by Bruce Middleton, March 25, 2008

What every fisherman needs to know about Barometric Pressure…

Lets start out this article with a little known fact: bass fishing can be just as bad if the barometric pressure is very low or very high. It’s the truth and I will take you on a tour of this World Wide fact and how it affects the way you need to approach bass fishing. But in order to understand how to fish under different times and weathers and the rising and falling barometric pressure you need to start at the beginning and understand what barometric pressure is and how we measure it.

Now we have all seen different types of barometers, mercury filled, mechanical and electrical. But basically they all work on the same principles so whichever kind you own or look at will give you the same readings.

Air Pressure is the weight of the air from hundreds of miles up in space all the way down to sea level. Air molecules are stacked upon each other and they exert pressure due to the force of gravity. An average air pressure of 14.7 pounds per square inch is found at sea level and it decreases as you climb higher up, say like up on top of a mountain. Changes in the weather also affect the barometric or air pressure.

As an example, take a hot air balloon and fill it with hot air. It will rise because the air pressure is less in the hot air balloon than all around it. This less dense air is lighter and that is an important factor in how barometers work. Also too you can understand that the air pressure would normally be low over a hot desert giving you a high barometric pressure and it would be high over an ice cap giving you a low barometric reading. Remember that a Barometer measures what the air pressure exerts on the column of air or liquid. This shows that heat or warmth is a driving factor. This air pressure changes all the time. It changes because of the rotation of the Earth, the winds around the world and the weather it causes. These differences can also affect we humans too. Many older people and people who suffer with arthritis can feel their joints ache with the weather changes and it can even cause headaches.

Now an old Barometer is a tube that is filled with mercury or sometimes other liquids. The tube is long and straight but at the top is a bulb to hold extra mercury. A small space is left so that the tube is not filled all the way to the top. It is then sealed and turned upside down so the bulb is now on the bottom. This causes the liquid to pull down on the mercury due to air pressure or the gravitational effect, which forms an empty space above the liquid, which is almost a vacuum now. It is a vacuum because it now has a larger air space in it than when you left the original air space. Mercury is used because it weighs a lot and even a small amount can be very heavy. A vacuum is used because it has zero air pressure. Mechanical and electrical units use a pressure transducer and/or bi-metal springs to accomplish the same task. But in an old barometer, as the air pressure moves up and down it causes the level of the mercury to rise or fall because it exerts less or more pressure per square inch on the column of fluid in the Barometer. Since a vacuum has zero air pressure, the weight of the fluid pulls down on the liquid until it reaches equilibrium. It is then marked in millibars so we can take a reading and see what the barometric pressure or air pressure is outside.

Now an average column of air extending up into space is equal to 14.7 pound per square inch of pressure. This is quite heavy, and water is even heavier and doubles its weight in air pressure every 33 feet. So if you dive to 33 feet the accumulated air pressure is equal to twice that of normal. And every 33 feet deeper you go it adds another atmosphere of pressure or another 14.7 pounds. Scuba diver are very attuned to this fact as it determines how deep they can dive and how they may or may not require to make staged stops on the way up to prevent the bends (nitrogen bubbles in the blood).

Now back to the Barometer, the average barometric pressure is about 30 millibars or 29.7 to be precise. The highest recorded air pressure was registered in North Carolina at 30.50 and the lowest was during a hurricane when it fell to 27.08 millibars. Now don’t worry about millibars, as they are really inconsequential. The numbers are all we need to know about. If the barometer is at 29.7 it will be a nice day and if it falls below that mark the weather will deteriorate. Conversely, if the barometric pressure starts from say 28.9 and moves up to 29.7 then the weather will improve and become warmer than it was.

Now how the barometric pressure of the air affects fishing is not totally understood by us yet but we do know quite a bit about how it affects fishing in shallow and freshwater. In water objects like zooplankton, phytoplankton and sediments rise, sink or suspend in the water column. But a sudden change of air pressure or the barometric pressure (which are the same thing for our purposes) is a change in the gravity of things on or in the water. Understanding the changes this causes on the tiniest creatures in the water up to the biggest has a major impact on how each of them act. With bass it has a minor affect on the pressure sensitive lateral lines the bass uses to hunt with. Bass are also built with an air bladder they use to hold at whatever level they choose. They are less affected than the tiniest creatures are.

A drop in air pressure (equal to a rise in barometric pressure) can cause sediment, plankton and other tiny and microscopic life in the water to rise up off the bottom. These living particles are the first rung in the food chain. Most all these creatures have some sort of mechanism that they use to allow themselves to raise of fall within the water column. But sudden changes in air pressure can cause them to float up and become easy prey for predators before they can sink to the bottom and make it harder for those predators to feed on them. Since tiny animals eat these creatures and then small fish eat them and finally bass eat the baitfish, the placement of these tiny animals (zooplankton and phytoplankton) is important to understand. As the tiny creatures rise they try very hard to equalize themselves but it still clouds the water up off the bottom with life and sediment. If they rise up a lot it makes it easier for small predators to feed on them. This feeding follows up the food chain. First the very tiny are fed on which draws in larger predators that feed on them. This in turn draws in bigger predators that feed on this lower level predators and so on and so on until you are up to the size of a bass.

Baitfish and bass all have internal air bladders that they use to regulate where they want to be in the water column. But a decrease in air pressure causes baitfish and bass to become more active because starting from the bottom of the food chain it is easier to find food. This energizes the fishing and usually means a good day on the water. Now as you can see this drop in air pressure has more affect on the tiniest creatures than it does the larger ones. The lower the air pressure and therefore the water pressure the higher these tiny animals rise. This makes it much easier for small predators to feed on them. You also have to understand that a tiny creature that is buoyant at three feet will sink if it is suddenly dropped to ten feet. Also if a lot of plankton is on or near the surface and the barometric pressure drops, it causes these tiny creatures to sink down to levels that the baitfishes normally live at. So even slight rising and fall of barometric pressure can cause good fishing days.

Now large changes in the barometric pressure have dramatic affects on all the creatures in the water from the largest to the smallest. With high air pressure or a low barometric pressure reading, all the tiniest animals and algae sink to the bottom where it is harder to be fed on. This causes baitfish to move out away from their shallow feeding grounds and out into open water where they suspend off the bottom where food may be more abundant in deeper water. This causes bass to sink to the bottom and wait out these changes as they too adjust to the pressure changes. And since bass are creatures of shallow cover, they seldom venture out to deep water where the baitfish are at this time.

So we have five different scenarios to understand about barometric pressure, the affect it has on bass and how we should be fishing in order to catch more bass. The first scenario is HIGH Barometric pressure or low air pressure. This is found under very clear skies and hot weather. Bass slow down, feed less and often head for deeper water where it is more comfortable, has a lot of shade and has less light. In order to fish for bass under these conditions it is best to use slow baits and lures, fish a little deeper than normal and try to find the bass in deep cover.

Scenario two is when the barometer is just starting to rise from below normal, 29.7 millibars. Improving weather and clearing skies accompany this. Bass become slightly more active and rise up in the water column. Good places to find them are around boulder piles and shallow hard bottom spots on the lake that warm up fast and hold the heat in the water around them. Bright lures are best in this situation. Intermediate fast lures and baits work now as well as many top water lures in shallower areas.

Scenario three is when the barometer is at average and steady. This is fair weather time when it is neither too hot nor too cold. This is the time when you can use all your favorite lures and baits and fish just about anywhere you prefer. These are the best fishing days to go out on. Long stretches of steady barometric pressure are tops for bass fishermen.

Scenario four is when the barometer starts to fall from normal. This is when cold air moves in and is denser and therefore adds more weight to the air column forcing the barometric pressure down. This is when the weather starts to degrade and clouds start to move in but you still have partly sunny times. This is the time when bass are the most active. They feed actively and often. Top water lures are a go to bait as well as jerk baits (hard and soft) and suspending lures or drop shotting techniques.

Scenario five is when the barometer is slightly lower. The clouds cover the sky and little sunshine breaks through. Again this is a great time to bass fish. All your favorite lures and baits will work but shallow water is better than deep at this time. Fast moving lures like Rattle-Traps™ ripped across the top of weed and grass beds is a great choice at this time.

Scenario six, low barometric pressure. This is when it is stormy outside and usually just plain nasty weather to fish in. Bring raingear, as you will need it. Bass will tend to be less active the longer it lasts. This is a time to slow down and go back to plastics fishing slow and methodical. The bass’s strike zone will shrink farther and farther as this type of weather wears on and the tough bite is the phrase of the day.

So now you know a little bit about air pressure and Barometric pressure. They measure the same thing but give you opposite readings. It may be confusing to some but if you don’t grasp the idea well, it would serve you to reread this article again. Also a lot of information can be found on the web about this subject and I recommend you research it if you’re still confused.

As the article stated in the beginning, we don’t have all the answers to how barometric pressure works when applied to fishing but what we do know is very important to understand. I hope this glimpse into air pressure can help you make better choices in lure selection and where to fish for bass as the weather changes and I hope it helps you land a few more fish.

Now as to my own theory about barometric pressure and bass fishing, I have mentioned that I don’t totally subscribe to these traditional theories 100%. I believe it is much more complicated than just fishing by a barometers readings. I believe a lot of it has to do with the light levels in the water at the depth the bass are found in. Since barometric pressure is an indicator of the weather it is also an indicator of the amount of light that exists in the water. So during high barometric pressure the sun is out and the bass will seek protection from the high light levels. As the barometer fall it also lowers the amount of light in the same place in the bass’s world. This decrease in light levels help the bass hunt as they can hide easier but still see quite well, so they rise up following the food chain. Since a bass has huge eyes as apposed to those of a baitfish they have the ability to see farther and better than the prey. This works to the bass’s advantage until the barometric pressure drops so low that the nothing is feeding along the food chain and the bass just hunkers down and waits for better times.

Since a bass can control where it wants to be in the water column it moves up and down this column of water pretty much like the barometric pressure. When the pressure is normal the bass are in water that is comfortable for them. But as the air pressure builds up and the bass becomes heavier it has a tendency to follow this change and drop lower in the water column.

Now while I have no scientific proof for my theory, it makes sense to me. I have seen a small barometric change affect bass in huge rivers verses a lake and its still water. I think that these barometric changes have less to do with how a bass feeds as opposed to the changing light levels. Light levels also influence heavily the baitfish’s ability to hide or be camouflaged in the water. The combination of these and other factors combine to tell you where the bass are likely to be and how active they are apt to be. So in my own mind I am completely comfortable following this theory of mine as it follows the norm but includes other contributing factors that add up to better fishing intuition and the lure choices I make.

Well, there are all the facts we know and a basic theory of my own for you to think about. But don’t take my word alone. Do some research, see how it effects the lures you use during the year and its weather changes and see if you too can understand which lures work best, at what times and what other factors you think have effects.

Tight lines to you and enjoy!


Bruce Middleton
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