The anvil cloud is really the description for the upper portion of a towering cumulonimbus cloud that flattens out like the picture to the right.

These clouds reach well above 40,000 feet, sometimes close to 60,000 feet. At that level in the atmosphere temperatures are well below freezing and that's why the anvil portion of this thunderstorm cloud is mostly if not all ice crystals.

The strong updrafts in a thunderstorm are what makes a storm cloud grow so tall. These updrafts can reach speeds of 100 MPH. That type of strong updraft eventually loses it's speed and the vertically growing cloud loses it's momentum. At that point, the moisture spreads outward horizontally forming the anvil shape of the cloud.

The fast moving winds aloft will often carry the moisture downwind. In the picture at the right, I can tell you the wind is moving from left to right. Notice how the anvil spread farther to the right.

Photos courtesy of the National Oceanic and Atmospheric Administration (NOAA)