In most deserts during the dry season the relative humidity during the daytime will usually be below 20%. It may be significantly higher during the rainy season, however.
I think your premise is wrong. A desert is a place with little or no rain or liquid moisture. Most of Antarctica is desert by this standard.
The Simpson Desert of Australia is a hot desert.
Moisture, the higher level of moisture in the air, the higher level of humidity it is. That's why when you walk into a green house it's hot and humid, the higher level of moisture in the air can sometimes make it difficult to breathe, ie. in a steam room.
Hot desert
The Gibson Desert is a hot desert.
A hot desert
The Arabian Desert is a hot desert.
The Kalahari is a subtropical hot desert.
The Kalahari is classified as a hot sub tropical desert.
Yes, the Simpson Desert is a hot desert.
The Arabian Desert is a hot desert.
The Gibson Desert is a hot desert.