Wichita Falls 10 Degrees With Heat Index Feeling Like 18

Every faucet in my domicile is in drip mode. My respiratory system also is in drip mode. I can turn off the faucet dripping, but my respiratory system dripping is not as easily plugged.

When the outer world is heated to 90 I can understand why high humidity can make those 90 degrees feel warmer, hence the concept of the Heat Index.

But, with the outer world chilled to 10 degrees, as is the current case at my current location, how does 82% Humidity make those 10 degrees feel like 18?

How can there be a Heat Index when the temperature is well below freezing?

I suspect some sort of weather reporting error is the explanation.