graduated by color
The radome for a WSR-88D unit houses a
rotating radar dish antenna
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Weather Radar:
Locating Precipitation
 Doppler Effect
– The shift in frequency of sound or electromagnetic waves that
accompanies the motion of the wave source(s) or wave receiver
– (A) = The sound wave source is stationary and wave frequency is
uniform everywhere
– (B) = The wave source is in motion so that wave frequency is greater
ahead of the source than behind the source
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Weather Radar:
Locating Precipitation
 Velocity (Doppler) mode
– Determines horizontal air motions within
a weather system
– There is a shift in frequency of sound and
electromagnetic waves coming from a
moving source
 Doppler radar can detect this frequency
shift, and displays it as motion
– Doppler radars monitor movement of
precipitation particles directly toward or
away from the radar unit
– Doppler radar allow advance notification
of severe approaching weather
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Weather Radar:
Locating Precipitation
 Doppler image – greens
and blues indicate motion
directly towards the radar,
while reds and yellows
indicate motion directly
away from the radar
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48
Phased Array Weather Radar
 Phased array radar
uses multiple beams
that are sent out
simultaneously
– Scans the atmosphere
much faster
– Can focus on a severe
weather feature
– More detailed
examination of storm
evolution
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49
Measuring Precipitation
 Rainfall and snowfall are routinely measured in terms of
accumulation depth over a specified time interval, usually
hourly and every 6 hrs and 24 hrs
 Measurements are made with gauges or remotely by
weather radar or satellite sensors
 Standard non-recording rain gauge
–
–
–
–
Cylinder with a cone-shaped funnel opening
Can resolve rainfall into increments as small as 0.01 in. (0.25 mm)
A graduated stick measures depth of water in cylinder
Rainfall is measured manually at a fixed time and then the gauge is
emptied
 Weighing-bucket rain gauge
– calibrates the weight of rainwater in terms of water depth
– Marks a chart on a clock-driven drum that sends an electronic
signal to a computer
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– May melt frozen precipitation with antifreeze or a heater
50
Measuring Precipitation
 Tipping-bucket rain gauge
– Consists of a free-swinging container
partitioned into 2 compartments, each of
which can collect 0.01 in. of rainfall
– Each compartment alternately fills with
water, tips, and spills its contents,
tripping an electronic switch that marks
a chart or sends a signal to a computer
Standard NWS nonrecording rain gauge
Weighing-bucket
rain gauge
 Weighing-bucket rain gauge
– Calibrates the weight of accumulating
rainwater in terms of water depth
– Marks chart on a clock-driven drum or
sends electronic signal to a computer for
processing
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51
Measuring Precipitation
 Snowfall Measurement
– Meteorologists are interested in the depth of snow that
falls over a certain time period, the melt water
equivalent of the snowfall, and the depth of snow on the
ground at observation time
– New snowfall accumulates on a board placed on top of
the old snow cover and is measured with a ruler
– The melt water equivalent can be determined by
weighing-bucket gauge measurements or melting snow
in a non-recording gauge
 As a very general rule, 10 cm of fresh snow melts to 1 cm of
water
 Actual ratio depends on crystalline form of snowflakes and the
temperature of the air through which the snow falls
– The depth of snow on the ground is determined by an
average of ruler measurements at several locations
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Measuring Precipitation
 Remote Sensing – Weather
Radar
– Reflectivity is proportional to
raindrop surface area and
rainfall rate is proportional to
the volume of the raindrops
– Reflectivity data is converted to
rainfall rates by estimating
raindrop size distribution
 Remote Sensing – Satellite
– Tropical Rainfall Measuring
Mission (TRMM) satellite uses
radar, a microwave imager, and
a visible/IR scanner to estimate
rainfall
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Cumulative rainfall as determined by a
computer analysis of radar echoes
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