Why ice forms on the top of lakes Water is one of the essential elements that makes up the Earth's atmosphere and drives the processes which determine our weather. Water is also one of the strangest substances on earth, in that ice forms on the top of a lake, rather than on the bottom. Conventional wisdom dictates that ice should form on the bottom of lakes and ponds. The reason why ice forms on the top of lakes and ponds is that once water cools below 39 degrees Fahrenheit, it becomes lighter and less dense the more it cools.
Water expands as it freezes into ice.
As a result, ice takes up more room than the water it was made from. This makes ice lighter and less dense than the water below it. This allows ice to float on top of the water, rather than sink to the bottom. It is this unique property of water that allows people to walk and skate on frozen lakes while at the same time fish are able to swim below the layer of ice. Ice can also cause problems if it forms in pipes.