"Water, water, everywhere, nor any drop to drink"

Water vapour is everywhere, and influences almost everything. Consequently, every metrologist ought to know a little bit about it. Humidity metrology - a field on the cusp between physics and chemistry - involves an interesting variety of measurands and measuring methods.

This talk will be a "whistle-stop tour" of water vapour phenomena, physics, measured quantities, units, sensing principles, standards, measurement challenges, and prospects for the future. There will also be mention, along the way, of NPL's activities in the field.