Here’s Why Air Or Wind Chill Temperatures Can Be Negative

I am typing this from the Atlanta area right now, and it’s cold. In fact, the current temperature at my house is 9 degrees F and the winds are roughly 24 mph. That means it feels like -5 degrees F outside. But what does a negative temperature really mean? After all, you never hear anyone say they weigh -45 pounds or that they are consuming -2 gallons of water. Here’s a quick little science “101” as much of the country endures brutally cold air and wind chill temperatures.

First, let’s start with the basics. Temperature is defined by the American Meteorological Society (AMS) Glossary as, “The quantity measured by a thermometer....molecular kinetic energy....” It is really a measure of the energy associated with molecular motion. That is the proper “physics-atmospheric sciences” definition. However, the definition by Britannica online provides clues as to why we have negative air or wind chill temperature values. That Britannica definition says that temperature is a, “Measure of hotness or coldness expressed in terms of any of several arbitrary scales and indicating the direction in which heat energy will spontaneously flow—i.e., from a hotter body (one at a higher temperature) to a colder body (one at a lower temperature).”

The key words in the Britannica dictionary entry are “arbitrary scales.” Most of the world uses the Celsius (°C) temperature scale, but here in the U.S (and a handful of other countries) the Fahrenheit (°F) temperature scale is still used. Writing in a National Magnetic Lab Organization blog, Tim Murphy explains that, “The Kelvin temperature scale is used by scientists because they wanted a temperature scale where zero reflects the complete absence of thermal energy.” Therefore at zero Kelvin (known as absolute zero), there is no thermal energy available. While not used in day-to-day weather or climate communication to the public, the Kelvin scale is used extensively in scientific calculations within my field and many others.

It should now be apparent to you that a -5 degree F temperature is simply a construct of a non-absolute scale with an arbitrary 0 reference point (see Carmen Olivares tweet above). The Celsius scale uses the freezing point of water as that reference. According to a NASA Glenn ResearchCenter website. Fahrenheit is a bit more complicated. The website notes, “It originally used the freezing point of sea water as the zero point and the freezing point of pure water as 30 degrees, which made the temperature of a healthy person equal to 96 degrees.” The NASA website goes on to say that the scale was adjusted, “To make the boiling point of pure water 212 and the freezing point of pure water 32, which gave 180 degrees between the two reference points.”

Ok, so what about negative wind chill temperatures? Well, wind chill temperature is also anchored to the same temperature scales and accounts for what it actually “feels” like when air temperature and wind are considered. The calculation is actually a bit more complicated than you might think. According to the National Weather Service, the following things are considered:

• wind speed calculation at roughly 5 feet
• a human face model
• heat transfer theory
• skin tissue resistance.

The wind chill temperature conversion chart is below. A simple wind chill calculator is also available at this link.