Thunderstorms (Tormenta) and Tornadoes After completing this section, students will discuss the formation of violent weather patterns such as thunderstorms and tornadoes (Standard PI – 060)
Storms come in various sizes. Hurricanes can cover several states, but thunderstorms and tornadoes are small-scale phenomenon
Thunderstorms (tormenta) are storms that produce lightning (rayo), thunder, rain and sometimes hail Tornadoes sometimes form in thunderstorms
All thunderstorms require three ingredients for their formation: Moisture Instability a lifting mechanism
Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico.
An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft. Air that is forced upward will continue to rise, and air that is forced downward will continue to sink.
F or a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. Fronts lift warm moist air.
Life Cycle of a Thunderstorms The thunderstorm cell has a distinct life-cycle that lasts about 30 minutes.
Cumulus Stage - A cumulus cloud begins to grow vertically, to a height of about 20,000 feet (6 km). Air within the cloud is dominated by updraft with some violent air currents around the edges
Mature Stage - The storm has considerable depth, often reaching 40,000 to 60,000 feet (12 to 18 km). This is the most dangerous stage when large hail, damaging winds, and flash flooding may occur
Dissipating stage – The storm no longer has a supply of warm moist air to maintain itself Light rain and weak outflow winds may remain for a while during this stage
Thunderstorms usually form along cold fronts These storms are associated with low pressure systems
Sometimes thunderstorms form before the cold front arrives – squall lines
Thunderstorms also produce lightning (rayo) – discharge of electricity from a cloud to the ground, to another cloud or some other spot
Tornadoes A violently rotating column of air that usually touches the ground
The most destructive tornadoes occur from supercells (celula super) - rotating thunderstorms with a well- defined radar circulation called a mesocyclone
Tornado season is generally March through August, although tornadoes can occur at any time of year. Over 80 percent of all tornadoes strike between noon and midnight.
Tornado intensity is measured by using the Fujita Scale F-0F-1F-2 F-3F-4F-5
The National Weather Service issues watches and warnings to give people time to find shelter Tornado watch – conditions are right for tornadoes to form Tornado warning – a tornado is actually spotted, by humans or on radar