Why do clothes dry faster when you open the window?

Why do washed clothes dry faster in open air than in a closed room? — A, Aizawl, India

What thrills me about your question is that while we’ve all noticed this effect, we’re never taught why it happens. Let me ask your question in another way: we know that opening a window makes the clothes dry faster, but how do the clothes know that the window is open? Who tells them?

The explanation is both simple and interesting: the rate at which water molecules leave the cloths doesn’t depend on whether the window is open or closed, but the rate at which water molecules return to the cloths certainly does. That return rate depends on the air’s moisture content and can range from zero in dry air to extremely fast in damp air. Air’s moisture content is usually characterized by its relative humidity, with 100% relative humidity meaning that air’s water molecules land on surfaces exactly as fast as water molecules in liquid water leave its surface. When you expose a glass of water to air at 100% relative humidity, the glass will neither lose nor gain water molecules because the rates at which water molecules leave the water and land on the water are equal. Below 100% relative humidity, the glass will gradually empty due to evaporation because leaving will outpace landing. Above 100% relative humidity, the glass will gradually fill due to condensation because landing will outpace leaving.

The same story holds true for wet clothes. The higher the air’s relative humidity, the harder it becomes for water to evaporate from the cloths. Landing is just too frequent in the humid air. At 100% relative humidity the clothes won’t dry at all, and above 100% relative humidity they’ll actually become damper with time.

When you dry clothes in a room with the window open and the relative humidity of the outdoor air is less than 100%, water molecules will leave the clothes more often than they’ll return, so the clothes will dry. But when the window is closed, the leaving water molecules will remain trapped in the room and will gradually increase the room air’s relative humidity. The drying process will slow down as the water-molecule return rate increases. When the room air’s relative humidity reaches 100%, drying will cease altogether.

Leave a Reply