There are two genera of rain clouds, cumulonimbus and nimbostratus. Nimbostratus clouds are usually mid level clouds, thick, dark, gray, and sometimes seen with "virga" or skirts of rain trailing down. These clouds are made of water droplets that produce either rain or snow. Cumulonimbus clouds are thunderstorm clouds, and arise from cumulus clouds that have reached a great height. When the cloud normally reaches the height of the tropopause above, it flattens out, resembling an anvil. All phases of water, gas, liquid, and solid, are contained in these clouds. There are powerful updrafts and downdrafts that can create violent storms.