Synoptic, or atmospheric boundary layer (ABL), winds occur due to
large-scale (~1000km) weather systems such as the high and low pressure
regions seen on weather maps. The vertical profile of
synoptic winds follows a logarithmic profile, with a rapid
increase with height near the ground and an asymptotic maximum at high
levels (Figure 1). In contrast, downburst winds reach a maximum
close to the ground (<100m) and then reduce with height. It can
also be seen from the figure that ABL winds are stationary - if short
samples are taken at from different points in the series then
they will be statistically negligibly different - whereas
downburst winds are highly non-stationary, with a short, high magnitude
peak which can be explained by the form which these winds take.
Figure 1: Comparison of Synoptic and Downburst (Non-Synoptic) Winds -
Vertical Profile (adapted from ) and
Downburst winds originate in rising, warm, moist air in a thunderstorm.
As it rises this air cools, becoming more dense, and the updraft
reverses to become a downdraft, which becomes an ouflow upon
contact with the ground. The downburst is therefore a short
duration event, as once this mechanism has exhausted the original volume
of warm air, the downdraft ends. There is, however, an additional
source of velocity in the outflow. In many (though not all)
downbursts, a ring vortex forms due to Kelvin-Helmholtz instabilities as
the downdraft commences.
Figure 2: Downburst Structure and the Primary Ring Vortex ()
This ring vortex, which at ground level is rotating in the same
direction as the outflow, increases the local velocity, resulting in the
exceptionally high-speed, short-lived winds found in a downburst.
 LIN, W. E., ORF, L. G., SAVORY, E. & NOVACCO, C. 2007.
Proposed large-scale modelling of the transient features of a downburst
outflow. Wind and Structures, 10 (4), 315-346.
 FUJITA, T. T. 1985. Downburst: Microburst and Macroburst, Chicago, Illinois, University of Chicago Press.