How the Water Warms and Crappies Respond

Does the sun warm lake water, or does the air warm it? What happens on cloudy days? Why do some backwater areas warm so much faster than others? Does the north side of a cove or lake always warm first?

Anglers want to know so they can locate where fish spawn and are active in spring, but most don’t fully understand the dynamics of solar heating and water warming. The sun and warm air heat lakes and reservoirs, and cold air cools and slows the process, of course; but the details are important to understanding which waters warm first.

Warming waters often attract late-winter and prespawn crappies, while cooler areas remain fishless. Moreover, the metabolism of fish is accelerated in warming conditions. In these areas fish feed and anglers flourish.

Solar power—The sun is the dominant source of the world’s heat. Sunlight heats water directly as light energy, mainly in the infrared (IR) wavelengths, and is converted into heat energy by absorption and dissipation. This heat is collected mainly near the surface as the infrared warming radiation is quickly absorbed by water molecules, silt, organic material, or plankton in the surface layer. IR radiation doesn’t penetrate as deeply as other light wavelengths.

Demonstrate the rapidity of IR absorption for yourself the next time you go swimming. Your hand feels the sun’s heat an inch or two under the surface, but move it deeper and the warming ends.

The air-water interface is a poor heat-exchanger. Air itself holds little heat—it’s mainly the moisture in air that warms and cools. During seasons when sunlight is ample, the amount of heat exchanged between water and air usually is much less than that absorbed from sunlight. Water temperatures respond more slowly to changes in air temperature than to direct sunlight warming. Find a shallow pocket that’s sheltered from wind on a warm early spring afternoon and you may be amazed by schools of sunfish, crappie, and bass seeming to bask in the sun’s rays.

Weather fronts and wind effects—When cold fronts bring air colder than the water, heat is lost, and the amount of heat received from the sun usually is reduced while frontal clouds cover a lake.

Wind modifies how much heat is gained or lost. By stirring water and increasing its surface area in contact with air, winds increase the rate of heat exchange by radiation and evaporation. Heat is lost when water evaporates. But the most important effect of wind is mixing the upper layers of warmed water with the cooler water in sub-surface layers. Wind factors can be responsible for erratic fishing results—fish are here one day and gone the next, as wind shuffles the warming layers of water.