You've been out in the rain for a while. At some point your clothes go from damp to soaked, and suddenly the temperature seems to have dropped several degrees. It hasn't. What changed is how fast your clothing is pulling heat away from your body.
Why wet fabric stops being insulation
Clothing insulates by trapping air in the spaces between fibres. Still air is a poor conductor of heat, which is what you want when you're trying to stay warm. Water is not a poor conductor of heat: it conducts thermal energy approximately 25 times better than air. When water displaces the air in your clothing, the insulating layer turns into a heat-extraction layer. The more saturated the fabric, the faster the heat loss.
Wind makes it significantly worse. Water on the surface of fabric evaporates, and evaporation is a cooling process. In still air this happens slowly. In wind it happens fast.
| Fabric state | Conditions | Heat loss relative to dry cotton |
|---|---|---|
| Dry cotton | No wind | Baseline |
| Wet cotton | No wind | 5-8x faster |
| Wet cotton | 20 km/h wind | 15-20x faster |
| Wet wool | No wind | 2-3x faster |
| Wet synthetic | No wind | 3-5x faster |
Wool holds onto more of its insulating properties when wet than cotton or most synthetics. This is why wool has historically been the material of choice for sailors, fishermen, and anyone else who expected to get thoroughly wet in cold conditions.
When this becomes more than discomfort
At 15°C, wet clothing will make you feel cold quite quickly. At 10°C it becomes a genuine concern rather than just an unpleasant experience. The body can lose heat faster than it generates it, and wet clothing accelerates that process. Prolonged exposure in wet clothing at low temperatures can lead to hypothermia even at temperatures that don't sound dangerous.
The combination of wind and wet fabric is the dangerous one. A 10°C still day in soaked clothing is uncomfortable. A 10°C windy day in soaked clothing is a situation worth taking seriously.
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What this means practically
If you're going to be outside for several hours and rain is forecast, a waterproof outer layer is worth carrying even if the temperature alone seems mild. Once the base layers are wet, temperature protection is substantially reduced regardless of how many layers you have on.
KorteBroekAan.nl includes rain and wind in the feels-like temperature calculation, which captures some of this. Fabric choice and layering strategy are down to you, but the article on dressing for changeable weather covers how to approach that.
Related articles in the Weather Explained section: rainfall amount, rain probability, and drizzle vs showers.
Further reading
Related articles in the Weather Explained section:
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