Barometric pressure can be an important part of the fish biting equation, but it's greatly misunderstood. Barometric pressure is the force of the atmosphere usually, expressed in terms of the height of a column of mercury in an instrument (barometer), used for measuring atmospheric pressure.

While trying to reach equilibrium, the air on top compresses the air that is lower or closer to the Earth. The compression of air caused by high-pressure causes heating of air molecules. This heating raises the temperature of the water in the air, causing it to evaporate.

On the other hand, in low-pressure areas the air closer to Earth becomes uncompressed and allows the air to cool. At this point the air temperature drops and allows the water in the air to fall below the saturation point causing clouds to form.

High pressure usually means few clouds and calm winds. Low pressure can spawn rain or even thunderstorms.

According to fish biologist Dr. Gary Van Gelder, when applying basic math it's easy to see that small changes in barometric pressure probably have little effect on fish directly. For example, a pressure change from 30 inches of mercury to 29 inches only represents a pressure change of about 13 inches of water.

“In this situation a fish would have to move one foot deeper to maintain the exact same pressure on its body,” Van Gelder said.

“Given that adult bass are neutral buoyant over plus or minus five feet over a 10-foot range, it's hard to see how this small difference would impact them.”

On the other hand, there may very well be something to falling and rising barometric pressure as this signals major changes in sky conditions, wind direction, and so on.

According to Van Gelder, there are two explanations that have at least some validity in explaining why fishing is sometimes more difficult on “bluebird days” such as those that follow a major cold or storm front.

“One explanation based on observations made by scuba divers that's consistent with fishing experience is that there is an active feeding period as the cold front moves into an area,” he said.

“The theory is that the fish gorge and are less active during the post-front ‘bluebird sky' period.

“The second explanation is that the higher levels of ultraviolet radiation adversely impact the smaller life forms in the food chain, and infrared radiation associated with sunlight under very clear sky conditions. It's possible the bigger fish have ‘learned' that feeding success is lower during these periods and thus maintain a lower level of activity until the food chain gets active and becomes more readily available.”

Many anglers seem to prefer the pressure to be around 30.00 or 30.10. Many professional fishing guides feel this is the peak biting period and say anything higher turns the bite off. There may be some science to back this up.

The Florida Game and Fish Commission put several species of saltwater fish, including speckled trout and redfish, in a large observation tank with a controlled atmosphere to study how pressure would affect their feeding habits.

At between 30.00 and 30.10 barometric pressure, the fish started to feed. When they turned the pressure up to 31.30, the fish died. It was believed the confined tank didn't allow the fish enough depth to equalize the pressure on their body.