There is also a TORRO scale, which ranges from zero to 11, which is used in some parts of the world to assess tornado strength. These are assessed using photogrammetry, Doppler radar information, and ground swirl patterns after the fact. Together, these help assign severity levels and ratings to a given tornado. Tornadoes do not have to be visible to the naked eye but they often are anyway. There are two reasons for this, including the development of debris in the tornado and the formation of water droplets. These droplets occur because of condensation occurring as the tornado cools. The rapid rotation will cause a significant negative pressure situation within them. The Bernoulli principle indicates why this low pressure develops inside the spinning vortex. Funnel clouds are not necessarily tornadoes but most tornadoes are funnel clouds. Funnel clouds are rotating clouds of any type. They do not necessarily mean there are strong winds linked with them. The term "condensation cloud" is a rotational cloud found below a cumulonimbus or cumulus cloud. Most tornadoes start as funnel clouds that do not have strong ground winds. Once these winds become evident, they are called tornadoes. You may see tornadoes occurring as families or in outbreaks. One storm can have more than one tornado associated with it. These are called tornado families. If there is no break in tornado activities, you will call this a tornado outbreak. You call the situation a tornado outbreak sequence if several days in a row are linked to the development of tornadoes. These are also called extended tornado outbreaks.
TORNADO CHARACTERISTICS Most tornadoes have a characteristic funnel shape up to few hundred yards across. You may not see a tornado because of the dust or rain in the area. These become dangerous because they are so hidden. If there is some wind on the surface because of the vortex, it is called a tornado. You need surface winds of at least 40 miles per hour to call it a tornado. There are several tornado sizes and shapes to consider. Short, cylindrical tornadoes are called stovepipe tornadoes, while wide, short tornadoes are known as wedge tornadoes. 124
