Temperature V Relative Humidity

Air contains a certain amount of water vapour and the amount any mass of air can contain depends on the temperature of that air: The warmer the air is, the more water it can hold. A low relative humidity (% rH) means that the air is dry and could hold a lot more moisture at that temperature.

For example, at 20˚C a cubic meter of air can hold a maximum of 18 grams of water and at 25˚C it can hold 22 grams of water. If the temperature is 25˚C and a cubic meter of air contains 22 grams of water, then the relative humidity is 100%. If it contains 11 grams of water, the relative humidity is 50%. If it contains 0 grams of water, relative humidity is 0%.

The relative humidity plays a large role in determining our comfort level. If the relative humidity is 100%, it means that water will not evaporate - the air is already saturated with moisture. Our bodies rely on the evaporation of moisture from our skin for cooling and the lower the relative humidity is the easier it is for moisture to evaporate, making us feel cooler.

You may have heard of the heat index. The chart below lists how hot a given temperature will feel to us in various relative/humidity levels.

If the relative humidity is 100%, we feel much hotter than the actual temperature indicates because our sweat does not evaporate at all. If the relative humidity is low, we feel cooler than the actual temperature because our sweat evaporates easily; we can also feel extremely dry.

Low humidity has at least three effects on human beings:

·         It dries out your skin and mucous membranes. If your home has low humidity, you will notice things like chapped lips, dry and itchy skin, and a dry sore throat when you wake up in the morning. (Low humidity also dries out plants and furniture.)

·         It increases static electricity.

·         It makes it seem colder than it actually is. In the summer, high humidity makes it seem warmer than it is because sweat cannot evaporate from your body. In the winter, low humidity has the opposite effect. If you take a look at the chart above, you'll see that if it is 21˚C inside your home and the humidity is 10%, it feels like it is 18˚C. Simply by bringing the humidity up to 70%, you can make it feel 3˚C warmer.

For best indoor comfort and health, a relative humidity of about 45% is ideal. At temperatures typically found indoors, this humidity level makes the air feels approximately what the temperature indicates, and your skin and lungs do not dry out and become irritated.

Most buildings cannot maintain this level of humidity without help. In the winter, relative humidity is often much lower than 45%, and in the summer it is sometimes higher.