Editing Hook echo (section)

== Interpretation ==
[[File:Supercell-above.svg|thumb|left|Diagram of air current in a supercell]]
Hook echoes are a reflection of the movement of air inside and around a supercell thunderstorm. Ahead of the base of the storm, the inflow from the environment is sucked in by the instability of the air mass. As it moves upward, it cools slower than the cloud environment, because it mixes very little with it, creating an echo free tube which ends at higher levels to form a [[bounded weak echo region]] or BWER.<ref name="review"/> At the same time, a mid-level flow of cool and drier air enters the thunderstorm cloud. Because it is drier than the environment, it is more dense and sinks down behind the cloud and forms the [[rear flank downdraft]], drying the mid-level portion of the back of the cloud. The two currents form a vertical windshear, which then develops rotation and can further interact to form a mesocyclone. Tightening of the rotation near the surface may create a tornado.<ref name="review" />

[[File:05june-dow7-wide.gif|thumb|upright=1.5|A [[Doppler on Wheels]] image of a tornadic thunderstorm near [[La Grange, Wyoming]] (USA) captured during the [[VORTEX2]] project. In the velocity image on the left, Blues/green represent winds moving towards the radar, and reds/yellows indicate winds moving away from the radar. In the reflectivity image on the right, the main body of the storm can be seen, with the appendage on the bottom of the storm being a hook echo.]]
Near the interaction zone at the surface, there will be a dry slot caused by the updraft on one side and the cloudy area below the rear flank downdraft on the other side. This is the source of the hook echo seen on radar near the surface. Hook echoes are thus a relatively reliable indicator of tornadic activity; however, they merely indicate the presence of a larger mesocyclone structure in the tornadic storm rather than directly detecting a tornado.<ref name="review"/> During some destructive tornadoes, debris lofted from the surface may be detected as a "[[debris ball]]" on the end of the hook structure. Not all thunderstorms exhibiting hook echoes produce tornadoes, and not all tornado-producing supercells contain hook echoes.

The use of Doppler weather radar systems, such as [[NEXRAD]], allows for the detection of strong, low-level mesocyclones that produce tornadoes even when the hook echo is not present and also grant greater certainty when a hook echo is present. By detecting [[hydrometeor]]s moving toward and away from the radar location, the relative velocities of air flowing within different parts of a storm are revealed. These areas of tight rotation known as "velocity couplets" are now the primary trigger for the issuance of a tornado warning. The [[tornado vortex signature]] is an algorithm-based detection of this.<ref>{{cite web| last = Paul Schlatter, Warning Decision Training Branch| title = WSR-88D Distance Learning Operations Course; Topic 5, Lesson 19| access-date = June 16, 2013| date = September 2009| url = http://www.wdtb.noaa.gov/courses/dloc/topic5/lesson19/player.html| archive-url = https://web.archive.org/web/20130227212912/http://www.wdtb.noaa.gov/courses/dloc/topic5/lesson19/player.html| archive-date = February 27, 2013| url-status = dead}}</ref>

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