Part 6—Determine Storm Speed and Size Using ImageJ
Step 1 – Download the ImageJ Stack for Hurricane Ivan
Use a combination of Control-Click (Mac) or right-click (PC) on the Ivan Stack link below to save the file to your computer. Store it in a convenient place such as your downloads folder or your desktop. This file is a prepared stack of Hurricane Ivan moving across the Gulf of Mexico and making landfall in Mobile Bay (September, 2004).
Ivan Stack (TIFF 2.1MB Jul25 10)
Step 2 – Open Hurricane Ivan Stack in ImageJ
- Return to ImageJ, choose File > Open, browse to where you stored the file ivan_stack.v2.tif, and open it.
- Use the animation control bar to animate the 7-slice stack, or use the greater than (>) and less than (<) keys on your keyboard to move through the stack one frame at a time. Another way to animate the stack is to use Image > Stacks > Tools > Start Animation...
Note: Image > Stacks > Tools > Stop Animation... stops the animation.
Step 3 – Set the Scale for the Stack
- Use the Straight Line Selections tool in the ImageJ toolbar to draw a line along a known distance in the image. In this example, use the east-west width of the state of New Mexico, which is a distance of 552 km.
- Under the Analyze option in the ImageJ menu bar, select Set Scale. In the window that opens, type 552 for Known Distance and km for Unit of Length. Then click OK.
- Select the Straight Line Selections tool in the ImageJ toolbar.
- Draw a line across the east-west dimension of New Mexico. Since this line will be used to establish scale in ImageJ, be as accurate as possible in starting and ending the line on the state borders.
- In the ImageJ menu bar, open the Analyze menu and select Set Scale...
- In the window that opens, type 552 for Known Distance and km for Unit of Length. Then click OK.
Step 4 – Use ImageJ to Create a Storm Track Line
- Make sure the animation bar is positioned at the left edge of the stack so that the first slice is showing.
- Right-click (PC) or control-click (Mac) on the Line tool in the ImageJ toolbar and select the Segmented Lines option.
- Position the cursor over the eye of the hurricane, and make a single click to establish the start of the segmented line.
- Advance the stack to slice 2 using the greater than key (>) on your keyboard, and make a single click in the newly positioned eye of the hurricane to advance the segmented line.
- Continue advancing to the next slice using the greater than key and making a single click in the eye of the hurricane, until you get to slice 7, where you need to make a double-click in the eye of the hurricane to end the segmented line. Once the segmented line is complete, avoid clicking anywhere on the image because doing so will delete the track line you just created.
Note: If every available GOES image (typically 30 minutes apart) had been inserted into the Hurricane Ivan stack, the animation would have 96 slices to show the 48-hour time span. The 7 slices that were used here allowed you to detect the motion and to measure distances. You may want to experiment a bit to discover the time interval that makes an effective animation while not requiring an enormous number of slices.
Important: Notice the time stamp, in Zulu time, in the upper-left corner of each slice of the stack. Most of the slices are 6 hours apart, with two 12-hour jumps when images were not available.
Step 5 – Measure the Distance and Calculate the Hurricane's Average Speed
Select Analyze > Measure. In the window that opens, you will see the length of any straight or segmented line that has been drawn on the image. Remember that you set the measurement units to kilometers. The length of the 48-hour storm track of Hurricane Ivan, as it appears in this animation, is approximately 1044 km.
Consider the following questions:
- What is the formula you need in order to calculate Hurricane Ivan's average speed during the 48 hours represented in this animation?
- Calculate Ivan's average speed over the entire course of this animation.
- Measure the diameter of Hurricane Ivan in slice 6 of the animation, shortly before it reaches land.
Speed avg. = Distance/Time. In this example, Speedavg. = 1044 km/48 hrs = 21.75 km/hr (13.5 mph).
Once you have established scale for the image, you can use a similar approach to measure the diameter of a hurricane, or the extent of any storm system.
Using the straight line option in the ImageJ Tool bar, and the Analyze > Measure command, the dense, circular body of Hurricane Ivan measures greater than 730 km, wider than the state of New Mexico!