Condensation and Dew

Condensation, on the inside of the kitchen window on a chilly morning or on a cold glass of water, and dew on the grass on a cool autumn evening, is a familiar sight, but what is it and what causes it?

About Water

Water molecules can be a solid (ice), a liquid (water) or a gas (water vapour), and this depends on temperature. Water evaporates into the air as water vapour from warm surfaces and then condenses back into droplets of water onto colder surfaces.

What Is Condensation?

Condensation is the film of droplets of water that forms when air that is full of moisture hits a surface that is colder than the air. This can be inside of a window in a warm room when it’s cold outside, the outside of a glass of ice-cold lemonade, or someone’s glasses when he or she opens an oven door or a dishwasher that’s just finished its cycle.

What Is Dew

Dew is a form of condensation – it happens when the surfaces outside are colder than the air, causing the droplets of water that are on a cobweb on an autumn morning, or the dampness of the grass on a late summer evening. The temperature when the dew forms is called the dew point.

Working Out the Dew Point

Try working out what the dew point is. Take the air temperature indoors (and the humidity if possible), and make a note of the weather. Fill a tin with warm water and take the temperature of the water. Add a lump of ice to the water and stir it, and take the temperature again. Keep adding ice, one lump at a time, stirring, and taking the temperature, until condensation begins to form on the outside of the tin. This temperature is the dew point.

Try this again outside – is the temperature of the dew point any different? Try it on different days – does the temperature of the dew point change with the weather or the relative humidity?

If you know the relative humidity, try calculating the dew point using the formula Td=T-((100-RH)/5)(Td=dew point; T=temperature; RH=relative humidity).

Is the calculated dew point similar to the dew point from the experiment?

The dew point is determined by how much water the air can hold as vapour before it has to turn back into liquid. When the air temperature is lower, the dew point will be lower, as warmer air can hold more water than colder air – this is why some hot days feel muggy and close, and why water vapour turns back to liquid on a cold surface. When humidity is high, the air is closer to the limit of the amount of water it can hold.

Why Is Condensation A Problem?

Condensation in the house is caused by water vapour from the environment, or from people breathing, drying clothes, cooking and heating. The water vapour condenses anywhere that is cool enough, especially windows and tiles, and can cause problems with dampness, mould and rotting in wood and fabric. Condensation in the house can be helped by ventilating areas well, and by using a dehumidifier, which draws moist area of cooled plates or pipes, allowing the water to condense out into a collection chamber.