Friday, February 5, 1999
In cold weather it can be important to wear enough clothes. As everyone knows, the chill factor depends not only on temperature but also on the strength of the wind.
But on a still day, if you are interested in the temperature alone, you can make a rough estimate without a thermometer by simply listening to the sound of your boots if you walk on packed snow.
Recently, I have done some research on this (with a very small "r"), prompted by something an English friend told me long ago, after he had spent three years in the Antarctic. He said he seldom looked at a thermometer, he just listened as he walked.
If the temperature of the snow is close to 32 degrees F, walking on it makes little sound. But if the temperature of packed snow lies in the range 0 degrees F to 20 degrees F, you hear crunching as you walk on it. At temperatures well below 0 degrees F, the sound becomes a squeak. This is about as far as my research has taken me.
Perhaps some Mosquito reader with a musically sensitive ear may care to calibrate the sounds more accurately against a reliable thermometer.
I imagine the cause of the acoustical behavior is connected with the decrease of the melting point of ice under pressure. At 300 atmospheres, ice melts at about 28 degrees F; at 2,000 atmospheres the melting point is about minus 4 degrees F.
When you tread on packed snow, most of the sole of your boot is in contact with air: your weight is supported by the very small area of ice (crushed snow) which is actually in contact with the sole. This ice, therefore, is momentarily under a pressure of many atmospheres and may melt. If the snow is not too cold, a water film forms which acts as a lubricant (as under skis) and one's footsteps are relatively quiet.
But if the snow is very cold, the pressure under your boots is not enough to melt it. Therefore, it moves as a solid on a solid; you hear a squeak because of the absence of the lubricating water.
This, at least, is my theory of what is happening. The sounds should depend on the weight of the walker. A child with big feet should hear squeaks at a higher temperature than a heavy adult.
A.E. (Ben) Benfield is a longtime Carlisle resident who lives on West Street. He wrote a column for the Carlisle Gazette and then for the Carlisle Mosquito when the two papers merged in 1983. Benfield was also chairman of the Carlisle Conservation Commission in the '60s and '70s when large pieces of conservation land were purchased by the town.
© 1999 The Carlisle Mosquito