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Chapter 5
Observing the Atmosphere
Figure CO: Chapter 5, Observing the Atmosphere--snowflake
© Steve Collender/ShutterStock, Inc.
Two Basic Approaches to
Observing the Atmosphere
• Direct, in situ, or in place methods
measure properties of the air that are in
contact with the instrument
• Indirect or remote sensing methods obtain
information without physical contact with
the atmosphere being measured
Direct Measurement of Surface
Conditions
• Accomplished by ASOS, the Automated
Surface Observing System
• Observations often visualized using the
station model and the meteogram
• Includes temperature, humidity, pressure,
wind, humidity and precipitation
• The United States’ primary surface
weather observing network
• Used by the NWS, FAA, DOD
Figure B01: Meteogram
Source: McIDAS-V
Temperature
• Mercury and alcohol liquid thermometers,
metallic thermometers have been used
• ASOS uses an electronic resistance
thermometer
– Thermometer is shielded from direct sunlight
– Thermometer is ventilated
– Measures the electrical resistance of a metal
wire, usually platinum
Figure 02: Stevenson Screen
Courtesy of OAR/ERL/National Severe Storms Laboratory (NSSL)/NOAA
Humidity—Dew Point
• ASOS uses a dew point hygrometer
• Based on the principle that a mirror fogs
when the temperature equals the dew points
• Uses a beam of light focused on a mirror
• The mirror is chilled
• Light is blocked from the detector when drops
or frost form on the mirror
• Mirror’s temperature measured with a wire
• False reading if something else covers the
mirror
Figure 03A: Laser dew point.
Figure 03B: Laser dew point.
Other Humidity Instruments
• The wet-bulb thermometer is a liquid
barometer with a wet wick around its bulb
• The wet-bulb thermometer measures wetbulb temperature
• A psychrometer has a wet-bulb and an
ordinary thermometer
– Aspirated psychrometer ventilated with a fan
– Sling psychrometer whirled by hand
• GPS satellites have been used to measure
humidity
Measuring Pressure
• The laboratory standard for measuring
pressure is the mercury thermometer, a
long glass tube
• The aneroid barometer is a partially
evacuated flexible metal box that changes
size with changing pressure
– Used by ASOS
– Smaller, more durable than a mercury
barometer
– Not poisonous
Figure 04: Barometer
© john rensten/Alamy Images
Wind Speed and Direction
• Anemometers measure wind speed
– Cup anemometer rotates in response to
pressure differences
• Wind vanes measure wind direction
– Typically a pointer in front and fins in the back
– The vane rotates until forces are balanced
– Wind vane points into the wind
• Anemometers and wind vanes mounted at
10 feet above the ground
Figure 05: Wind vane and cup anemometer
Courtesy of Lewis Kozlosky, NWS/NOAA
Wind—Other Devices
• Propellers measure wind speed because
blades rotate at a speed proportional to
the wind
• A windsock moves with wind direction and
fills proportionally with wind speed
• Sonic anemometers use ultrasonic sound
waves
– No moving parts
– Measures wind speed by time difference
between transmission and reception of a
Measuring Precipitation
• ASOS uses the rain gauge
– A funnel-like collector above a bucket is
heated to melt snow and/or ice
– Water is funneled into a tipping bucket
– Tipping bucket measures in 0.01 inch
increments
– Has errors due to splash, wind blowing across
the gauge
• ASOS doesn’t measure snow
– A snowboard is white painted wood
Direct Upper-Air Observations
• Radiosondes are radio-equipped
meteorological instrument packages carried
aloft by a helium-filled balloon
– Measure vertical profiles from the surface to 30
km
– Temperature and relative humidity measured
electronically
– Pressure measured with an aneroid barometer
– Tracking the position of the balloon gives wind
speed and direction, and gives the observation
the name rawinsonde
– Soundings taken world-wide twice a day
Figure 07: Rawinsonde
Courtesy of NWS/NOAA
Indirect Weather Observations
• Active sensors emit energy and measure
the energy that returns
– Example: radar
• Passive sensors measure radiation
emitted by the atmosphere, surface, or the
sun
– Example: visible satellite data
• Indirect methods mostly involve light
interacting with molecules or objects
• Must review laws of optics
Reflection and Refraction
• Law of reflection: the angle at which light
strikes a surface is the same as the angle
of reflection
• Refraction is bending of light as it passes
through a transparent substance like water
– The index of refraction is the ratio of the
speed of light in a vacuum to the speed of
light in a substance
– Refraction explains why stars twinkle, and
causes objects partly immersed in water to
look bent or broken in two
Figure 08: Reflection/refraction
Figure 09: Critical angle
Figure B03: Twinkle of a star
Scattering
• Scattering is change of direction of light
rays when they encounter small particles
• Solved mathematically by Mie in 1908
• Rayleigh scattering
– Particles small compared to the wavelength of
incident radiation
• Geometric scattering
– Larger particles, like cloud droplets
• Explains blue sky, red sunsets, white haze
Figure 10: Red sky
Courtesy of Pam Knox
Figure 11: Crepuscular rays
© Wong Chee Yen/Dreamstime.com
Figure 12A: Thick clouds appear darker on the bottom
© tonobalaguerf/ShutterStock, Inc.
Figure 12B: Thick clouds appear darker on the bottom
Measuring Visibility
• Visibility is the horizontal distance a
person with normal vision can see and
identify specified objects
– Reduced when particles in between scatter or
absorb light
– ASOS uses an active remote sensing method
– A flash of light over a very short distance
– Scattered light flash measured by a receiver
and converted into visibility
– Only one direction, and not identical to human
Measuring Cloud Ceiling
• ASOS uses a ceilometer
– An active remote sensing instrument
– Uses a laser beam which sends pulses of
radiation
– Designed with aircraft take-offs and landings
in mind
– Thin and high clouds are invisible to ASOS
– Clouds at the horizon are invisible too
Figure 13: Ceilometer
Meteorological Satellite
Observations
• Two basic orbits
– Geostationary Earth orbit (GEO)
•
•
•
•
Orbits at the speed of Earth’s rotation
Stays above the same point on the Earth
Height of orbit is 36,000 km
Must be located over the Equator
– Low Earth orbit (LEO)
• Often pass over polar regions
• Altitude of 850 km
• Each orbit slightly to west of previous orbit
Figure 14: Satellite orbits
Tradeoffs for GEO and LEO
• GEO
– Continuous view of tropics and mid-latitudes
– Poor view of polar regions
– Tracks storm systems continuously
– GOES, METEOSAT
• LEO
– Good polar coverage
– Excellent detailed snapshots of weather
events
– Good for studying global weather—2 views
Interpreting Satellite Imagery
• Radiometers: passive remote sensing
instruments
– One type measures visible light reflected from
Earth to space
• Brightest images are clouds
– Second type measures radiation emitted by
the surface or clouds (IR)
• Measures heat
• Data 24 hours per day
• Thick cold clouds appear bright white
Figure 15: Satellite Visible image
Courtesy of SSEC and CIMSS, University of Wisconsin-Madison
Figure 16: Satellite IR image
Courtesy of SSEC and CIMSS, University of Wisconsin-Madison
Figure 17: A matrix for how different types of clouds appear in visible versus
IR images
Water Vapor Imagery
• Use radiometers that measure radiation
between 6.5 and 6.9 microns
• Give information about upper and middle
troposphere
• Give information in both clear and cloudy
regions
• Black for low concentrations, milky white
for high concentrations, and bright white
for thunderstorms
Figure 18: Satellite water vapor image
Courtesy of SSEC and CIMSS, University of Wisconsin-Madison
Radar Observations
• Radar is an active remote-sensing
instrument
– Sends out pulses of energy
– Measures energy scattered back to the
transmitting point
– Received signal is called the radar echo
• Radar echo indicates the location and intensity of
precipitation
• Range is a maximum of about 240 km
– Scans through 360 degrees and at several
different elevation angles
Figure 20: Weather radars send out a narrow-beam radio wave that is
scattered off precipitation
Radar Displays
• A nearly horizontal scan is a Plan Position
Indicator or PPI
• A vertical slice is a Range Height Indicator or
RHI
• The intensity of scattered radiation is called
reflectivity and is displayed using a
logarithmic scale in units of decibels or dBZ
– 20 dBZ or greater is rain
– Above 55 dBZ is usually hail
– New data are available about every 5 minutes
Figure 21: Radar image
Courtesy of NOAA
Doppler Radar Data
• Motions of particles towards/away from the
radar are detected
– Displays use cool colors (green, blue) for
approaching
– Displays use warm colors (red, orange) for
receding
• Particular signatures indicate rotation
(supercell updrafts, tornadoes),
divergence (downbursts)
• It takes 2 Doppler radars to put together a
Figure 22A: The Doppler Effect
Figure 22B: The Doppler Effect
Figure 23: A radar display in Doppler mode of a thunderstorm
Courtesy of NOAA
Other Radar Capabilities
• Dual polarization radars
– Can change orientation of electromagnetic
fields
– Can give information about shape and
orientation of particles
– Help distinguish type of precipitation (rain,
hail, snow)
• Wind profilers
– An application of Doppler technology
– Can determine wind speed without
Figure 25: Hourly wind profiler measurements
Courtesy of College of DuPage, NEXLAB
Atmospheric Optics
• Phenomena you can view with your eyes
• Mirages
– Images formed by the refraction of light
– Strong temperature gradients cause changes
in the index of refraction
– Example: hot summer day over pavement
– Upside down images are called inferior
mirages
– Superior mirages when the surface is much
colder than the air above
Figure 26: Inferior mirage
© John King/Alamy Images
Figure 27: Sketch of mirage
Figure 28A: Superior mirage
Figure 28B: Superior mirage
Courtesy of Jason Pineau
Halos
• Whitish rings that encircle but do not touch
the Sun or Moon
– Due to refraction of light by ice crystals
– Usually with cirrostratus cloud
– Most common is the 22° halo caused by small
columnar ice crystals
– Different crystal habits, orientations, and
different solar zenith angles can produce a
variety of halos
Figure 29: 22° halo
© Tatiana Grozetskaya/ShutterStock, Inc.
Figure 30: Diagram showing how light is refracted through a crystal to form
a 22° halo
Effects of Dispersion with
Refraction
• Separation of colors is called dispersion
• Caused by prisms, the atmosphere, water
drops and ice crystals
• Green Flash
– When the sun is setting or rising
– A sliver of green appears for a second or two
as the Sun disappears over the horizon
– Green refracted more than red but not
scattered as effectively as blue
Figure 31: Light through a prism
© Comstock Images/Jupiterimages
Figure 32: Setting sun with green flash
© Stephen & Donna O'Meara/Photo Researchers, Inc.
Sundogs and Sun Pillars
• Sundogs are caused by refraction and
dispersion
– Shiny colored regions on either side of the
Sun
– Usually 22° away from the Sun
– Produced by both cirrus and cirrostratus
– Hexagonal ice crystals drift down with bases
flat
• A Sun pillar is a narrow column of reddish
light straight above a setting sun
Figure 33: Sun with a sun dog
© Van Truan/Dreamstime.com
Rainbows
• The lower rainbow is the primary rainbow
with red outermost
• The higher colored arc is the secondary
rainbow with the colors reversed
• Rainbows are caused when raindrops act
as prisms
• Rainbows are located opposite the sun in
the sky
Figure 34: Double rainbow
© Alexey Stiop/ShutterStock, Inc.
Figure 35: Formation of a rainbow
Adapted from Greenler, R. Rainbows, Halos and Glories. Cambridge
University Press, 1980.
Figure 36: When viewing a rainbow, the Sun must always be behind you
Diffraction
• Diffraction occurs when light is bent
around small objects
• Cloud droplets diffract light
• Iridescence is the color produced by
diffraction
• Glories are a complex combination of
refraction, reflection, diffraction, and
surface waves
– Glories are concentric rings visible on water
clouds from above, as from an aircraft around
Figure 37: Brocken Bow
© Richard Berry/age fotostock
Figure T01: Atmospheric Optical Phenomena Categorized by Cause