If you've ever noticed a pond, lake or river freeze, you've seen a sheet of ice begin to form over the top of the water.
Do you realize that if ponds, lakes and rivers froze from the bottom up, instead of from the top down, many important things about our life would be quite different? Not only would it change the climate of the world, but certain creatures who live in the water would disappear altogether!
Here is how the water in a pond changes into ice: When the air above the pond grows cold, it cools off the top layer of water, too. This coldness makes the water heavier than the warmer layers underneath and the cold water sinks down. This process goes on and on until all the water in the pond has reached a temperature of about 39 degrees Fahrenheit.
But our temperature is still dropping! When the top layer of water becomes colder than 39 degrees, it remains on top. The reason is that water cooled to below 39 degrees actually becomes lighter!
We now have the top layer of water ready to start freezing. So when the temperature remains at the freezing point of 32 degrees, or if it goes below that, tiny crystals begin to form.
Each of these crystals has six rays, or points. As they join together they form ice and soon a whole sheet of ice appears on top of the water. Sometimes the ice is clear, sometimes its cloudy. Why? The reason is that when each drop of water freezes, it sets free a tiny bubble of air. This bubble sticks to the arms of the crystal. As more crystals form, the bubble caught and we have cloudy ice.
If the water under the ice is moving, the little air bubbles are grouped together into clear ice.
Water is one of the few substances that do not shrink when changed from liquid so solid. When water is frozen into ice it expends by one-ninth, so that nine quarts of water give you ten quarts of solid ice! When automobile radiators and water lines crack in winter, it's because the water freezes and there is no room for the ice!
