A barometer hygrometer is a weather instrument that measures both the atmospheric pressure and the relative humidity of a location. The instrument is typically calibrated to work at a certain range of altitude, such as 0 to 3,000 feet (0 to 0.9 km). It is commonly manufactured as either a dial model or a digital model. The barometer portion is used for forecasting weather, and the hygrometer shows if the relative humidity level is in what is considered to be the comfortable zone for most people.
The dial model of a barometer hygrometer has two circular faces with needles — one that measures atmospheric pressure and one that measures relative humidity. Air pressure is measured with an aneroid type of barometer that is composed of an aneroid cell that expands or contracts as pressure changes. A lever makes the needle on the face move to point to the current amount of air pressure. The hygrometer dial works with a human or animal hair that expands or contracts with changes in humidity, which makes a spring move the needle on the face of the dial.
A digital barometer hygrometer displays the numeric air pressure and relative humidity on a small screen, and the device generally runs on batteries or plugs into an electrical outlet. It uses electrical charges to determine the current air pressure. The relative humidity is calculated from changes in electrical resistance or capacity that is caused by moisture in the air. Some digital versions will have additional features such as displaying the temperature, wind speed, and dew point.
One of the abilities of a barometer hygrometer is to aid with forecasting weather in the short term by showing the atmospheric pressure changes. Atmospheric pressure is the weight of the air molecules above the point of measurement. High pressure typically means the weather is dry and fair, and low pressure can indicate rain or storms. When air pressure rapidly changes, it can indicate severe weather. A barometer will generally measure air pressure in terms of millibars or pascals.
A barometer hygrometer also measures relative humidity, which allows one to see if the humidity is in the comfort zone. Relative humidity is the amount of water vapor in the air compared to how much water vapor the air can hold at that temperature, as warm air can hold more moisture than cold air. It is usually expressed as a percent, with 100% meaning that the air is completely saturated. The relative humidity comfort zone for a human is typically from 30% to 70% for one to feel the most comfortable. In the summer, low humidity can make it feel cooler and dries out the skin, and high humidity makes it feel hotter since sweat cannot evaporate off the skin.