During a Chicago winter, there are two numbers that most people look for in the forecast, aside from snowfall totals: the actual air temperature and the predicted wind chill value.
The two numbers can be drastically different. The record-breaking high for Thursday is in the single digits above zero, but wind chill values could fall as low as -30 degrees.
What's the difference between these two temperatures?
As suggested by the name, wind chill is directly related to how windy and cold it is outside. Whereas an actual temperature reading measures the air using only a thermometer, wind chill measures how the combination of wind and cold feels on a person's skin.
Wind chill values drop below actual temperature readings when the wind increases. The faster the wind is blowing, the faster your body heat is carried away. Even as the actual air temperature remains the same, your body will feel much colder because it has lost more of its own heat.
Wind chill values today are measured different than the formula meteorologists used before 2001. They used to be measured using a device similar to a metal can, but because metal loses heat faster than the human body, wind chill values were recorded at much lower temperatures.
Now, meteorologists use the human body as a more accurate measure for wind chill values. The coldest day in Chicago was recorded on Jan. 20, 1985, when air temperatures were -27 degrees and wind chill values were set at a brutal -80 degrees. Today, that number would be closer to -55 degrees below zero.
High winds and record-breaking cold temperatures this week will make the air temperature feel much colder than it is, but fortunately wind chill values are not expected to shatter that record set in 1985.
Although the actual temperature will not fall too far below zero, a Wind Chill Advisory has been issued. It is set to begin at 6 p.m. and last until noon Thursday.
The National Weather Service defines a Wind Chill Advisory by values in the -20 to -30-degree range. The advisory is upgraded to a warning when wind chill values drop below -30 degrees.