When we describe some occurrences or
nature, often we need to describe a momentary state of an atmospheric
fluid. Vorticity is
among the ‘state variables’ like temperature, density, pressure,
which are commonly used to describe a
fluid motion. So to understand the dynamics of tornado, we should have
idea about what ‘vorticity’ is.
Vorticity can be defined as “the
angular velocity of
matter at a point in continuum space” [Ref: Lugt
(1983)].In a fluid flow, vorticity is a measure of the local rotation and
= Curl(Velocity vector)
That means, a flow
becomes ‘irrotational’ if there are no local rotations in the fluid
The definition of
vortex is not very
simple. In one way we can call it as the region in a fluid where
higher compared to the surrounding.
vortex: for a vortex, if the diameter is much larger
than the axial scale it is a ‘disk-like’ vortex. Hurricane is one
of this type of vortex.
Columnar Vortex: On
other hand if the diameter
is much smaller than
axial scale – it is called
‘columnar’ vortex. A
tornado falls into this
In general, we can say, when there is a
has some vorticity. But,the reverse is not necessarily true. With all
vorticities, there may not be a
vortex attached. A parallel shear
flow, which is not a vortex, has vorticity too.
Vortices are of
different dimensions. Example of one such extremely
small vortex is "quantized votex" in
helium (diameter ~ 10-10
m). Extermely large
are the Galaxies, diameters of which are of the order of light-years.
the falling leaves are of the order of
1cm. Dust devils
are larger cousins (~ 1m - 10m) to these small vortices.
waterspouts are much larger
vortices with much larger diameter (
100m – 1000m).
The following examples are in order of
diameter: hurricanes, ocean circulations, great red spot of Jupiter.