A water barometer is a device that uses water to measure barometric pressure. The device uses the force of gravity, combined with pressure in the air, to create a vacuum, which in turn gives a reading that indicates the barometric pressure changes that are taking place in a specific area. Water barometers, which have been used for thousands of years, can be used to predict weather pattern changes. Low atmospheric pressure indicates precipitation, or rain, while high atmospheric pressure indicates dry weather.
Sometimes referred to as a "weather glass," a water barometer typically consists of a glass container that is half filled with water. The glass has a spout that sits below the water line and is open to the environment. The pressure from the atmosphere pushes the water down as it tries to rise from the spout; when an area of high atmospheric pressure occurs, the water level drops. Conversely, when an area of low pressure moves in, the water level rises in the glass.
By reading the water level, a forecast for short-term weather events can be made, and a gauge can also be affixed to the barometer to allow readings to be taken. Water barometers have been used by hobbyist boaters and commercial sailors alike to forecast the weather. While not providing the most accurate measurement possible, the water barometer provided early meteorologists with valuable information in regard to predicting weather phenomenon.
Water barometers have several drawbacks that have added to their decrease in use. First, because water barometers use water as a means of measurement, the water needs to be monitored and replaced on a regular basis. This is because the water evaporates as a result of being openly exposed to the atmosphere. Second, a water barometer cannot possibly offer the accurate results that modern forecasting demands. A water barometer is meant only to give a general idea of an approaching weather pattern.
While water barometers are not obsolete, more modern methods, materials, and technologies have, by and large, replaced the weather glass model. Among other weather tools, mercury barometers and aneroid barometers provide more accurate and specific results. Computer simulation software further assists forecasters in predicting upcoming weather.
Many schools still use the water barometer as a science experiment in classrooms around the world. A simple water barometer can be made by sealing a glass tube on one end and then filling it with water. The sealed end of the tube is then submerged in a glass of water and unsealed. The tube will need to be clamped in place so it doesn't move. Once unsealed, a vacuum will be formed within the tube and the water level will change along with the barometric pressure in the atmosphere.