Article and photo provided courtesy of ACRES Land Trust, an organization that acquires and preserves natural areas in northeast Indiana, northwest Ohio, and southern Michigan.
As the temperature decreases, ice forms on the surface of ponds and lakes and gets thicker. It's a phenomenon that we are all familiar with and take for granted. Have you ever wondered why bodies of water freeze from the top down instead of bottom up? This is especially curious when you consider that normally cold liquids sink and warm liquids rise.
To understand the process of water freezing, we must first understand a unique fact about water. As temperature decreases virtually all liquids increase in density, and when temperature is low enough they solidify (freeze). Water, the exception, is most dense at 39.2° F, and as its temperature drops below this it becomes less dense. Since water freezes at 32°F, the less dense, colder water turns to ice and floats on the more dense, warmer water underneath it.
Imagine if water behaved like other liquids – the coldest water would be the densest, and ponds and lakes would freeze from the bottom up until they turned into solid blocks of ice! This disruption would dramatically affect the ecology of lakes and ponds, creating dramatic discontinuities between seasons.
This physical property of water also explains the phenomenon of lakes “turning over” twice a year (ponds being shallow, don't behave this way). Let's start with a frozen lake. In the spring, the ice will melt due to rising temperatures. As this dense oxygen rich surface water approaches 39.2°F, it sinks to the bottom. This forces the oxygen-deficient water that has been sitting on the bottom all winter to the surface where it gets re-oxygenated as it mixes with the air.
In the summer months temperature stratification in lakes becomes extreme. The upper layer, known as the epilimnion warms to around 75°F. Since water is a poor conductor of heat, the temperature of the upper 10 to 15 feet is maintained by convection or mixing. The degree of mixing is related to turbulence which in undisturbed lakes (those with no jet skis or motorboats) is related to the amount and velocity of the winds. This means that the depth of the epilimnion is determined by turbulence. The more turbulence, the more mixing and the deeper the epilimnion!
Below the epilimnion “beyond the reach of turbulence,” the temperature changes quickly forming the thermocline. In the thermocline water temperatures decrease rapidly with depth terminating at about 20 feet with the beginning of the bottom layer or hypolimnion; uniformly dense, dark and (39.2°F).
Turbulence will circulate the water throughout the summer in the epilimnion, but rarely, under natural conditions, will these changes initiate complete top to bottom mixing. In fall the warm surface water cools. Its density increases and at 39.2°F it silently drops down to the bottom, recharged with oxygen.
Understanding that water is most dense at 39.2°F has solved two mysteries: water in ponds and lakes freezes from the top down, and lakes experience spring and fall turnovers. Bet you won't look at a frozen pond or lake the same again!
Thank you to Dr. Richard Hurley for his review and edits to this article.