Before tornadoes, there exist mesocyclones: giant, rotating columns of low-pressure air that sink below the height of their surrounding cloud basins. Sometimes subsidiary vortices called tornadoes spawn under them. The already swiftly-moving air circulating above is restrained into a smaller diameter and, due to the conservation of angular momentum, wind speed is increased. If the ground level under a tornado is disregarded, a tornado’s width is related to the depth to which it can sink.
Generally, tornadoes are born when the downdraft of higher pressure, colder air in front of the associated storm twists and approaches the similar downdraft at the end. At first, a funnel cloud emerges from a wall cloud: a section of a cloud that is allowed to descend to unusually low heights. Funnel clouds are twisters in the making – they haven’t yet reached the surface. The moment surface debris is disturbed, a tornado has formed. Typically, they grow diameters of 50 to 800 feet but on occasion they can stretch over a mile. If they live long enough to reach maturity, they become more erect because the updraft in each of their cores approaches its greatest strength. Once the rear downdraft of the storm cell fully wraps around and cuts off the updraft, a tornado perishes.
Sometimes the low-pressure zone in the center of a tornado becomes so low, the tornado loses its hydrostatic balance and air begins to descend down the middle, making a central downdraft. Pressure at the center may drop 100 millibars and the tornado can expand to half a mile wide. Often, this situation gives rise to suction vortices: subsidiary tornadoes within the larger tornado, a.k.a. sister twisters (see photo below from NOAA.
Tornadoes: Nature’s Deadliest Surprises
Nature’s deadliest surprises, tornadoes can spontaneously develop in supercell thunderstorms, along squall lines, near the ends of bow echoes, and within hurricanes after they’ve moved ashore. They may generate the strongest winds of any atmospheric phenomenon: over 200 mph. Annually, they kill dozens and injure hundreds. Typically they are responsible for around $100 million in property damage annually.
The science of pinpointing where tornadoes are about to spawn is currently little understood, so it is important to heed the watches and warnings of meteorologists, who know when the right conditions will exist for violent weather. If a Tornado Watch is issued, the right atmospheric conditions for the formation of tornadoes will soon exist or presently exist. If a Tornado Warning is issued, a tornado has been spotted either on radar or by a spotter. If a Warning has been issued for your area, head to a basement, shelter or other safe place with sturdy walls, no dangerous debris and no windows.
Below is a diagram of the damage paths of the worst tornado outbreak in recorded history “Palm Sunday Outbreak.” Photo courtesy of NOAA.
Article written by Meteorologist Geoff Linsley