AS SEEN FROM THE CHICAGO WSR-88D DOPPLER RADAR
"SMOKE PLUME OF A STEEL MILL"
Atmospheric conditions and the natural sensitivity of the Doppler radar occasionally combine to reveal a visual perspective of events around us.
In this particular instance it is a smoke plume from a steel mill just northwest of Gary Indiana, on the Lake Michigan shoreline. The Doppler beam is quite sensitive even to small scattering particles at great distances. Often, however, the concentrated smoke (in this case) is dispersed through winds and mixing in the atmosphere thus becoming too diffuse to be seen in a defined pattern.
On this day, however, the temperature structure of the lower portion of the atmosphere was such that the plume was trapped below a thermal inversion (temperature rising with height rather than falling). Mixing in the vertical was halted and the stack discharge remained somewhat concentrated, at least enough to be detected by the WSR-88D beam.
The accompanying vertical sounding chart was taken from instrumentation on a plane departing O'Hare International Airport about the time of the radar image.
O'Hare is about 28 miles northwest of the steel complex. The sounding shows a temperature inversion beginning just under 4000 ft AGL. This the effective 'lid' on any vertical mixing. A stable layer where the temperature becomes almost uniform with height is lower, beginning at under 3000 ft AGL.
Along the left side of the chart the wind barbs show a northeast wind at 10 to 15 knots within this low layer. The plume is drifting off to the southwest, reflecting the northeast wind.
The beam elevation from the radar site is about 2900 ft AGL which matches very well with where the thermal structure would suggest this smoke to be trapped.