DataStreme Activity 3A:

WEATHER SATELLITE IMAGERY

Do Now:

  1. Print this file.
  2. Print the Monday Image 1, Image 2 and Image 3 Files.
  3. Print (when available) the Tuesday, 25 September 2001, Daily Summary File

To Do Activity:

  1. Read Chapter 3 in STUDY GUIDE, Part A: Narrative.
  2. Go to STUDY GUIDE, Part B: Applications. Start Activity 3A.
  3. Return here (Monday Activity A File) when told to do so.

Go To STUDY GUIDE - Activity 3A Now


WELCOME BACK: Procedure continued from STUDY GUIDE.

As noted in the Monday, 24 September 2001, Daily Summary, the autumnal or fall equinox occurred at last Saturday evening (at 2305Z, or 7:05 PM EDT, 6:05 PM CDT, etc.). At that time the sun was positioned directly above the equator. During this time of year, the length of daylight becomes dramatically shorter by several minutes per day. The lessening of solar heating, due to both the lowering of the sun's path in our skies and the decrease in the duration of daylight period, will be felt in a general cooling of the Northern Hemisphere and its air masses over the coming weeks.

  1. Image 1 is the visible satellite display for Saturday evening, 0015Z on 23 SEP 2001 (7:15 PM Central Daylight Time), labelled across the upper margin "Visible Image" (listed "Visible - Latest" on the DataStreme Homepage). This visible satellite image was made just about one hour after the equinox occurred. The image is for the time when sunset was occurring across the central US on Saturday evening.

    The visible image shows the cloud pattern in the western US. Blotchy clouds from Kansas to Texas were associated with thunderstorms, some of which became severe. The remainder of the West was relatively quiet. For details of the weather patterns at this time, see the Monday Daily Summary. [Note: The printed satellite image usually displays more detail than is seen on screen. The on-screen image may be enhanced by adjusting the screen brightness and/or contrast.]

    At the time of the visible satellite image, sunlight was reaching the US from the general direction of [(east) (west)]. Cloud conditions, if any existed at this time, are not seen across the eastern half of the image because [(of nighttime conditions) (the satellite is beyond the horizon)].

  2. At the Image 1 time of 0015Z it was near sunset time at St. Cloud, Minnesota (7:14 PM at longitude 94.2 °W, in the center of the state) and at Beaumont, Texas (7:13 PM at longitude 94.1 °W, on the Gulf near the Texas-Louisiana border). Mark these locations with dots on the map. Draw a straight line between these two cities and extend it beyond the borders of the satellite image. The line you drew represents the "terminator" or line separating day and night at the time of Image 1.

    The north-south line segment shown as the western borders of Missouri and Arkansas border represents an approximate north-south longitude line. The terminator line is [(approximately parallel) (at an angle)] to the north-south border longitude line. The terminator's orientation relative to north-south longitude lines changes throughout the year and will be discussed in this week's second activity.

  3. Image 2 is the infrared satellite image ("Infrared - Latest") for the same time (0015Z 23 SEP 2001). This image shows much [(less) (more)] cloudiness, particularly in the eastern US, than was seen in the visible image. Infrared images are basically temperature maps of the surfaces "seen" by the satellite sensor. Warm surfaces (land during most of the year and low clouds) would appear relatively [(bright white) (dark)] and cold surfaces emitting little infrared radiation would appear [(bright white) (dark)]. Surfaces with intermediate temperatures appear in gray shadings.

  4. The Monday 24 SEP 2001 Daily Summary mentions the storm system centered near southwestern Minnesota and the cold front curving southwestward toward northeastern New Mexico at that time (positions from surface map, not shown). Several tornadoes formed near this time near Hastings in southeastern Nebraska. Additional damaging thunderstorm winds occurred in southeast Georgia and Florida about this time.

    The brightness of clouds in the infrared image located in central Florida, south-central Texas, southeast Nebraska, and southwestern Kansas, for example, showed the cloud tops to be generally [("warm") ("cold")]. This implies that the cloud tops associated with thunderstorm convection are generally very [(low) (high)]. Comparatively, the cloud tops located just off the US in the Pacific Ocean area from Washington State along the coast to Baja California are generally [("warm") ("cold")]. These oceanic clouds are at [(lower) (higher)] altitudes.

  5. Now compare the Image 1 visible satellite view with the Image 2 infrared view. On the visible image, the thunderstorm cloud tops from Texas to Kansas illuminated by the late evening sunlight appear generally [("grainy") (smooth)]. By comparison, the visible image shows the Pacific Ocean clouds to be generally [("grainy") (smooth)].

  6. If you wished to create a 24-hour time-lapse of the cloud patterns across the US using the DataStreme satellite images from each hour, you should choose the [(visible) (infrared)] images because the [(visible) (infrared)] images would appear black during nighttime hours. (Most satellite images seen on television are infrared images.)

  7. Image 3 is an enlarged image from the web site, http://www.time.gov, at 7:15 PM CDT. The depiction in Image 3 is from a model showing the positions of the sunrise and sunset terminators and the portion of the entire Earth's surface that would be illuminated by the sun at that time. This site gives the official time for the United States from the National Institute of Standards and Technology (formerly the National Bureau of Standards) in Boulder, Colorado. Selecting a time zone displays the current exact time and date for that zone, its relationship to UTC, and an image that is a smaller version of Image 3.

    The Image 3 sunset terminator line across the US [(did) (did not)] closely coincide with the sunset line you drew between the two cities whose sunsets were about 7:15 CDT. The widths of the lighted portions of horizontal lines (latitude) at comparable distances north and south of the Equator (shown by the dark horizontal line bisecting the map view), are proportional to the lengths of daylight at that latitude. These widths show that, at this time of the year, 22 September, the equinox, the daylight periods are [(longer in the Northern Hemisphere) (longer in the Southern Hemisphere) (equal in both hemispheres)] .

Routinely compare the latest satellite views with the latest analyzed surface map available via the DataStreme Homepage. Compare what you see on-screen with your local weather. You can ask yourself such questions as: Are your skies clear or cloudy? Do the satellite views show the same? A sequence of these map and satellite views posted in your classroom or shared with other teachers allows everyone to relate information provided by these differing depictions of weather conditions.

(Sunrise times, along with other astronomical information, can be obtained from the U. S. Naval Observatory site, http://aa.usno.navy.mil/AA/data/docs/RS_OneDay.html.)


Hold this activity until you have completed all applications for this week. Instructions for faxing your LIT mentor will appear at the end of this week's Activity B.


Return to DataStreme Homepage

URL: datastreme/learn/a_act.html
©Copyright, 2001, American Meteorological Society