Barometric pressure is the weight of an overlying air mass in a particular area. It’s synonymous with atmospheric pressure. Meteorologists monitor barometric readings to predict weather changes; shifts in the barometer can indicate the approach of a high-pressure (usually fair-weather) or low-pressure (more unsettled) system. While less familiar than the concept of temperature, barometric pressure is a useful—and fairly straightforward—statistic for the average, weather-wise person to keep track of. The condition is measured by a barometer, where air pressure affects water or mercury levels, or pushes on a spring-equipped cell, in a device that then translates this into a number (inches or millibars).
Obtain the reading from a weather forecasting resource. This could include the report on your local TV news or a newspaper spread. The data is readily available online from any number of prediction websites, including the National Weather Service’s. From these professional sources, the pressure may be displayed in either inches or millibars (a more technical unit of atmospheric pressure)—or both. Pressure readings usually range from 29 to 31 inches, or 980 to 1050 millibars. This is the barometric pressure at any given time. Keep in mind, however, that what is most important is barometric tendency—the variation in pressure over three-hour intervals. The exact measure matters less than the fluctuations; you should pay closest attention to whether pressure is increasing or decreasing, and at what relative rate.
Use a commercial barometer. As with any piece of equipment, you can spend a hefty sum on this tool if you so choose—but less expensive models can often be acquired from many larger retail stores and online. Aneroid barometers—which don't contain fluid—are most practical for the serious amateur. A little cell in the gadget includes a spring that is compressed or decompressed depending on air pressure; the measure is translated through a pointer to a value.
Create a water barometer. These are the oldest forms of this pressure-reader, developed in the 17th century. While commercial units are available, the National Weather Service includes a simple set of instructions on its website for constructing one yourself; see the link in the "Resources" section. Briefly, a clear piece of tubing is affixed to a ruler that has been taped to the inside edge of a beaker (with the numbers clearly visible). The beaker is half-filled with water. The observer then suctions water into the tube and caps it with clay or gum. Over 24-hour periods, the water levels in the tube will rise or fall with increasing or decreasing air pressure, respectively. The readings won't be as accurate as with professional models, but weather patterns should be discernible.