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Measuring Global Sea Surface Temperatures

Learner Outcomes

After completing this activity, the learner will:

Background

Water covers approximately 80% of the Earth's surface. The oceans in particular play a major role in driving the Earth's weather systems. Long term patterns in the weather, called climate, interact with other environmental factors to make one place a lush rain forest and another a desert. These patterns may seem eternal in nature, but they are not. In the past, catastrophic changes in climate have had profound impacts on both sea life and land-based life. For instance, one theory holds that the dinosaurs died from starvation following an asteroid or comet impact that darkened the sky for years with ash and dust, killing the plants that the dinosaurs used for food.

The Ice Ages offer a different lesson in global climate change ... it can happen slowly. Some scientists are concerned that the Earth may be about to enter another period of significant climatic change. Others believe that the changes we are now seeing are normal and should not be seen as threatening. In truth, it's too early to tell. But it's not too early to start understanding the scientific issues involved. This activity will open a window on one aspect of this problem, global sea surface temperature, and suggest ways that you can find out more about the engine that drives our weather, the sea.

The oceans have been monitored for the past 17 years using remote sensing satellites that measure the temperature of the sea very accurately from space. Current satellites do this to within .3 degrees Celsius of the actual sea surface temperature. Accuracy is important because it is believed that small changes in the temperature of the sea can have significant impacts on world-wide weather patterns. The scientific data that you will use was collected by the Advanced Very High Resolution Radiometer (AVHRR) on board the NOAA-7 (National Oceanic and Atmospheric Administration) satellite during the period 1981 - 1986. Using a scientific visualization tool called NIH Image 1.55, your goal is to display and make sense of a small part of this data and to communicate your findings to one another and your teacher.


Images You Will Need to Save Before Beginning

Investigation

Start NIH Image by double clicking on its icon. Select Open from the File pull-down menu. When the dialog box appears, double click on the file called Earth.

1. This is the Earth seen from space. If you were an alien visitor, what would your first impression be?
Select Close from the File pull-down menu to erase the Earth image. Select Open from the File pull-down menu. When the dialog box appears, double click on the file called SD86365I.BRW. The image that appears represents the average daytime sea surface temperatures during the month of December, 1986.

In this image, water temperature is color-coded. The land is colored black. The location of each pixel (picture element) is also coded. Your first task is to understand the location code. Select the Cross-Hairs icon in the Tools window near the bottom of the right hand column. Position the Cross-Hairs near the lower left-hand corner of the image and observe what happens to the x-coordinate and y-coordinate in the Info window as you move the cross-hairs around the image.

2. Find the (x,y) coordinates of each corner of the image.
Lower-left corner
Upper-left corner
Lower-right corner
Upper-right corner
3. What are the dimensions of the image? Rows (horizontal): Columns (vertical)

4. How many degrees of longitude per pixel?

5. How many degrees of latitude per pixel?

6. Do all the pixels represent equal-angle sections of the Earth?

7. Do all the pixels represent equal-area sections of the Earth?

In order to understand the relationship between a pixel's coordinates in the image and its corresponding longitude and latitude, it helps to rearrange the image as shown here. The vertical white line is a line of longitude called the prime meridian. The horizontal while line is a line of latitude called the equator.

As shown in the following link, the pixels may be thought of as equal-angle tiles covering the Earth. These tiles spread out from the prime meridian and equator. The lines of longitude and latitude that name each tile are marked with heavy lines.
 

Table 1 gives the rules for converting the (x,y) coordinates in image SD86365I.BRW to longitude and latitude. This is necesessary since most satellite images are not centered on the intersection of the prime meridian and the equator.

8. Find the following cities using the cross-hairs tool and the (x,y) coordinates. Use Table 1 to convert the cities' (x,y) coordinates to longitude and latitude:
 
(X,Y) City Longitude   Latitude
(104,90) Singapore _ _
(302,54) Buenos Aires  _ _
(116,57) Perth _ _
(11,149)  Oslo _ _

student chart

The Value of each pixel, shown in the lower left Values window, is called the data number or DN. You can convert the DN (Value) to degrees Celsius using the formula Celsius = 0.15*DN - 2.1
9. Zoom in on each location and find the DN (Value) of the sea pixel closest to each city (black pixels represent land area). Use the DN to find the Celsius temperature, using the above formula. You may convert Celsius to Fahrenheit degrees using the formula below, but first try to estimate the Fahrenheit from the Celsius temperature. Fahrenheit = 1.8*Celsius + 32
 
(X,Y) City Longitude   Latitude
(104,90) Singapore _ _
(302,54) Buenos Aires  _ _
(116,57) Perth _ _
(11,149)  Oslo _ _
 student charts
By now you should have some idea what the colors mean in the global sea surface temperature image. Select the Cross-Hairs icon and move the cursor into the LUT (Look Up Table) window. As you move the marker from the top (purple) to the bottom (red) of the color table, the DNs (called Index in the Values window) change from 0 to 254, corresponding to temperature changes from -2.1 Celsius to 36 Celsius. Think of this color scale as a key to the temperatures in the image, where cold things are purple and warm things are red. Colors in between purple and red represent temperatures in between cold and hot. Note that the land is black, and its DN is 255, regardless of its temperature.

In the Tools window, the 6th object in the left column is the Draw Line icon. Use it to draw a horizontal line starting at image coordinates (0,89). When drawing the line release the mouse button when the Info window shows DX = 359 and DY = 0.

10. On a globe, what would you call this line? Using the same Draw Line icon, draw a vertical line starting at image coordinates (179,0). When drawing the line release the mouse button when the Info window shows DX = 0 and DY = 179.

11. On a globe, what would this line be called ?

Select the Rectangle Tool from the Tools window. Use it to draw a rectangle starting at (155,89) and extending to (234,137). You will have to calculate the location of the second point by looking at the width and height in the Info window. This rectangle takes in much of the North Pacific.

Select Load Macros from the Special pull-down menu. When the dialog box appears, select the Macros folder. When it opens, select the macro entitled Plotting Macros. Pull-down the Special menu again and select Plot Histogram. Drag the Histogram off the image so you can see both clearly. Click on the image to get the rectangle back.

The histogram shows the relative frequency of various colors (temperatures) in the rectangle. The most frequent color has the greatest height in the histogram. Click on the histogram and position the Cross-Hairs on the highest spike.

12. What is the most frequent DN (Value) in your rectangle? Convert this DN to degrees Celsius. Note: Horizontal line values at either end of the histogram are misleading. Ignore them.

13. What is the highest DN (Value) in your rectangle? Where is it found in the histogram? Convert it to degrees Celsius.

14. What is the lowest DN (Value) in your rectangle? Where is it found in the histogram? Convert it to degrees Celsius.

15. Write a few sentences in which you interpret the general shape of the histogram in terms of the sea temperature within the rectangle.

Click on the histogram. In the upper left hand corner is a small box. Click the box. You will be asked if you wish to save changes in Histogram. Click Yes. A dialog box will appear with Histogram highlighted in the Save As box. Give a name to your histogram, such as North Pacific Histogram and click Save. This will save your histogram for later use.

You should still see your rectangle in the image. If not, click on the image and it should reappear. Select the Hand tool from the Tools window, move the hand into your rectangle, and drag it straight down so the top edge of the rectangle lies on the equator and the left hand edge of the rectangle is on the point (155,89).

Pull down the Special menu and click on Plot Histogram to get a histogram of the South Pacific. As before, drag the histogram off the image so you can see both clearly. Click on the image to show your rectangle.

16. What is the most frequent DN (Value) and temperature in your rectangle?
Note: Click on the histogram to get the DN values.

17. What is the highest DN (Value) and temperature?

18. What is the lowest DN (Value) and temperature?

19. Write a few sentences interpreting the histogram.

As before, save the histogram and name it South Pacific Histogram. Pull down the File menu and click on Open. When the dialog box appears, select North Pacific Histogram. Pull down the File menu again and Open South Pacific Histogram. Move the two histograms so that their edges line up, permitting an easy comparison.
20. Write a few sentences comparing the two histograms and interpreting their differences in terms of water temperature.
Pull down the Enhance menu and select Image Math. When the dialog box appears, select North Pacific Histogram - South Pacific Hemisphere and click OK.
21. Write a few sentences describing and interpreting the result. ·
Repeat this procedure selecting South Pacific Histogram - North Pacific Histogram
22. What does this result represent?

23. Why are the results from items 20 and 21 different?

Pull down the File menu and Close all the open files. Pull down the File menu and Open SD86181I.BRW. This image represents global sea surface temperatures during the month of June, 1986.
24. Write a series of questions that you could answer about global sea surface temperature using the techniques you have learned.

25. What comparisons would you like to make between the data from the month of December and the month of June in 1986? What differences would you expect to find?

26. Conduct your investigation and summarize your findings in a paragraph or two.

Suggested Follow-Up Activities