Since it is winter, it is a good time to read so I thought a good subject to write about would be how the barometric pressure affects fish and fishing. In the following threads, I will be presenting a discussion on the barometer, barometric pressure, weather conditions, types of fish swimbladders, anatomy and physiology of swimbladders and buoyancy.
Mercury barometers rather than water barometers are used because the density of mercury (Hg) is 13.6 times that of water. Mercury barometers read pressure in in/HG. If water is used instead of Hg then the barometer tube would be 34 feet long. Standard barometric pressure at sea level is 29.92 in/Hg which corresponds to 14.7 psi or 33.9 feet of water (10.3m). There are barometers called aneroid barometers which utilize a bellows made of thin metal that expands and contracts as the air pressure changes which is communicated to a needle/scale dial. The "standard value" for the average atmospheric pressure is 760 mmHg. But, barometric pressure (BP) varies from day to day and where you may live on earth. If you divide 76 cm by 2.54 (2.54 cm/inch) to convert to inches, then you would have 29.921 inches of Hg. If you were to dive down under water to 33 feet, you would experience a pressure change of 1 ATM or 760 mmHg.
If the weight of Hg is less than atmospheric pressure, the barometer Hg level will rise. If the weight of Hg is more than atmospheric pressure, the Hg level in the barometer will drop. In areas of low pressure, air is rising away from the surface of the earth more quickly that it can be replaced by air flowing in from surrounding areas. This reduces the weight of air above the barometer so the Hg level drops to a lower level. In constrast, in areas of high pressure, air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas. There is more air above the barometer, so the weight of air is high and the Hg rises to a higher level.
Atmospheric or barometer pressure is the force per unit area exerted against a surface by the weight of air above that surface at any given point in the earth's atmosphere. In other words, BP is the pressure of the atmosphere pushing down on the earth, and this pressure is measured by a barometer. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass. As elevation increases, there is less overlying atmospheric mass. We live at the bottom of the atmosphere on earth just as fish living at the bottom of a lake are subjected to the pressure exerted on them by water. One cubic yard of air at sea level pressure at a temperature of 70 degrees F weights almost 2 pounds so air is heavy. If you were to weigh the air in a room 12ftX14ftX8ft, it would weigh almost 100 pounds. Modern technology now allows the BP to be measured by digital barometers which use an electricl charge.
As we climb mountains, go from the basement to upstairs in our homes or swim upward, we experience a decrease in pressure. You expect the air in the atmosphereto distribute itself uniformly over the surface of the earth so that the BP at the same elevation would be the same everywhere, but this not the case. One of the things that changes weather is the uneven heating of the earth's surface. Warm air is lighter than cold air, so it rises, thus creating wind. It is this ascending warm air and descending cold air that causes the BP to fludctuate. The rotation of the earth and uneven heating of the earth's surface causes the air to slosh around so some places are heavier or lighter than others. Valleys and mountains vary in height as well. Where the air is piled up, the BP on the ground is greater, which is called a high pressure area. Where there is a valley or the air is thinner, it is called a low pressure trough.
Air also flows from high pressure regions to low pressure regions. However, the rotation of the earth influences the direction of the air flow. When this occurs, rather than the air moving away from the high region, it will circle around the high region in clockwise direction. Where air flows into a low pressure region, it will circulate ina counter-clockwise pattern, with a small inward movement. The direction of air circulation will be reversed in the southern hemisphere. Winds tend to blow parallel to the contours of equal pressure (isobars) drawn on weather maps. When the BP is high, the air is sinking and pushing on the ground. That same air must eventually rise when weather conditions change. When it rises, air cools and moisture condenses and causes the PB to drop. Weather partterns constantly change over the earth. When a mountain of air floats over us, we experience high BP readings, and when a valley of air moves over us, we get lower BP readings. Range of variation is not great, generally 0.5 in/Hg or less above or below the average. The lowest BP ever recorded was Hurricane Gilbert in 1988, which was 26.22 inHg.
Most lake water, as well as the arterial blood of fish, has a partial pressure of oxygen of approximately 0.2 atmospheres and a partial pressure of nitrogen of around 0.8 atmospheres, but in the swimbladder, partial pressure may vary from 100 atmospheres to 20 atmospheres. Some deep-sea fish are able to create and maintain pressures as great as 200 atmospheres.
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Originally Posted by justlookingn
