Ice is water in its frozen, solid form. Ice often forms on lakes, rivers and the ocean in cold weather. It can be very thick or very thin. It occurs as frost, snow, sleet and hail.

Water will freeze at zero degrees Celsius (32 degrees Fahrenheit). Once it gets close to its freezing point, water molecules begin to expand. In a small space, these expanding molecules can create a lot of pressure. This is why freezing water can burst even the heaviest of metal pipes in the winter. This is also the reason your ice tray at home can look like its overflowing with ice, even though you only filled it halfway with water. If water freezes in a crack in rock, the ice can eventually break the rock apart. Because of these powerful properties, ice is very important in the processes of weathering, where rocks are broken into smaller bits, and erosion, where rocks and earth are washed or moved to other locations.

The expanded molecules make ice a lot lighter than liquid water, which is why ice floats. An iceberg that weighs several tons can still float easily in the ocean, just like a piece of ice floats easily in your cup of water or soda.

Water covers more than 70 percent of Earth's surface. Slightly more than two percent of Earth's water is frozen into ice; almost all of this ice is in glaciers, which are huge masses of ice. Today, glaciers are found in many mountainous areas and in the polar regions of Earth.

Ice Sheets
Glaciers that cover more than 50,000 square kilometers (19,305 square miles) of land are called ice sheets. The North and South Poles of Earth are covered with ice sheets. In fact, almost the entire continent of Antarctica is covered with ice. The Antarctic Ice Sheet is about the size of the United States and Mexico combined. The Greenland Ice Sheet near the North Pole is three times the size of the U.S. state of Texas. During past ice ages, continental ice sheets like the ones at the poles covered huge portions of North America and Europe.

Large areas covered with ice that are less than 50,000 square kilometers are called ice fields.

Sea Ice
Frozen ocean water is called sea ice. While icebergs form on land and then break off into the sea, sea ice forms in the ocean. Ice that has survived one melt season is called old ice. Every fall, large sections of northeastern Canada's Hudson Bay freeze over. Polar bears (Ursus maritimus) hunt for seals under the cover of the Hudson Bay's old sea ice. When the ice melts in spring, the polar bears return to land. They eat very little until the ocean freezes again in the fall.

Mobile sea ice that floats freely in the ocean, not attached to any shoreline, is called drift ice. There are many different kinds of drift ice. A collection of icebergs and ice no more than two meters (6.5 feet) wide, for instance, is called brash ice.

Frazil ice is a thin layer of freshwater crystals formed as ice interacts with the surface of the ocean. Frazil also forms through cold, quickly flowing streams.

Grease ice is a thin accumulation of frazil. Grease ice is thicker than frazil, but not solid enough to be a true iceberg or ice floe. Grease ice makes the ocean look somewhat like an oil slick. A thin, frozen layer of grease ice is called an ice rind. A slushy, spongy collection of grease ice is called shuga.

Frazil and grease ice create nilas ice, which is newly formed and usually transparent. Sometimes, nilas ice and various ages of old ice freeze together, forming breccia ice.

Unlike drift ice, fast ice is attached to the shore or sea floor, and it doesnt move with the wind or currents. A large area of accumulated drift ice is called pack ice. An ice floe is a floating chunk of ice that is less than 10 kilometers (6.2 miles) wide.

Black Ice
When water hits a cold road and quickly freezes, a thin, clear layer of ice called black ice forms. Its called black ice because unlike snow, which is white, black ice is transparent, revealing the black road below.

Anchor Ice
In extremely turbulent conditions, water can drop below its freezing point without actually freezing solid. Under these conditions, tiny individual ice crystals form, creating a slushy mixture. These slushy pieces of ice can build up and stick to the bottom of rivers or oceans, becoming anchor ice.

Dry Ice
Frozen carbon dioxide, called dry ice, is unique because it melts directly into a gas, skipping the liquid stage. With a temperature measuring more than 73 degrees degrees below zero Celsius Fahrenheit (-100 degrees Fahrenheit), dry ice cools things very effectively, but can also pose a danger. Touching dry ice can cause frostbite.