Weather and Water Temperature

Weather affects water temperature in a lake every day and throughout the year. On a daily basis, weather conditions can cause subtle changes in water temperature. Seasonal changes in weather temperature can cause dramatic variations in a lake’s water temperature—even causing waters to mix.

Daily Changes

Strong winds can cause large waves that can mix a lake’s water. Cloud cover is also important. Skies can be cloudy, clear or a mixture. On cloudy days or foggy days, when visibility is low, the sun cannot warm top waters as quickly as on clear days. On clear, sunny days, a lake’s top waters may become warmer than bottom waters.

Seasonal Changes

Winter
Low weather temperatures cause a lake’s water to become cold. Sometimes the water near the surface gets so cold, it freezes. During winter, nearly the whole water column (the depth from surface to bottom) becomes uniformly cold and near freezing.

Spring
Sun begins to warm the cold water near a lake’s surface. When water reaches a certain temperature—exactly 4 degrees C or 39.2 F—it reaches its maximum density or heaviness, and it sinks. This process causes a lake’s waters to mix. Wind also plays a role. Winds get stronger during spring and help to mix the whole water column, from top to bottom. This seasonal mixing, called turnover, also occurs in the fall. This mixing also helps circulate nutrients throughout the lake.

Summer
Sun warms the surface waters of a lake. Winds die down and are no longer strong enough to mix the whole water column, or depth of water. Surface water becomes very warm, but the bottom water remains cold. Swimmers may notice this sharp temperature difference, called a thermocline.

Fall
In the fall, Great Lakes surface waters begin to cool. When the temperature of the water reaches 4 degrees C or 39.2 F (as it does in the spring), it reaches its maximum density or heaviness, and it sinks. This seasonal process causes a lake’s waters to mix. Wind also plays a role. Winds get stronger during fall and help to mix the whole water column, from top to bottom. This seasonal mixing, called turnover, also occurs in the spring.

13-407-Water-Stratification-Illustration

(Click to See Larger)

When waters in a lake separate into warm and cold “layers” during summer, scientists refer to the lakes as being stratified. During the process of becoming layered, the lakes are undergoing thermal stratification.

Warm water near a lake’s surface (the top layer) can be several inches deep or several feet deep depending on the size of the lake and the amount of sun it receives. A lake’s surface layer of warm water is called the epilimnion.

Cold, deep waters of a lake (the bottom layer) form the hypolimnion. During summer, these cold, bottom waters do not mix with the warm surface water. The hypolimnion contains less oxygen and nutrients than the warmer waters above. Deep water fish such as salmon and lake trout generally swim in the cold waters of the hypolimnion.

These warm and cold layers of water are separated by a thin layer of water called the metalimnion (also called a thermocline). Water temperatures in the metalimnion drop noticeably.

Centigrade or Fahrenheit?
Converting centigrade to Fahrenheit: Multiply centigrade reading by 1.8 (or 9/5) and add 32.
Example: 20C x 1.8) + 32 = 68F