40 below zero: One coincidence & two oddities
We've just experienced (endured) one of our coldest nights of the season in the mid-Atlantic, but prospects are increasing for a very mild period ahead. As a result, this may be one of my last opportunities this winter to say "How low did it go?" after a very cold night (for the record: Reagan National 22, BWI 13, Dulles 11). According to Infoplease, 27 states, including Maryland* (but not Virginia and obviously not D.C.), have seen low temperatures of forty degrees below zero or lower.
Which brings me to:
You may have noticed that when referring to a temperature of "forty below zero," I never mentioned the temperature scale. The reason, of course, as most of our weather-savvy readers probably know, is that forty below zero Fahrenheit equals approximately forty below zero Celsius, the only point where the temperature coincides on both scales.
Keep reading to learn two oddities related to forty below zero...
The invention of the thermometer--originally called the thermoscope--is attributed to a number of people, including Galileo, the famous astronomer and physicist (1564 -1642), Santorio Santorio, an Italian physician (1561-1636), and Ferdinand II, the Grand Duke of Tuscany (1610 - 1670), who, in 1654, used alcohol as the medium.
Alcohol was later thought to be impractical for public use, however, due to its visibility problems, its slow reaction time, and other reasons.** About 70 years later, Gabriel Fahrenheit invented the more accurate and more practical mercury thermometer. But later, it was discovered that mercury freezes at minus 40 degrees (F or C), rendering the mercury thermometer useless when it got that cold. The solution: back to some variation of (color-dyed) alcohol, the various forms of which have a far lower freezing point.
The oddity, of course, is not that mercury freezes at -40 degrees. There has to be some point at which it freezes. The oddity is that it is the same temperature at which another transformation occurs. I call that:
Scientists and meteorologists know that, in most cases, rain and snow are produced from supercooled, microscopic water droplets, which coalesce and form around tiny nuclei of dust, smoke, or salt particles. These droplets can, in fact, remain in liquid form far below the freezing point. That is, unless the temperature is 40 degrees below zero (F or C) or lower, when the nuclei are unneeded. At that temperature, supercooled droplets freeze spontaneously even in super clean air, which probably accounts for the "ice fog" or "diamond dust" commonly seen in parts of Alaska in winter when it's extremely cold.
* Jan. 13, 1912
**A few other disadvantages of alcohol are: Inaccuracy, because alcohol residue can sometimes stick to the tube; flammability; and non-linear expansion
| February 11, 2011; 10:15 AM ET
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