What humidity is too high to dry clothes outside?
Above 70% relative humidity, outdoor drying gets unreliable. Above 80% it mostly stops working, and thick items like towels and jeans may still be damp after a full day on the line.
The thresholds worth remembering
Under 50% is a gift. Washing dries fast, thick items included, and you can hang a full load without thinking about the order.
Between 50 and 65% is the normal working range. Everything dries, just at different speeds, so put towels and jeans in the windiest spot and thin items anywhere.
Between 65 and 75% is marginal. Thin cotton will dry, bedding probably will, towels probably will not. This is the band where wind decides the outcome.
Above 75% and you are fighting the air. Above 85%, washing can genuinely come in wetter than it went out if the temperature drops in the evening.
Why humidity beats sunshine
Think of the air as a sponge. At 30% humidity it is nearly dry and soaks up moisture from your washing happily. At 90% it is already dripping, and evaporation slows to a crawl no matter how brightly the sun is shining on it.
This is why a bright, muggy day after rain is such a common trap. It looks like perfect drying weather from the kitchen window, and the air has no capacity left at all.
Dew point is the honest number
Relative humidity wanders as the temperature wanders, because warm air can hold more water than cold air. The same amount of moisture reads as 60% at midday and 90% at dusk without anything actually changing.
Dew point does not move like that. It is the temperature the air would have to cool to before it is completely full, so it is a plain measure of how much water is genuinely in there. A dew point well below the air temperature means dry, thirsty air. A dew point close to the air temperature means your washing is going nowhere.
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