Leaving washing out after dark carries a real risk in temperate climates: dew can settle and re-wet fabric that spent the whole day drying. Whether that risk is high or low on a given evening depends on specific atmospheric conditions you can learn to read before making the call.
What dew actually is and why it matters for washing
Dew forms when a surface cools to a temperature below the dew point of the surrounding air. At that point, water vapour in the air condenses into liquid droplets on the surface, in the same way a cold glass sweats on a warm day. According to the Met Office , up to 0.5 mm of dew can form on a surface in a single night under the right conditions. That may not sound like much, but it is enough to leave fabric noticeably damp by morning.
The dew point itself is a measure of how much water vapour the air holds. As the Met Office humidity guide explains, warmer air can hold more water vapour than cooler air. When the air temperature drops overnight toward the dew point, the gap between the two narrows. Once the surface of your washing cools past the dew point, condensation begins.
The conditions that raise dew risk overnight
Not all nights carry the same risk. According to the Met Office , dew forms most readily during the calm, settled conditions associated with high-pressure systems. A clear sky allows surfaces to radiate heat away freely, so they cool faster and further. A cloudy sky acts like a blanket, slowing that radiative cooling and often keeping surface temperatures above the dew point through the night.
Wind also plays a role. Moving air mixes warmer and cooler layers together, which can limit how far a surface cools. On a still night, a thin layer of air sits directly against the fabric with nothing to disturb it, allowing the surface temperature to fall further.
In practical terms, a calm, clear evening is when dew risk is at its highest. A cloudy, breezy evening carries lower risk, though not zero. In autumn, when overnight temperatures fall more sharply, the gap between air temperature and dew point can close more quickly than in summer, so the same forecast cues may warrant more caution depending on the season.
Reading a forecast for dew risk before nightfall
You do not need specialist equipment to make a reasonable assessment. A few forecast indicators are worth checking.
Cloud cover overnight: A forecast showing clear or mostly clear skies through the night is a signal that radiative cooling will be efficient. Significant cloud cover breaks that process and reduces risk.
Wind speed: A forecast showing near-calm or very light winds overnight supports conditions where surface temperatures can drop furthest. Any meaningful breeze tends to reduce that effect.
Dew point temperature: Some forecast apps and the Met Office’s own tools show dew point alongside air temperature. When the overnight low and the dew point are close together, the risk is higher. Weather forecasts combine observations from satellites, surface instruments and upper-air devices to generate these figures, as the Met Office forecasting guide describes, but all forecasts carry some uncertainty, so treat the figures as a guide rather than a guarantee.
Relative humidity: As the Met Office humidity guide explains, relative humidity indicates how close the air is to being saturated with water vapour at a given temperature. High overnight relative humidity means the air is already close to saturation. Combined with surface cooling, this creates conditions where even a small temperature drop can push a surface below the dew point.
For more on reading forecast details before making outdoor drying decisions, see how to read forecast uncertainty before hanging washing outside .
How to judge whether to bring washing in
If a forecast shows a clear, calm, humid night with an overnight low close to the dew point, the practical suggestion is to bring washing in before the temperature drops significantly in the evening. The surface of hanging fabric, particularly on a metal rail or line, can cool quickly once the sun is gone.
If the night looks cloudy and breezy, the risk is lower and leaving lighter fabrics out a little longer may be a reasonable choice, depending on how dry they already are. Thicker items like towels retain moisture longer and are less likely to finish drying overnight in any conditions.
The Met Office notes that dew forms most easily on surfaces that do not conduct heat from the ground. A washing line or drying rack holds fabric away from the ground, exposing it to open air on all sides, which may make it more susceptible to dew than fabric on a warmer surface might be. That is worth bearing in mind when deciding whether to leave items on a rack in the garden versus under a covered porch, where direct sky exposure is limited.
For guidance on the broader decision of leaving washing out after dark, see Can you leave washing out after dark in autumn? and when should you bring washing in before the forecast changes? .