Friday, August 22, 2008

Doing Science in History Class

First I learned that lightning is seen before the thunder is heard because light travels much faster than sound. But I was really fascinated when I learned that the distance between the lightning and the observer could be measured by the time between the lightning and the thunder -- five seconds correspond to about a mile. I was fascinated because I figured that by making such measurements and plotting them on a graph, one could track the movement of an approaching thunderstorm, and could estimate the time of its arrival. The graph would look something like this:

The vertical scale would measure the lightning-to-thunder delay in seconds (inferring distance), and the horizontal scale would record the time of each measurement. As the storm approaches, the distance would decrease, so the graph would show a downward trend. If all the lightning came from the exact center of the storm, and the storm came toward me with constant speed, the graph would show a straight line. But, of course, the lightning strikes would be scattered throughout the storm cell, so the plotted measurements would also be scattered. However, by estimating a straight line through the center of the plotted points, the path of the center of the storm could be estimated.

I wanted to try this idea the next time that I heard the thunder of an approaching storm. To be prepared to record measurements immediately, I prepared a blank chart and kept it inside one of my textbooks so that I would be prepared whether at school or at home.

The opportunity came when I was in History class. The sky outside was darkening, and soon I began to hear thunder in the distance. I pulled out my chart, and started counting the seconds between lightning and thunder while trying to listen to the teacher -- or at least try to look like I was listening. But now and then I would glance toward the clock and my head would dip as I recorded another measurement.

As the storm approached, the measurements became more frequent, and I became more absorbed in my science project. At some point, I suddenly realized that the history teacher had stopped talking, and when I looked toward the front of the classroom, the teacher was not there.

Then I heard the teacher's voice right behind me, asking "what are you doing?" As I turned to look over my shoulder, I saw that she was looking over my shoulder with a puzzled look, trying to figure out what my chart was all about.

It was too late to hide my chart. I might as well explain what I was doing, I thought, especially since she seemed a bit curious. I hoped that I might get by with just a warning. As I explained my chart, the teacher asked me to speak up so all of the class could hear. I ended by pleading that I really didn't plan to do this during history class, but since that was when the storm came, I didn't have any other choice.

To my surprise, the teacher told me to continue my experiment! Furthermore, she said that when I had enough data to predict when the rain would start, to raise my hand and announce my prediction, announcing this to the rest of the class.

With a sense of relief, I returned to my counting and recording in earnest, no longer worried about hiding my activity. At some point, I had enough points plotted to be able to hold a transparent straight-edge over the graph and estimate a best-fit straight line. The point where this line intersected the bottom edge of the graph (representing zero distance) indicated the arrival time of the storm.

I raised my hand, and the teacher interrupted her lecture. "Two minutes after the hour" I declared, hoping that I wouldn't be embarrassed by a big error. I continued with more data recording, hoping to confirm this estimate as I completed the experiment.

When the rain started, it didn't creep up gradually with an uncertain start time. It suddenly crashed against the tall windows along the entire left side of the classroom, as though some giant had thrown a huge bucketful of water against the windows. Everyone was startled and first looked to the left at the rain suddenly pouring down the windows, than all heads turned in unison to the right, toward the clock. It was two minutes after the hour! exactly! and cheering erupted spontaneously. I was surprised by the accuracy of the prediction, but felt completely exonerated.

I did the same experiment later, at other opportunities, and learned that there was generally a difference between the arrival of the average center of the lightning and the arrival of the leading edge of the rain. Also, if the storm passes by one side of the observer, the graph would tend to be curved rather than follow a straight line. As I looked back at my first experiment, I realized that I was lucky that a number of errors happened to cancel, resulting in an unusually accurate prediction.


Sabrina said...

This was very interesting, Mr. Clark! Reading this reminded me of a scene from one of my favorite horror movies where the father character is comforting his 7-year-old son during a thunderstorm. The father tells his son that when he sees lightning to start counting 'one one thousand, two one thousand' etc. until he hears the thunder, and if he can count higher each time then that means the storm is moving away.

Scott said...

In junior high I read a book called The Weather Makers. It had been inspired by the author hearing a prediction of when it would rain and going outside at that time and seeing it begin to rain.