Tornadoes can be detected in a number of different ways: forecasters, storm spotters, algorithms, and doppler radar.
Both forecasters and storm spotters have learnt to recognise particular thunderstorm features and structure that make the formation of a tornado more likely.
Storm spotters have been trained to recognise tornado conditions and report what they see to the National Weather Service. Storm spotters can be anyone with a keen interest in tornadoes (and severe weather) who have taken formal training in their local area.
What do storm spotters look for when trying to identify a tornado?
There are 6 features that storm spotters look for when trying to identify a tornado: inflow bands, beaver’s tail, wall cloud, rain-free base, rear flank downdraft (RFD), and a condensation funnel.
Inflow bands are ragged bands of low cumulus clouds that extend from the main storm tower usually to the southeast or south. The presence of inflow bands tends to suggest that the thunderstorm is collecting low-level air from a few miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of rotation.
The beaver’s tail is a smooth, flat cloud band that extends from the edge of the rain-free base to the east or northeast. It normally skirts around the southern edge of the precipitation area. It also suggests the presence of rotation.
A wall cloud is an isolated cloud lowering attached to the rain-free base of the thunderstorm. The wall cloud is normally to the rear of the visible precipitation area. A wall cloud that will produce a tornado can exist for 10 – 20 minutes before a tornado touches down, but it doesn’t always occur.
A wall cloud may also rotate continuously, have strong surface winds flowing into it, and could have rapid vertical motion indicated by small cloud elements rising into the rain-free base.
As the thunderstorm strengthens, the updraft in low-level air from miles around. Some low-level air is pulled into the updraft from the rain area. The rain-cooled air is very humid; the moisture in the rain-cooled air swiftly condenses below the rain-free base to form the wall cloud.
Rear Flank Downdraft (RFD)
The rear flank downdraft (RFD) is a downward rush of air on the backside of the storm that descends along with the tornado. Experts say the RFD looks like a “clear slot” or a “bright slot” just to the rear of the wall cloud. They also say it can look like curtains of rain wrapping around the cloud base circulation. The RFD causes gusty surface winds that occasionally have downbursts embedded. The rear flank downdraft is the motion in the storm that causes the hook echo feature on the radar (find out more below).
A condensation funnel is made up of water droplets and extends downward from the base of the thunderstorm. If it makes contact with the ground it is a tornado; otherwise it’s a funnel cloud. Dust and debris beneath the condensation funnel confirm a tornado’s presence.
Algorithms and doppler radar
Algorithms analyse Doppler radar data and display it in a way that makes it easier for forecasters to identify dangerous weather. A storm which has a tornado observed by radar has certain features and forecasters are trained to recognise them.
When Doppler radar detects a large rotating updraft that occurs within a supercell, it’s called a mesocyclone. The mesocyclone is normally 2 – 6 miles in diameter, and significantly larger than the tornado that may develop within it.
A hook echo describes a pattern in radar reflectivity images that looks like a hook extending from the radar echo, normally in the right-rear part of the storm (relative to the motion of a storm).
The hook is usually associated with a mesocyclone and indicates favourable conditions for the formation of a tornado. A hook is caused by the rear flank downdraft (RFD) and is the result of precipitation wrapping around the backside of the updraft.
Dual-polarisation radar technology can detect the presence of randomly shaped and sized objects like leaves, insulation or other debris. This gives meteorologists a high degree of confidence that a tornado is on the ground – very helpful for tornadoes at night.