Meteorologists affiliated with the World Meteorological Organization (WMO) officially classify nimbostratus as mid-level for synoptic purposes while informally characterizing it as multi-level. Independent meteorologists and educators appear split between those who largely follow the WMO model and those who classify nimbostratus as low-level, despite its considerable vertical extent and its usual initial formation in the middle altitude range. Species cumulus mediocris – These cumuliform clouds of free convection have clear-cut, medium-grey, flat bases and white, domed tops in the form of small sproutings and generally do not produce precipitation. They usually form in the low level of the troposphere except during conditions of very low relative humidity, when the clouds bases can rise into the middle-altitude range. Cumulus mediocris is officially classified as low-level and more informally characterized as having moderate vertical extent that can involve more than one altitude level. Towering vertical These very large cumuliform and cumulonimbiform types have cloud bases in the same low- to mid-level range as the multi-level and moderate vertical types, but the tops nearly always extend into the high levels. Unlike less vertically developed clouds, they are required to be identified by their standard names or abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulence. Species cumulus congestus – Increasing airmass instability can cause free-convective cumulus to grow very tall to the extent that the vertical height from base to top is greater than the base-width of the cloud. The cloud base takes on a darker grey coloration and the top commonly resembles a cauliflower. This cloud type can produce moderate to heavy showers and is designated Towering cumulus (Tcu) by the International Civil Aviation Organization (ICAO). Genus cumulonimbus (Cb) – This genus type is a heavy, towering, cumulonimbiform mass of free-convective cloud with a dark-grey to nearly black base and a very high top in the form of a mountain or huge tower. Cumulonimbus can produce thunderstorms, local very heavy downpours of rain that may cause flash floods, and a variety of types of lightning including cloud-to-ground that can cause wildfires. Other convective severe weather may or may not be associated with thunderstorms and include heavy snow showers, hail, strong wind shear, downbursts, and tornadoes. Of all these possible cumulonimbus-related events, lightning is the only one of these that requires a thunderstorm to be taking place since it is the lightning that creates