When most fly fishers think about any weather related aspect of their time on the water, generally atmospheric pressure is the first thing that comes to mind. Much has been written about pressure and fishing, including a little ditty like “Wind from the west – Fishing best – Wind from the east – Fishing least.” Like so many axioms, there is a degree of truth to it. Winds in the northern hemisphere flow counter clock wise around a low pressure area. If the winds are coming from the west, a low pressure area is approaching and the pressure is starting to fall. Fishing tends to be the best with a falling barometer. A falling barometer is a harbinger of a weather change. More often than not the change is rain or snow.
Rain has two effects on the waters we fish. The first one we always think of is a raise in water level from the runoff. For more information to help understand runoff follow these links: http://streamsideadventures.com/tips-and-techniques/stream-flow-and-fly-fishing-runoff-hydrographs.html and http://streamsideadventures.com/tips-and-techniques/stream-flow-and-fly-fishing.html. The second effect, and the one we rarely think of, is on water temperature.
Nimbus clouds are rain clouds. The mother of all nimbus clouds is the cumulonimbus clouds. These are the clouds associated severe thunderstorms and the “pop up” afternoon storms of the mountain west. While nimbus clouds can form at any elevation, many reach an altitude of up 50,000 feet. It keeps getting colder with increased altitude. A general rule of thumb is that for each 1,000 feet of elevation the temperature falls by 3 degrees F. The water vapor falling as rain from those clouds can be very cold, cold enough to dramatically cool lakes, ponds and streams. I first started thinking about this a couple of years ago while fishing my favorite bluegill pond. For the previous week the fishing had been fantastic. The spawn was going on. The big males were building and guarding nests and the females were hanging just out near the drop off. Then we got a thunder storm – one of those short but violent and intense ones. The pond level rose only a little bit and didn’t get off color. The fishing the day after the storm was terrible. I didn’t have a thermometer with me but just from the feel I was sure the water temperature had dropped a few degrees – enough to lower the ponds temperature to below the preferred spawning temperature.
The summer of 2012 was hot and dry out west. The snow pack was way below normal and temperatures we normally see in mid-August were reached in late June. Add to that very little rain and water temperatures were dangerously high. Morning temperatures were not too bad but by noon you could feel the water being much warmer. Each day as the water temperature rose above the trout’s preferred temperature, the fishing slowed to a stop. Then one day it didn’t happen. There had been a little rain up in the mountains, the water was cooler and the flow increase slightly. That single rain event had made a big difference in
the water temperatures. Unlike the bluegill fishing, in this case a few degrees decrease in temperature keep the fishing good all day!
In this case the rise in water level was to minimal - only about 5 inches - and didn't effect water clarity enough it be a significant cause of the improved fishing. But in other areas, the Missouri Ozarks for example, A rain that results in an increase in a few inches of stream depth will result in a noticeable increase in available habitat and a significantly more food available. Add a decrease in water clarity, making the fish a bit less spooky and it is the prime time to fish.