1.2.3.2. Updraft depending on the wind

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Wind shear present, wind changes speed and/or direction with height

The environment that is particularly conductive for severe thunderstorms is an environment where the wind changes speed and/or direction with height. This is called directional shear. If the wind changes direction clockwise with increasing height, it produces a veering vertical wind profile. A counterclockwise change in direction produces a backing vertical wind profile. Backing wind profiles are rarely associated with thunderstorms as they are typically associated with inflow of cold air at the surface. Veering profiles, on the other hand, are typically associated with inflow of warm air at the surface and often with severe thunderstorms.

An environment where wind direction changes with height produces a rotating updraft. The entire updraft rotates around its vertical (tilted) axis. A thunderstorm with a persistently rotating updraft is called a supercell. Rotation often produces visually apparent rotational features, such as striations.

Rotating updraft on a storm (supercell). Rotation produces a striated appearance to the updraft, with visible rotational features. The updraft on such a storm is also tilted, however, in this case the direction of view is directly towards the tilt, so it appears vertical. This storm formed near Concordia-Sagittaria, northeast Italy on July 12, 2016. Photo: Jure Atanackov.

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The same thunderstorm from a somewhat larger distance and at a slightly different angle. The striated, rotational appearance of the updraft is apparent. The tilt is better revealed from this angle (the angle was still not optimal and the updraft had a larger tilt than apparent in this photo). Photo: Marko Korošec.

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Rotating updraft (supercell) over Kaposfo, Hungary. July 11, 2017. Best seen after 1:30 mark. Video: Rafael Schmall..

Some rotating updraft show clear signs of rotation, as apparent in the case above, while in other cases rotation is not readily apparent. We will go into much more details on rotating updrafts in the upcoming chapter “Supercells”.