Barometers & Barometric Pressure

Barometric Pressure

• Barometric pressure is a measure of air pressure, the weight with which air presses down on earth. The amount of pressure exerted is directly related to the air's density. Air density is mainly a function of three major factors: temperature, moisture and altitude. According to Boyle's Law, as a gas' pressure increases, its density decreases and vice-versa. For weather, this means that as barometric pressure increases, air density decreases. Days with is high air pressure generally have good weather, while low pressure days are often stormy.

Barometers

• Barometers are tools used by meteorologists to measure air pressure. The first was invented in 1643 by Evangelista Torricelli when he put mercury into an airless glass tube -- in absence of air pressure from within the tube, the height of mercury in the tube fluctuated with outside air pressure. For this reason, surface barometric pressure is commonly measured in inches of mercury (inHg), though scientists use the metric hectopascals (hPa). Milibars are used to measure air pressure aloft. Whichever system is used, the equivalent of 30 inHg is normal air pressure. If there is more it means air pressure is high, foreboding good weather, if low it indicates bad weather.

Fishing

• Barometric pressure can also be useful for helping fishermen to predict fish behavior, especially in fresh and shallow waters where the effects of air pressure are felt the most. When skies are clear and air pressure is high many fish slow down and seek cover in deeper waters, and when it is rising they are often slightly more active than usual. As pressure falls, fish also become more active; and when it is lower than usual they become aggressive while seeking shallower waters. When weather is bad and pressure is very low, however, fish tend to be less active.

Aches and Pains

• Rising air temperature and pressure cause some people to get "barometric pressure headaches." Although the exact cause of these headaches is unknown, it is thought they are caused by the changes in oxygen levels that come with changes in barometric pressure. If you seem to get headaches or other body aches based on weather, it is recommended that you buy a barometer to gauge how your pain corresponds with air pressure. If there seems to be a connection, talk with your doctor.