WMOD00500

• Title: Observations of Silver Iodide Plumes over the Grand Mesa of Colorado
• Date: 10/10/1988
• State: CO
• Weather Modification - Doc Type: Report

OCTOBER 1988

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FIG. 4, Vertical cross sections of plumes and terrain along the axis of the plumes. The solid lines show upper plume boundaries that have
been reasonably well determined; the dotted lines show approximate boundaries, The shaded rectangles at the left indicate stable layers,

and a calming of the winds. The aircraft was able to
detect the IN only near or below treetop level, less than
70 m above the ground, but only trace amounts of IN
were found there, and the temperature sensors on the
aircraft did not show that the layer of cooling air was
penetrated. Nighttime releases of AgI in mountainous
terrain may have a similar problem with drainage winds
and radiation inversions, although the normal cloudy
conditions accompanying the best seeding opportu-
nities would not support the rapid radiational cooling
that sets up drainage winds.
These observations show that, from sites as low as
700 m below the crest, ground-released AgI plumes
can often be expected to rise to altitudes over 500 m
above the crest. The plumes rise as long as the warmest
potential temperature (or equivalent potential tem-

perature in cloudy conditions) at the seeding site is
equal to or warmer than the coldest potential temper-
ature at a level aloft. Furthermore, the terrain may
force vertical movements of air, thus changing the al-
titudes of the stable layers. Only trace amounts of IN
rise into air that is everywhere warmer than at the
seeding site. The level at which the plume stops cannot
be well predicted by an upwind sounding because such
soundings do not measure the temperature fluctuations
at vaJious altitudes.
The vertical extents of plumes measured over the
Grand Mesa are similar to those of plumes typically
observed over the Bridger Range (Super 1974; Super
and Heimbach 1988). Both mountain ranges rise about
1.5 km above the surrounding valleys; which suggests
that the range of values from the Grand Mesa experi-