What are the highest and lowest temperatures at which relative humidity can no longer form?

Dear Tom,
What are the highest and lowest temperatures at which relative humidity can no longer form? What are the lowest humidity values ever recorded here?
— Margaret Robertson, Chicago
Dear Margaret,
Relative humidity is greatly misunderstood. It is the ratio of the actual amount of water vapor present in the air at a given temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. As such, there are no temperature limits to relative humidity.
It’s true that very cold air can hold much less water vapor than hot air, but relative humidity has no temperature limits. Humidity values of 100 percent (the highest value) occur occasionally during rain or fog, and sometimes (though rarely) even in clear air. Chicago’s lowest relative humidity is 13 percent, recorded May 10, 1934; April 11, 1956; April 8, 1971 and April 14, 2015.