Dew point and sticky sweat: why humidity is so uncomfortable

‘If you can’t evaporate that sweat to keep cool, then it induces discomfort.’ Sydney’s high humidity has been giving residents grief. Photograph: Brent Lewins/AAP

Sydney has been sweating through oppressive mugginess for days after hitting Singapore-high levels of humidity earlier this month.

Feeling hot and feeling humid are different beasts. The latter, defined by sticky sweat and shortness of breath, is unbearable. But why?