Part 2—Open and Explore Ozone Images
Step 1 – Open Images in ImageJ
- Launch ImageJ by double-clicking its icon on your desktop (Mac or PC) or by clicking the icon in the dock (Mac) or the Start menu (PC).
- Start ImageJ, choose File > Import > Image Sequence.... Browse to the location where you saved the TOMS images, then select the first image. Go with the default values in the Image Sequence dialog.
- The Import Image Sequence command puts all the images into a single window named Stack. You can flip between the images by dragging the slider button along the bottom of the stack window, or by pressing the comma (,) or period (.) keys on your keyboard.
- Save the stack as a TIFF file.
Step 2 – Explore the Images
The images you've opened show the amount of ozone overhead, measured in Dobson Units (DU). The measurements can be thought of as showing the "thickness" of the ozone layer: if all of the ozone molecules overhead could be brought down to Earth's surface, the "layer" of ozone would only be about 3 millimeters thickabout the same height as two stacked pennies. This amount of ozone is assigned a value of 300 DU. The ozone "hole" is defined as the area where the total amount of ozone is less than 220 DU. The color scale in these images has a boundary at 225 DU, so you will use that value as the threshold value for the ozone hole.
- Look at the color scale below the first image to figure out where ozone is "thickest" and "thinnest." Can you identify the area that would be considered as part of the ozone hole?
- Use the , and . keys to flip through the stack of images and examine them. Watch how the ozone levels and the size of the low-ozone area change over time.
Images centered on the South Pole allow us to see the shape and size of the ozone hole in comparison with the rest of the globe. The surface area represented by each pixel in the images is not equal, but as all the yearly images use the same viewpoint, we can measure changes the hole by counting the number of pixels that are part of the hole each year. You won't have to count the pixels by handImageJ gives you a convenient way to highlight and measure the number of pixels that represent the hole each year.