Seasonal Thermal Stratification in Lakes

In this article we discuss lakes as a cold water and ice-diving enviroment. In future articles we will look at oceans, rivers, sea ice and fresh water ice.

Lakes in the temperate zone have a general seasonal pattern of thermal stratification. During the summer months, the upper two meters of lake water will absorb more than one-half of the sun’s radiation and will be warmed. A typical summer stratification has three distinct water layers. The epilimnion is the surface layer of warm water, typically 55°F (13°C) to 75°F (24°C), which reaches a depth of about 30-50 feet (10-15 m). Below this layer is a region of sharp decline in temperature, termed the thermocline or metalimnion. The temperature here can drop by as much as 30°F (15°C) in just a few meters of depth. The lowest layer of water is called the hypolimnion, which is a deep, cold, undisturbed layer of water that approaches the temperature of maximum density for freshwater: 39°F (4°C).

In the fall, as air temperatures begin to cool and the sun is not as high in the sky, the water cools to about 43°F (6°C), and wind-induced mixing of the water layers begins. This so-called fall turnover results in an isothermal condition in which the water temperature is about 39°F (4°C) from the surface to the bottom.

As winter ensues, further cooling of surface water occurs, and ice begins to form. A reverse stratification can occur in which colder (less dense) water overlies warmer (more dense) water.

In the spring, as ice melts and day length increases, the wind can induce a spring turnover that produces relatively isothermal water conditions of around 39°F (4°C). This is a general description of stratification in large temperate lakes, and many variations will be found due to climate, lake morphology, and movement of water masses.

Excerpted with permission from the publisher. Source: Cold Water Diving: A Guide to Ice Diving, Second Edition by John Heine.

Learn more about cold water diving!

About the Book Cold water and ice diving can be extremely challenging and require planning, preparation, training, and safety. This book by John Heine, a scientific diving safety officer and an experienced ice diver for more than 25 years, has detailed the requirements for safe and comfortable ice diving. This book covers cold water and ice-diving environments; training; equipment; thermal protection; evaluating, preparing, and planning dives; and safety and emergency procedures. About the Author John Heine, past President of the American Academy of Underwater Sciences, accomplished writer and photographer has served as a member of the Diving Control Board for the National Science Foundation, Officer of Polar Programs. His diving has taken him to many areas of the world, including both poles and tropical areas in between. A certified instructor in ice diving, he has conducted many scientific and sport dives under the ice. He spent three seasons in the Antarctic doing research dives through ice that was ten feet thick, in water temperatures of 28.6ºF, and air temperatures as low as -60ºF. This second edition is an excellent book for those who dive or plan to dive in a cold environment.

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