What is the dew point temperature?

CHICAGO (CBS) -- With heat indexes expected to soar into the triple digits, meteorologists talk a lot about dew point temperatures.

What is that exactly?

Answer: It is the temperature at which the air reaches 100% relative humidity. The higher the dew point rises, the greater the air's moisture.

This directly affects how "comfortable" it will feel outside. A dew point above 65 means a lot of moisture in the air, and temperatures begin to feel very uncomfortable. 

It's very, very oppressive outside right now.

According to the National Weather Service, the dew point at O'Hare hit 80 degrees at 10 a.m. on Wednesday.

That number is quite rare. It has only happened on seven other days in Chicago since 1871 when records were first charted. That last time was on July 30, 1999, when the dew point reached 83 degrees. 

The dew point and actual temperate at 10 a.m. at O'Hare raised the heat index (the "feels like" temperature) to a dangerous 103 degrees. That index figure is expected to rise to as high as 115 degrees today.

As of noon, the heat index, with a dew point of 80 degrees and temperature of 94 degrees, reached 114. That's the highest index number since July 30, 1999, according to the National Weather Service.  

And what about the hottest temperatures ever in Chicago?

The record air temperature, not factoring a heat index, is 105 degrees, set on July 24, 1934. The record for consecutive 100-degree days is three, set on July 4-6, 2012.