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Posted at 10:15 AM ET, 02/11/2011

40 below zero: One coincidence & two oddities

By Don Lipman
A thermometer in Yellowstone National Park dips to 32 below zero, not quite 40 below, C or F. Credit: Jim Peaco via National Park Service

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

By Don Lipman  | February 11, 2011; 10:15 AM ET
Categories:  Latest, Local Climate, Science  
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Thank God the temperature is now above freezing. I thought I was gonna die this morning walking to my doctor's appointment in 12 degree temps. Well it was more like half-running, to get there quicker.

Posted by: rwalker66 | February 11, 2011 10:27 AM | Report abuse

Interesting article. Here in the Blue Ridge Mtns between Warren and Fauquier Counties, our low last night was 14.2F. Coldest I've seen in the few years I've lived here is -2.6 on 1/17/2009.

Currently 32.2F and sunny.

Posted by: spgass1 | February 11, 2011 11:27 AM | Report abuse

Funny, this was the first morning in three weeks that I wore shorts out the door in the morning at 5:00am for the car trip to the gym.

Posted by: bigolpoofter | February 11, 2011 11:35 AM | Report abuse

Wow, interesting fact about no nuclei needed at 40 below. Thanks for sharing, Don.

Posted by: Ann-CapitalWeatherGang | February 11, 2011 12:23 PM | Report abuse

Forty below...generally as cold as it ever gets in Wisconsin...though Mondovi has recorded 45 below; Danbury, 54 below, and the Rice Lake area an unofficial 65 below.

Colder places on the globe include the Yukon, interior Alaska, many areas of Siberia [most notably Verkhoyansk and Oymyakon], Summit Station, Greenland and much of Antarctica. The extremely low temperatures recorded at low as 120 below zero...may in part be due to elevation, ca. 11,000 ft.

You might also add Mt. Washington, NH where wind chill often makes it feel as though you are in Vostok or Summit Station. When it's cold in Siberia, the air is often still...and ice fog may be prevalent...but Siberia also gets the dreaded "buran", a windy whiteout often in subzero [Fahrenheit] temperatures.

Posted by: Bombo47jea | February 11, 2011 12:37 PM | Report abuse

-40C is in fact precisely equal to -40F, not approximately.

C=(9/5)F + 32

so if there's a temperature T where C=F, then we might express that as:

T = (9/5)T + 32
-(4/5) T = 32
T = -40

Posted by: gary55 | February 11, 2011 1:39 PM | Report abuse

don, you said,
"A few other disadvantages of alcohol are: Inaccuracy, because alcohol residue can sometimes stick to the tube; flammability; and non-linear expansion"

to say nothing of the slurred speech, impaired motor skills, and hangovers...

Posted by: walter-in-fallschurch | February 11, 2011 2:03 PM | Report abuse

gary55: Yes, you're right. It would have been better if I had said "mercury freezes at APPROXIMATELY 40 below zero (F or C), which is the case.

Posted by: Don-Capital Weather Gang | February 11, 2011 4:18 PM | Report abuse

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