AMS Interactive Infrared Weather Satellite Image
Making the Invisible Visible
Teacher's Answer Key

BACKGROUND

Any object at a temperature greater than absolute zero emits electromagnetic radiation. The intensity of the energy radiated is related to the temperature of the radiating body: relatively "cool" bodies, like the Earth, radiate (emit) less electromagnetic radiation at longer wavelengths; relatively hot bodies, like the Sun, emit more radiation at shorter wavelengths. Radiant energy (light) is transformed into thermal energy (heat) when absorbed by a substance. E.g., radiant energy from the Sun is transformed into thermal energy when absorbed by the Earth. The Earth--including land and water on and above Earth's surface--emits radiation in the infrared part of the electromagnetic spectrum.

Special weather satellite sensors measure infrared radiation (heat) emitted at a wavelength of 10.7 µm (micrometers), in a region of the electromagnetic spectrum known as "near infrared" (i.e., electromagnetic radiation just beyond visible red light). [Note: A micrometer, a.k.a., a micron (an obsolete term), is one millionth of a meter (1 x 10-6 m or 0.000 001 m) or one thousandth of a millimeter (0.001 mm) or 1/25,000 inch. A human hair is approximately 50 micrometers in diameter.]

Teacher's Note: As an "advance organizer" for the Radiation Station activity extension, show the following videodisc movies [Optical Data Corporation "Windows on Science" (WoS), "Physical Science, Volume I" (see WoS Lesson Manager/Print Directory)]:

ACTIVITY QUESTIONS

  1. The date and time appear in the data bar at the bottom of the image: the date is shown in dd/mm/yy format; time is shown in hh:mm format, UTC (Universal Time, Coordinated). To convert from UTC to Eastern Standard Time (EST), subtract five hours from UTC; subtract four hours during Daylight Saving Time (DST). Eastern Daylight Time is in effect from the first Sunday in April to the last Sunday in October.

  2. Answers will vary depending upon time of day, season, and whether the metropolitan region is either cloud-free or cloud-covered, among other factors.

  3. Unenhanced IR weather satellite imagery uses a gray scale (from black to white) to display temperature: the darker the color, the warmer the temperature; the lighter the color, the colder the temperature. Black is hot; gray is warm; white is cold.

  4. Higher, colder cloud-tops are brighter white; lower, warmer cloud-tops are shades of light gray to medium gray. Fog (a low-level cloud touching the ground) may be nearly the same color (temperature) as the land surface.

    [Note: The focal point of this question is, how can you be sure you're really "seeing" the Earth's surface (rather than clouds)? Colder land (and water) surface temperatures may be difficult to differentiate from the temperatures of relatively warmer low-level cloud-tops, especially during the winter. In order to clearly distinguish land- and water surfaces from low-level clouds, it may be helpful to compare & contrast the IR weather satellite image with the latest visible image (similar to a black & white photograph of Earth). Verify your observations by a process known as "ground-truthing."]

    Satellite-derived land surface temperatures may be "ground-truthed" by cross-checking observations reported by ~200 National Oceanic and Atmospheric Administration National Weather Service (NOAA/NWS) stations located across the United States (geographically distributed for maximum coverage). For a map of "Available Surface Stations," shown by three-letter location identifiers (LOCIDs), see the following Web page:

    Available surface stations in Virginia include: IAD (Dulles International Airport, Sterling, VA); and RIC (Richmond, VA). [Note: For valid comparison, be sure the times coincide for both the "AMS interactive infrared satellite image" and "Temperatures" weather map. The National Weather Surface (NWS) reports surface temperatures in degrees Fahrenheit; quickly convert these temperatures to degrees Celsius using the "weather calculator" (see "References").]

    See question No. 7 (below) for guidance regarding ground-truthing satellite-derived sea surface temperatures (SSTs).

  5. In general, the following relationship is true during the day (and in summer): land temperatures should be warmer than water temperatures at the same latitude; during the night (and in winter) the opposite is true. For a specific example, comparing air temperatures over land and water, see the following NOAA NWS NDBC Web page:

    In general, surface temperatures (for both land and water) decrease with increasing latitude and vice-versa.

  6. For some storms, especially convective storms such as thunderstorms, there is an association between storm intensity and the height of storm cloud-tops: more intense weather is often--but not always--associated with higher, colder cloud-tops. Use weather RADAR imagery to determine whether there is a correlation between higher precipitation intensity and higher cloud-tops.

    Weather satellites and weather RADARs use electromagnetic radiation to detect light (or heat) and precipitation, respectively. As stated previously, the IR sensors on-board NOAA/NWS weather satellites detect infrared radiation at a wavelength of 10.7 micrometers (10.7 x 10-6 m). In contrast, weather RADARs use microwave radiation to "see" precipitation-sized particles within- and falling from clouds. The wavelength of the National Weather Service Doppler Radar (also known as WSR-88D) is approximately 10 centimeters (10 x 10-2 m).

    As a side-bar, note the word RADAR is an acronym that stands for RAdio Detecting And Ranging. Some pundits jokingly say the acronym NOAA (National Oceanic and Atmospheric Administration) stands for "National Organization for the Advancement of Acronyms!"

    [Note: Not all clouds produce precipitation. For example, high, thin cirrus clouds (made almost entirely of ice crystals) are cold clouds that DO NOT produce precipitation.]

  7. Warm ocean surface currents should appear darker than nearby waters; cold ocean currents should appear lighter.

    Computer-generated one-, three-, five-, and seven-day average weather satellite images enable the user to "see" virtually cloud-free views of the ocean surface. Ocean features (e.g., surface currents) are dynamic, but usually change more slowly than atmospheric features (e.g., clouds and weather systems).

    Satellite-derived sea surface temperatures (SSTs) may be "ground-truthed" (verified) by cross-checking observations reported by ocean buoys operated by the National Oceanic and Atmospheric Administration National Weather Service (NOAA/NWS) and the U.S. Coast Guard (USCG). Refer to NOAA/NWS National Data Buoy Center (NDBC) website:

    Be sure to access buoys that report "Water Temperature" (WTMP): click on the square blue symbols shown on the NDBC station location map (NDBC Moored Buoys). [Note: The exact location (latitude/longitude) of each buoy is provided on its page of "Recent Observations." The NOAA/NWS NDBC reports surface air and water temperatures in degrees Fahrenheit; quickly convert these temperatures to degrees Celsius using the "weather calculator" (see "References").]

    For a brief explanation of the process of "ground-truthing," see the National Aeronautics and Space Administration (NASA) "Ground Truth" Web page:

    The concept of "ground truth" could lead to a discussion of the relative value of in-situ- versus indirect observations (remote sensing). All things being equal, in-situ observations are more reliable; indirect observations are more cost-effective.

REFERENCES

© Copyright 2005-2012 Walter Sanford. All rights reserved.