Relative humidity answers a strange question: how much moisture is in the air compared to the most it could hold right now? That "right now" is the problem. Warm air can hold dramatically more water than cold air, so the denominator moves all day long.
Which means the percentage can drop by half while the air gets no drier at all. It just got hotter.
A muggy summer morning at 94% and a sweltering afternoon at 46% can contain exactly the same water. Nothing dried out. The air simply expanded its capacity, and the ratio fell.
Where it does matter
Relative humidity isn't useless — it's just misused as a comfort gauge. It genuinely matters for things that respond to saturation rather than absolute moisture:
- Fog and cloud formation, which happen as air approaches 100%
- Drying times — laundry, paint, hay, concrete
- Static electricity and wood movement indoors in winter
- How quickly your skin and sinuses feel dried out in heated air
What to read instead
For "how will it feel outside," read the dew point. It's an absolute measure that doesn't move when the thermometer does, and it maps to comfort almost perfectly. Above 65° is sticky; above 70° is oppressive; below 55° is crisp.
For "is fog or low cloud likely," read the spread — the gap between temperature and dew point. A small gap means nearly saturated air, whatever the percentage happens to say at that moment.
Airport observations report temperature and dew point directly, side by side, precisely because the people using them need moisture information that doesn't lie when the sun comes up.