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.

Factors Influencing Warming

The seasons—The angle of the sun and day length are determined by season and latitude. Higher sun angles and longer days increase the amount of heat the sun provides. From mid-December through mid-June, the sun’s heat increases; from mid-June through mid-December, it decreases. Solar heating also increases during February and March, but arctic fronts repeatedly bring in air masses cold enough to slow or briefly reverse the solar heating process.

From April through August the sun’s angle and day length are sufficient to steadily warm and maintain water temperatures. In many southeastern waters, the temperature can be in the high-80°F range. The dissipation of heat through evaporation and losses to cooler air put a flexible upper limit on maximum temperatures in most lakes and reservoirs that are not artificially heated by power plants. So, waters seldom exceed 95°F at the surface, even in direct desert sun.

Clouds—Water vapor of clouds absorbs some of the sun’s heat. Clouds capture a portion of the warming radiation directly and reflect light, preventing some IR radiation from reaching the water. A thin cloud layer may allow solar heating to continue, while a thick overcast may block almost all solar input, allowing average air temperatures to become the dominant factor influencing daily heating or cooling of lakes. When the sun is blocked, and at night, winds and the exchange of heat between air and water become the major influences on lake surface temperatures.

Like the air-water interface, the substrate-water interface is a comparatively poor source for water warming. The sun’s heat is reflected by light surfaces and absorbed by dark surfaces. Dark, shallow bottoms and rocks radiate heat rapidly into cooler air or water. This radiation can warm adjacent water slightly, but even small currents rapidly dissipate this heat, and overall water temperature isn’t usually much affected in waters deeper than 2 to 3 feet during the day.

Bottom warming effects usually are significant only in clear backwaters less than 4 feet deep that have little wind current or inflowing water. Water clarity affects solar heating, too: The bottom releases significant heat only if the water is clear enough to let the sun’s rays reach it. Murky water traps solar heat, warming the shallow water more directly.

Lake size and shape—The size and shape of a lake greatly affect how solar heating and exchanges of heat with the air and the lake bottom influence water temperature. Large volumes of water warm and cool slowly. Ponds and shallow, isolated backwaters react much more rapidly to increases in air temperature and sunlight than main-lake areas or rivers. Main-lake waters warm more slowly and steadily as the sun’s power gradually increases in spring, because winds keep stirring down the warmed surface layers and there’s cold water below to absorb the warmth. Isolated coves may warm quickly, but may also cool substantially at night or during cooler weather.

Winds—Wind strength and direction are critical to whether or not temperatures of ponds and shallow coves warm or cool. When strong winds mix the surface layers of water, daily heat gains are rapidly dispersed and surface water appears to heat slowly. The overall amount of heating is the same as in wind-blocked areas, but the heat is more evenly distributed and the immediate surface layer seems cooler.

Calm days allow heat to collect in the top layer of water, so near-surface temperature measurements show much greater increases in temperature under calm conditions. But water 3 or more feet below the surface remains much cooler. If winds eventually stir down these warm surface layers, surface temperatures drop noticeably, even if air temperatures are increasing.

Strong winds often are associated with clouds and fronts. Wind effects can make it appear that cloudy frontal days are allowing no heat input, but there still can be a net heat gain if the average air temperature isn’t much cooler than the surface.