How the Water Warms and Crappies Respond

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.