CWCB Wintertime Weather Mod Report

• Project Type: General OWC
• Applicant: Weather Modificaton
• Project Name: Colorado Weather Modification Program
• Title: CWCB Wintertime Weather Modification Report Fall 2002 - Spring 2003
• Date: 7/1/2003
• Water Conservation - Doc Type: Final Report

2.5 Atmospheric Stability

Temperatures in the free atmosphere typically decrease with increasing heights.
An atmospheric inversion occurs when the temperatures above the earth's surface at a
given point actually increases in temperature. These inversions lead to trapping of
materials below the inversion. Inversions that persist over cities can often lead to the
formation of smog beneath the inversion (i.e. Los Angeles). Atmospheric stability refers
to the rate at which the temperature decreases with height. A standard rate (normally
referred to as a lapse rate) of decrease is approximately 2 0 C per 1000'. In cloud seeding
applications, we are interested in knowing if atmospheric inversions or stable lapse rates
(rates less than the standard lapse rate) are present in the area of interest during storm
periods. If so, they wi11likely restrict the vertical rise of the seeding material which may
render seeding under these circumstances ineffective if the silver iodide particles do not
!
-------reach-the--~~C level-in the-atmosphere.
. 1-- -- - - - -- - - --- - - --- . - .-- -.

The following lower-level stability analysis, which utilized the balloon sounding
information from Grand Junction, has some limitations since the soundings from Grand
Junction are in an area of different topography from that of the project area. Stability
below about 600 mb (-14,000 feet) was examined in these soundings, and the results
summarized in Table 2-3. The atmospheric temperature profile was well-mixed in 72
cases, or about 43% of the time; there were minor stable layers or slight static stability in
49 cases, or about 30% of the time; and a more definitely stable atmosphere was observed
in 45 cases, or 27% of the time, The atmosphere was most stable during the month of
January, with half of the soundings quite stable and over three-quarters having some
stability. March and April soundings had the least stability, with over half being well-
mixed. Figure 2.10 provides a plot ofthis stability information by month.

A stability analysis comparing wind direction sectors does not seem to show a
clear relationship between stability and 700-mb wind direction, although the atmosphere
was perhaps most well-mixed when 700-mb winds were westerly. Of the few cases with
an easterly component, most had at least some stability below 600 mb.

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