Impressive Mammatus formation on the underside of a cumulonimbus in Halfmoon, New York on July 23, 2016.
About Mammatus Clouds: Cumulonimbus, from the Latin cumulus ("heap") and nimbus ("rainstorm", "storm cloud"), is a dense towering vertical cloud  associated with thunderstorms and atmospheric instability, forming from water vapor carried by powerful upward air currents. If observed during a storm, these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes. Cumulonimbus progress from overdeveloped cumulus congestus cloudsand may further develop as part of a supercell. Cumulonimbus is abbreviated Cb and are designated in the D2 family. [via Wikipedia]