The most sticky, hot, and humid places in the world tend to be found in Southeast Asia, near coastal regions around the equator. Anyone who is not used to the heavy, damp, often motionless air can find them to be very uncomfortable places to live. Humidity, the moisture content of the air, tends to be high in these places because the heat of the sun causes the air to absorb increased moisture from the surrounding seas and oceans—the air in cold latitudes is relatively dry. But it was not until the 1600s that people were able to measure air humidity.
Technically, Leonardo da Vinci designed the first crude hygrometer in the 1440s, but in 1664 the first practical hygrometer, used to measure the moisture content of air, was invented by the Italian scientist Francesco Folli (1624-1685). Folli's invention was a finely decorated device, made of brass, that contained a mounted paper ribbon acting as a hygroscopic (moisture-absorbing) indicator. When the ribbon changed in length as a result of changes in its water volume, a simple mechanical system moved a pointer on a central brass dial marked with a graduated scale. The pointer indicated variations in humidity.
Some modern hygrometers still use principles very similar to those of Folli's original design. One commonly seen improvement of the original is that blond human hair, rather than paper ribbon, is the medium used to expand and contract in response to variations in atmospheric moisture.
However, there are now many different types of hygrometer. The most common is the dry and wet- bulb psychrometer, which compares readings of dry and water-immersed thermometers. Others use semiconductors to measure changes in electrical resistance, which is affected by humidity.
