No, the wall cloud is a lowered section of the cloud base from which a tornado or funnel cloud descends. The dark cloud at the base of a tornado is called the debris cloud.
yes they are called
First, the storm itself has to be rotating. A downdraft from another part of the rotating air and tightens and intensifies it into a tornado.
The dark gray cloud that blankets the sky and often generates precipitation is called a nimbostratus cloud. These are also clouds that extend far both vertically and horizontally.
cumulonimbus
A Thunderstorm cloud is called cumulonimbus cloud. They can range in height from 30,000 feet to 70,000 feet into the atmosphere. The taller the cloud, the more power that has built up within it.
The visible funnel of a tornado is composed of water droplets like an ordinary cloud, and so usually is the same color as the clouds that it extends from. Not all tornadoes are gray, however. Depending on lighting some may appear white or black. Tornadoes can also be colored by soil that they lift and become gray, black, brown, tan, or even red.
Tornadoes occur during severe thunderstorms, typically supercells. So you will typically see thick, often very dark storm clouds. The clouds may take on unusual colors, such as green, yellow, or turquoise. The cloud base itself may appear ragged, and there will usually be a rotating lowering of the cloud base called a wall cloud. The tornado will descend from the wall cloud. See the links for examples of wall clouds.
Sometimes, but not always. In a typical situation you will see a low-hanging cloud beneath a thunderstorm called a wall cloud. As the circulation of the tornado develops, a smaller rotating cloud called a funnel cloud will develop in the wall cloud and extend toward the ground. If the funnel cloud reaches the ground or if there is a whirl of dust beneath the funnel cloud, then the tornado has touched down. This is the classic depiction, however, it does not always happen this way. In many cases, the area where the tornado is forming is obscured by rain. If the air beneath a thunderstorm is dry, the the tornado may develop without a visible funnel. Finally, many tornadoes occur at night, when it is too dark to see them.
The funnel cloud of a tornado is full of water droplets just like an ordinary cloud. As a result it blocks light and may appear dark. A tornado may also appear dark if it is lifting large amounts of dark soil into the air. However, not all tornadoes are dark in color. Front-lit tornadoes may appear light gray or even white. Depending on the soil some tornadoes may be light tan, to reddish brown, to gray in color.
It varies, but it will always occur during a thunderstorm. If that area of the storm is not obscured by rain or the dark of night you will usually see a rotating block of clouds lowered from the main cloud base called a wall cloud. Near this a downdraft will blast a hole in the clouds. Not long afterwards there will likely be strong winds. Then the funnel begins to lower from the wall cloud and a swirl of dust or debris may appear on the ground below it as the tornado connects to the ground.
It is often dark during a tornado not because of the tornado itself, but becasue of the parent thunderstorm. The thunderstorm consists of a very tall cumulonimbus cloud, which blocks out most sunlight.
First, the storm itself has to be rotating. A downdraft from another part of the rotating air and tightens and intensifies it into a tornado.
cirrus
These sagging pouches are likely a type of cloud known as mammatus. Such clouds are not necessarily associated with tornadoes, but are a feature of thunderstorms. They are generally more common during severe storms.
A cirrus cloud.
They are often dark gray or even black, but not always. In terms of color they could look like fairly ordinary storm clouds.
Nimbostratus Cloud
* Dark, often greenish sky. Sometimes one or more of the clouds turns greenish (a phenomenon caused by hail) indicating a tornado may develop. * Wall cloud, an isolated lowering of the base of a thunderstorm. The wall cloud is particularly suspect if it is rotating. * Large hail. Tornadoes are spawned from powerful thunderstorms and the most powerful thunderstorms produce large hail. Tornadoes frequently emerge from near the hail-producing portion of the storm. * Cloud of debris. An approaching cloud of debris can mark the location of a tornado even if a funnel is not visible. * Funnel cloud. A visible rotating extension of the cloud base is a sign that a tornado may develop. * Roaring noise. The high winds of a tornado can cause a roar that is often compared with the sound of a freight train. * Tornadoes may occur near the trailing edge of a thunderstorm and be quite visible. It is not uncommon to see clear, sunlit skies behind a tornado. They may also be embedded in rain and not visible at all.