Weather forecasters, meteorologists and other scientists use a variety of instruments to measure data. The data is then analyzed, compared to existing knowledge of past weather patterns, and forecasts built from the information. Two instruments that provide essential data are the hygrometer and the barometer. The major difference between the two is that a barometer is used to measure atmospheric pressure of the surrounding air and a hygrometer is used to measure atmospheric humidity.
Beginnings of the Barometer
Italian mathematician and physicist, Evangelista Torricelli (1608 to 1648) invented the first known useful barometer. Torricelli's apparatus, based on the siphoning characteristics of a straw, used a column of mercury suspended in a tube to measure ambient air pressure. French mathematician and physicist, Blaise Pascal (1623 to 1662) had Torricelli's device tested at various altitudes to prove that atmospheric pressure was indeed what supported the column of mercury in the measuring instrument.
Beginnings of the Hygrometer
A hygrometer can be defined as any instrument used to measure humidity in the air. The prefix "hygro-" derives from the Greek word for "moist." Swiss scientist Horace Benedict de Saussure (1740 to 99) is credited with the invention of the first useful hygrometer. Saussure observed that hair expands and lengthens when wet and contracts and shortens during the drying process. Based on this knowledge, Saussure created a hygrometric device that utilized human hair to measure humidity in the atmosphere based on this knowledge.
By the late 19th century, most meteorologists had replaced Saussure's device with what is called a sling psychrometer, or more commonly a wet-and-dry-bulb thermometer. A sling psychrometer is a type of hygrometer that measures relative humidity in the atmosphere by using the fact that the lower the humidity, the faster moisture evaporates. This weather instrument uses two thermometers, one is moistened and one remains dry. Measuring the cooling effect of evaporation and speed at which the wet-bulb thermometer dries can be used to calculate the amount of humidity in the ambient air.
An aneroid barometer measures atmospheric pressure without the use of liquids. Instead, an aneroid barometer uses a metal cell that shrinks or expands depending on the pressure of ambient air. Today the use of the barometer has become so ingrained in weather forecasting that meteorologists now refer to barometric pressure in lieu of atmospheric pressure. Recording barometric pressure simultaneously at various weather stations around the world allows meteorologists to measure the size and movements of air masses. This information is critical in formulating an accurate weather forecast.