Example Lesson - Relating IR Temperature Derived from Satellites to Cloud Top Heights

Michael R. Taber
3010 Agronomy Hall
Iowa State University
Ames, Iowa 50011
mtaber@iastate.edu
Office hours: 2-4 PM

Objective

Statement of the Problem

When looking at an IR satellite image, how can we tell the height of the cloud tops, particularly for thunderstorms?

Procedure

You should keep notes while working on the "Exploration" section. You are only required to turn in your responses to the "Evaluation" and "Summary" sections.
Exploration
  1. View the latest infrared satellite image from theUCAR Real-Time Weather Page. What do the different computer-generated colors represent?
  2. Explore the Interactive Weather Visualizer from the Department of Atmospheric Sciences - University of Illinois at Urbana-Champaign (UIUC). As you move the "cursor" around the image, note the reported IR temperature. Make note of the different "intensities" of clouds and the relationship between temperatures. Also note the longitude and latitude location as you move the cursor around.
  3. Watch the COMET movie on the growth of a cumulus cloud. When the simulation ends, what is the approximate temperature of the cloud top? At what approximate height (in mb) does this temperature correspond? Compare your answers with other students.
  4. Click on the pressure vs. temperature graph. The blue line is a graphical representation of an extremely complicated equation called the saturated adiabatic equation. There are many other "sister" lines for this equation, but for the purpose of this exercise only one line is used, based on standard temperature and pressure (20 oC and 1000 mb). With this graph, simple graphical calculations can be made for determining pressure and temperature for any given point along the blue line.
  5. Using the COMET movie and the pressure vs. temperature graph, predict the cloud top temperature for the height of the cloud in the movie. How well did your estimate for temperature in part 3 above correspond to your predicted temperature? Share your findings with a classmate. Do you agree? If you had to come to a consensus, how did you resolve your differences?
Evaluation
  1. Using all the information at your disposal (Latest infrared satellite image, the Interactive Weather Visualizer, and the pressure vs. temperature graph) predict the cloud top heights for at least three different locations. (Use the latitude and longitude numbers provided in in Interactive Weather Visualizer.) Report your findings using the format described by the table below.

    Example
    StateLatitudeLongitudeCloud Top TemperatureHeight
    Northwestern Nebraska43N103W-63 oC?
    North Carolina36N80W-28 oC?
    Southwestern Pennsylvania40N80W-38 oC?

  2. Based on what you know about IR temperature and cloud top heights, where is severe weather most likely occurring?
Summary
Concept maps are extremely useful in summarizing the key concepts and "visualizing" their relationships. Below is a skeleton map for this lesson. Your task is to complete the concept map by first copying the map on paper, then finishing the map where "?s" are located. You have the option of not using the skeleton map below and creating your own concept map. Clicking on particular regions of the map will offer some additional information to aid you in completing your map

Further exploration

Once you understand the basics of IR satellite image and the relationship to cloud height, you might want to explore how visible and water vapor satellite images can be used in furthering your understanding of cloud development. Once again, visit UIUC's Online guide to satellite imagery, "Visible vs. Infrared vs. Water Vapor."

Concept Map Hints

Infrared satellite images

Infrared satellite images measure the long wave radiation from a particular surface. The amount of long wave radiation emitted depends on the temperature of the surface.

Relating IR temperature of the cloud surface to cloud height

 In this section, think about the IR satellite map from UIUC. What do the darker areas represent? Look in southern California in the vicinity of the Mojave Desert. Do you see dark or bright areas? Think about the likely surface temperature in the Mojave Desert. Now, what do the darker areas on the Satellite map likely represent, a warmer or colder surface?

What units are we using for cloud height?

You may have noticed that we are not measuring cloud tops in kilometers, or any other standard unit of length. Meteorologists commonly measure surfaces in the atmosphere in millibars (mb), a measure of pressure. Think of the atmosphere being made up of many "sheets," each with a corresponding pressure. You may have noticed in the COMET movie that the horizontal pressure/temperature lines are curved upwards in the cloud. Because this phenomenon happens at all levels in the atmosphere, meteorologists have found it to be much easier to use pressure to indicate height.