Barometric Basics

Earth’s weather is affected by a never-ending series of high- and low-pressure fronts that generally move from west to east in that part of central North America sandwiched between the Rocky Mountains and the Appalachians. The term “front” applies to the leading edge of these air masses, which may be warmer and of lower pressure (warm fronts) or cooler with high pressure (cold fronts).

A barometer measures the pressure exerted on the local environment by the atmosphere. It reports pressure as inches of mercury called millibars—an inch of mercury, for instance, equals 33,864 millibars. This system dates to the late 19th century when Norwegian meteorologist Vilhelm Bjerknes defined a formula for measuring atmospheric pressure. Barometric pressure is greatest at sea level and decreases with elevation, thus the need to adjust instruments for local elevation.

As commonly reported on the nightly TV weather news, barometric readings are noted as inches of mercury, ranging from 28.5 (extremely low) to 31 (extremely high). Avid weather watcher and In-Fisherman reader Irvin Decker of Issaquah, Washington, notes that the midpoint of atmospheric pressure is 29.92 inches of mercury.

During June 2000, Decker recorded barometric readings made by the National Weather Service in Seattle every three hours. “Barometric pressure varied between 29.65 and 30.32,” he reports. “A major front passed near the middle of the month, resulting in a pressure drop of 0.67 inches of mercury. To put that in perspective, the highest barometric pressure ever recorded was 32.0 in Siberia in 1968. The lowest occurred in the eye of a typhoon in 1979 at 25.9.”

Following several days of stable, mild weather with increasingly hazy skies, the approach of a cold front brings a falling barometer, along with increasing clouds and wind. According to the typical scenario, bass at this time feed actively, and good catches are common as a weather system approaches. After the front passes, the barometer rises rapidly, accompanied by clearing skies, northwesterly winds, and lower temperatures, particularly at night.

During these conditions, best termed “postfrontal,” the bite is tougher. Good catches can still be made, but typically on smaller baits fished slowly in thick cover, where groups of bass have gathered. While trolling small crankbaits and midsized tubes may have worked as the front approached, miniature plastics and finesse hair jigs fished adjacent to weed clumps or in thick brushpiles may be the best way to tempt bites from crappies on the back side of the system. Anglers persisting with faster-moving horizontal baits likely catch smaller crappies, and not too many of those. Every experienced angler has experienced such changes.