You are watching: Why does a lake freeze from the top down instead of from the bottom up?
Warm water typically gets much more dense together it it s okay colder, and also therefore sinks. This truth may command you to believe that ice cream should form on the bottom of a lake first. However a funny thing happens to water as it gets even colder. Colder than 4° Celsius (39° Fahrenheit), water begins expanding and becomes less thick as it it s okay colder. As a result, close to freezing, colder water floats come the top and also the warmer water sinks to the bottom. The thickness of water together a function of temperature can be seen in the plot on the right. Eventually, the coldest water, which has actually floated come the top of the lake in wintry conditions, freezes to kind a class of ice. Right once the water freezes come ice, the ice becomes substantially less dense than the water and continues to float on the lake"s surface.
Ice is less dense than water since of the way it develops a hexagonal crystalline structure. Each water molecule is composed of two hydrogen atoms bonded come the bottom of one oxygen atom. When ice forms, the hydrogen atoms of one water molecule type weak hydrogen bonds with the height of the oxygen atom of two various other water molecules. Lining increase the water molecules in this sample takes up much more space than having them jumbled randomly with each other (as is the case in liquid water). And also because the same mass of molecules takes up an ext space once frozen, ice is less dense than fluid water. Because that this very same reason, water listed below 4° Celsius becomes progressively less thick as it it s okay colder. Nearby to freezing temperatures, the molecule in the liquid water start to line up into the space-filling hexagonal structure.
In the winter, temperatures in a lake get close sufficient to freezing the the cooler water is much less dense and also floats to the top. Public domain image, source: Christopher S. Baird.
See more: Gilgamesh King Of Uruk Epic Hero Holding Lion Statue Of Gilgamesh !
The textbook River and Lake Ice engineering by George D. Ashton states, "As a lake cools from above 4° C, the surface ar water loser heat, becomes an ext dense and sinks. This procedure continues till all the water in the lake is in ~ 4° C, as soon as the density of water is at its maximum. With more cooling (and without mechanical mixing) a stable, lighter class of water develops at the surface. Together this class cools to its freezing point, ice starts to form on the surface of the lake."
In deep lakes, water pressure may additionally play a role. The gravitational load of every the water higher up in the lake presses under on the water deep in the lake. The pressure permits the water near the bottom that the lake to obtain cold without expanding and rising. Because of the pressure, the water in ~ the bottom that deep lakes can end up being cold there is no freezing to ice.
Topics: density, freezing, frozen, ice, lake, lake ice, water, weather
To submit a question, email me:chrisbaird.ma
To re-publishing comments and get updates, join us on various other platforms:FacebookTwitter
AntimatterAtomBlack HoleBrainCancerCentrifugalColdColorConservation the EnergyConservation the MomentumElectricityElectromagnetismElectronEnergyEvolutionFireFishGravityHeatHigh Blood PressureIonizationLicoriceLightMagnetismMassMomentumMoonOrbitPlanetary AlignmentPlanetsQuantumRadiationRainRefractionRelativitySkin CancerSolar SystemSoundSpace ShipSpeed of LightSunThermal RadiationTidesTornadoUltravioletVaccinesVisionWaterWaveWeightlessness