Why does water freeze from the top downward?

Answer 1

A fresh-water lake (say in New York state?) has a temperature distribution from top to bottom of perhaps 75F to about 46F at the bottom. Lakes are fed by streams and springs at near groundwater temperature and the cold water being more dense sinks to the bottom. almost everything that cools shrinks and becomes more dense. Groundwater temperature is the average temperature of all the water that enters the ground during a typical year which may be as high as 90F in the summer but no lower than 32F in the winter (because ice can't flow into the ground). As fall and winter arrive the cooler then colder air cools the surface of the lake to below groundwater temperature even down to nearly 32F. The denser 32F water sinks and the 46F (warmer water) rises to the surface. The entire mass of lake water must 'turn over' cooling all the water to 32F before any ice can form otherwise warmer water would rise and melt the ice. As ice forms, heat of fusion is removed from the surface of the water and ice crystals begin to form. The ice crystals are chains of water molecules that use up space less efficiently than liquid water molecules. Try gluing a box of golf balls together into curved chains then try to get them back in the box! The ice is less dense than water and floats on water like an ice cube in a drink.

Answer 2

Mainly because it loses heat from the surface faster both because it is exposed and because of evaporation (which continues even below 0C - snow actually can sublime).

In an ice box you will often find ice cubes freeze around the outside first in all directions - not just the exposed top surface. This then insulates the liquid water inside which takes longer to freeze.

The lower density of ice is irrelevant except that it ensure that in open water the insulating layer of ice that forms on the surface stays there rather than sinking.

Answer 3

Because cold water (aprox 0 deg Celsius) is lighter than water with higher temperatures (but lower than 8 deg Celsius). The maximum density of water is around 4 deg Celsius.