ANVIL CLOUD

The top portion of a cumulonimbus cloud which spreads horizontally outward from the block-line updraft. It spreads out, faster if there are strong jet stream winds, near the equilibrium level where the updraft air is no longer positively buoyant. The cloud, which can occur upwards of 18,000 to 22,000 meters (60,000 to 75.000 feet), is primarily made of water droplets near the updraft; those that move farther away turn to ice crystals. The cloud got its name because the shape is similar to a blacksmith's anvil.