Humidity
From Swikipedia
Introduction
Humidity is the amount of moisture in the air. It can be measured in various ways, but the most usual is to describe it as 'relative' humidity. This is expressed as a percentage. A relative humidity of 100% means the moisture content of the air is the maximum possible at any particular temperature. The hotter the air, the more moisture it can hold.
Brief Description
When relative humidity is low, evaporation is rapid. Soil dries out, wet clothes dry quickly and perspiration evaporates from the skin. When relative humidity is high, clothes dry slowly and body sweat cannot evaporate easily. We feel sticky and hot and because our perspiration cannot evaporate quickly, we feel uncomfortable. We can stand dry heat much better than damp heat which makes us feel listless and takes our energy away.
Recommendations for comfort:
Humans control their body temperature by sweating and shivering. The United States Environmental Protection Agency cites the ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control dust mites. At high humidity sweating is less effective so we feel hotter; thus the desire to remove humidity from air with air conditioning in the summer. In the winter, heating cold outdoor air can decrease indoor relative humidity levels to below 30%, leading to discomfort such as dry skin and excessive thirst.
Humidity in construction:
Traditional building designs typically had weak insulation, and this allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation forming in and around walls, which encourages the development of mold and mildew. Solutions for energy-efficient buildings that avoid condensation are a current topic of architecture.
References:
http://www.bbc.co.uk/weather/weatherwise/activities/weatherstation/humidity_what.shtml
