There is water vapor , in an air-less volume , thus , its humidity can be calculated .
Relative Humidity is defined in terms of only the physical properties of water and thus is unrelated to the notion of air holding water.  : //www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCloudsFAQ.html In fact , an air-less volume can contain water vapor and therefore the humidity of this volume can be readily determined.
Relative humidity