Ideas for Teaching
Why Radar Can't (Usually) See Tornadoes
Page 1 | Page 2 | Page 3 | Page 4
Beamwidth Size vs. Object Size
Beamwidth
What can a radar see? Beamwidth is one consideration. Earth curvature and height of the feature is another (addressed on the next page). We'll soon understand why researchers built mobile radars...
For the moment, we'll keep the problem in two dimensions and ignore height above ground. The geometry is an isoceles triangle. Be sure to note which beamwidth you are calculating for (i.e. 1 degree).
| Distance from radar | Width of the "beam" |
|---|---|
| 20 mi | |
| 40 mi | |
| 60 mi | |
| 80 mi |
Object Size
How wide and tall are various things we want to see?
| Object | Width | Height or Depth |
|---|---|---|
| Supercell thunderstorm | 10-30 mi | 28,000-55,000 ft |
| Circulation within the supercell thunderstorm | 2-8 mi | 2,000-55,000 ft |
| Tornado | 0.1 - 1.0 mi | Cloud base - 0.5 - 1.5 mi* |
| Individual storm cell within a squall line | 2-8 mi | 4,000-55,000 ft |
| Circulation embedded within a squall line | 2-5 mi | 4,000-40,000 ft |
How Many Data Points?
As you consider the numbers you've calculated, also consider how many data points you'd like to have to detect the supercell thunderstorm, tornado, etc. Is one point enough? Probably not, especially if you want to see motion within a circulation.
Mobile radars
Now can you see why scientists put radars onto trucks? What kind of resolution can they gain by driving up near to the storm? Consider making another table with several distances ranging from 10-50 mi.
On to page 4 and earth curvature...
*How tall is a tornado? Good question. I noted cloud base because that is visually the "top" of a tornado. However, a true supercell tornado circulation will go right up through the center of the mesocyclone, and that can be anywhere from a few thousand feet to the entire depth of the storm.
Last updated: March
28, 2002
Created by: DSZ