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A Unique Temperature
Temperature has been front and centre in my mind the past few weeks. Here in Victoria, temperatures have fallen to below freezing for a string of nights, and snow has even fallen in town. Back east in my former homelands, temperatures are quite mild for late January: Chicago hitting 60 oF (15.6 oC) and Southern Ontario 10-13 oC (50-55 oF). And as I was writing an article on the coldest temperature to every be officially recorded in North America (-81.4 oF/-63 oC, Snag, Yukon), Key Lake, Saskatchewan has been shivering at the minus 50 oC (minus 58 oF) mark.
In writing on temperature, I always think in two temperature scales: Fahrenheit and Celsius. This is in part because my North American audience requires the use of both, and many of my older Canadian friends, like I, relate more easily to Fahrenheit since it was the standard across all of North America when we were young.
But there is one temperature I like to write about because it is so unique: Minus 40.
"Minus 40 what?" you ask?
Ah-ha!! That is the most unique property of minus 40. It is the same temperature on both the Fahrenheit and Celsius scales. But there are several more interesting facts about minus 40.
Around minus 40, (-38.8 oC to be exact), mercury freezes. And since it solidifies at that temperatures, it becomes useless as a filling for thermometers in areas where the air temperatures may drop this cold or colder. Meteorologists/climatologists, therefore, replace mercury thermometers with alcohol thermometers when they are to be exposed in cold climates. Alcohol doesn't freeze until temperatures reach a very bitter minus 73.3 oC (minus 100 oF).
So whenever you read a headline or sentence saying: "Mercury plunges to Minus 50" or some other very cold temperature, you can bet the writer is not up on their science.
When we talk about the freezing of water, most of us understand that it occurs at a temperature of 0 oC (32 oF). But, in actual fact, 0 oC (32 oF) is, more correctly, the melting point of ice. Liquid water may begin freezing at that temperature when it's pure and exposed as a fairly flat surface. However, for small, curved, water surfaces, such as raindrops or cloud droplets, the freezing point can be lowered as the water surface becomes more curved, that is, as it becomes smaller. The smaller the droplet, the colder the temperature required for very pure water to freeze -- at least until a threshold is reached, a process called spontaneous nucleation. Have you guessed what that ultimate threshold temperature is? Minus 40!
Minus 40 is also a convenient temperature to remember for cold safety, particularly when outdoors in the winter. An air temperature or windchill temperature of minus 40 will freeze exposed skin almost instantly (less than 10 minutes).
A final unique piece of information about minus 40. According to Dave Thurlow, former host of The Weather Notebook radio program, if you calculate the average of the record lows for each state in the United States, it comes to minus 40.
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