Anvil Cloud- The flat top of a thunderstorm (cumulonimbus) cloud that spreads out in front of or behind the parent cloud, making the entire system resemble an anvil (pictured at right). The anvil will usually mark the tropopause in the lower atmosphere above which moisture and clouds cannot rise any further. Thunderstorm anvils are often blown up to hundreds of miles away from the parent thunderstorm by upper-level winds, creating a thin, wispy layer of cirrus clouds that signal the storms' presence at great distances.