Barometric pressure is the weight of the overlying air pressing down on the earth. It is also known as air pressure. Low barometric pressure means the overlying air is rising, whereas high pressure means the overlying air is sinking.
Barometric pressure has important effects on water chemistry and weather conditions.
It affects the amount of gas that can dissolve in water. More gas, such as oxygen, can dissolve in water under higher pressure when compared to lower air pressure. For instance, more oxygen is dissolved in water at sea level than at high altitudes.
The pressure of the overlying air forces more gas into solution. As the overlying pressure decreases, gas is released from solution. A common example of this is when someone opens a carbonated beverage.
High barometric pressure supports sunny, clear, and favorable weather conditions, but lower levels promotes rainy and cloudy weather conditions.
This atmospheric parameter has been used for hundreds of years to forecast weather conditions. Differences in barometric pressure over large inland lakes can produce seiches, which affect water levels in the lake and adjoining estuaries.
Barometric pressure is typically reported in inches of mercury or in millibars. 1 inch of mercury equals about 33.9 millibars,
Inches of mercury is the traditional way of reporting air pressure. Greater air pressure pushes a column of mercury higher in a barometer. Millibars are a more direct measure of barometric pressure and are the units used in SWMP.
Traditionally, a barometer was used for measuring barometric pressure. SWMP uses a more modern and accurate device called a capacitative pressure sensor.
The barometer was first devised in 1644. It 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.