Can any one give me a short description on the hygrometer the aneroid barometer and the rain gauge?
A hygrometer is simply made of something organic, like a human hair, that stretches when the relative humidity increases. Note that it only shows the relative, not the absolute humidity. As the temperature increase, the air can contain more moisture and the relative humidity, i.e. the percent of saturation, decrease. An Aneroid barometer is simply a sealed box where the pressure on one surface is sensed and displayed. Like the hygrometer, it must be calibrated. The average pressure at sea level is 1013.25 hPa. In the USA, on still holds to the inches of Mercury. The Mercury barometer is open to one end and closed to the other. As the atmospheric pressure chances, it changes the vacuum at the other hand. The average pressure then correspond to a column of Mercury that is 760 mm high, or 29.9 inches. The latter still is the calibration of aviation altimeters in the USA. Elsewhere, the unit is the hecto-Pascal (hPa) which is the same as a millibar. A rain gauge is simply a container. One
