Overview
Barometric pressure is the weight of the overlying air pressing down on any given point. It is also known as air pressure. Low barometric pressure can mean the overlying air is rising, whereas high pressure can mean the overlying air is sinking.
Influencing Factors
Barometric pressure is mainly influenced by temperature and altitude. Barometric pressure is lower at higher altitudes because there is less air pushing down on the earth. Barometric pressure also decreases with increasing temperature. This is because air is less dense at warmer temperatures. This makes warm air rise, and cold air sink.
Low-pressure regions are masses of rising, warmer air, and high-pressure regions are masses of cooler, more dense, sinking air. As warmer air masses rise, they cool, allowing water to condense and form clouds. This is why low pressure is often associated with the formation of clouds and rain. High-pressure regions are typically associated with fair, sunny weather.
Importance
Barometric pressure has important effects on water chemistry and weather conditions. Barometric pressure affects the amount of gas that can dissolve in water. More gas, such as oxygen, can dissolve in water under higher barometric pressure than under lower barometric pressure. Therefore, more oxygen is dissolved in water at sea level than at high altitudes. Differences in barometric pressure over large inland lakes can produce seiches, which affect water levels in the lake and adjoining estuaries.
The pressure of the overlying air forces more gas into solution. As the overlying pressure decreases, gas is released from solution, such as when you open a carbonated beverage.
High barometric pressure supports sunny, clear, and favorable weather conditions. Low barometric pressure promotes rainy and cloudy weather conditions. Barometric pressure has been used for hundreds of years to forecast weather conditions.
Measurement
Barometric pressure is typically reported in inches of mercury or in millibars. One inch of mercury equals about 33.9 millibars. Inches of mercury (inHg) are the traditional way of reporting air pressure, while millibars are a more direct measure of barometric pressure.
Traditionally, a barometer was used for measuring barometric pressure. A barometer consists of a glass tube that is closed at one end and open at the other. The tube sits vertically, with the open end sitting in a pool of mercury. Changes in pressure will change the level of mercury in the tube. Increased air pressure pushes the mercury higher into the tube, whereas decreased air pressure allows the mercury in the tube to drop.
A more modern and accurate device is called a capacitative pressure sensor. With capacitative pressure sensors, changes in air pressure cause a silicon diaphragm within the sensor to bend, which changes the ability of the sensor to store an electrical charge. This change is measured and converted into a pressure reading.