North American Weather Consultants, Inc.
________________________________________________________________________
SUMMARY OF A TRIAL WINTER FOG
CLEARING PROGRAM
MISSOULA INTERNATIONAL AIRPORT
2006-2007 WINTER SEASON
Prepared for
Missoula International Airport
by
North American Weather Consultants, Inc.
8180 South Highland Dr., Suite B-2
Sandy, Utah 84093
Report No. WM 07-1
Project No. 06-197
June 2007
TABLE OF CONTENTS
1.0
2.0
3.0
4.0
5.0
6.0
Introduction …………………………………………………………………. 1
Background on Fog …………………………………………………………. 2
Design of Missoula Trial Fog Clearing Program …………………………. 4
Operations …………………………………………………………………… 14
University of Utah Patent …………………………………………………… 15
Summary and Recommendations …………………………………………….. 16
Figures
1
2
3
4
5
6
7
8
9
10
Occurrence of “Cold” and “Warm” Fog Events for Three Winter Seasons,
October through April ………………………………………………………. 6
Occurrence of “Cold” and “Warm” Fog Events for Three Winter Seasons,
November through February ………………………………………………… 6
Landmarks and Terrain Surrounding the Missoula International Airport …… 7
Map of the Northwest End of the Runway ………………………………….. 8
Liquid Carbon Dioxide Dispensing System ………………………………… 9
Close-up of Nozzle used to Atomize the Liquid CO2 ……………………….. 9
Missoula International Airport ……………………………………………… 12
National Weather Service ASOS Weather Observing System at the Missoula
International Airport ………………………………………………………… 12
Cris Jensen at the Runway Visibility Observation Site, Missoula International
Airport ……………………………………………………………………….. 13
Fog Approaching the Missoula International Airport during the morning of
November 9, 2006 …………………………………………………………… 13
Tables
1
2
3
4
5
Fog Statistics for Three Winter Seasons ……………………………………. 4
Time of Occurrence of Fog Events ………………………………………….. 5
Results of 15-Minute Tests on Release Rate of CO2 Nozzles ………………. 10
Fog Seeding Operations during the 2006-2007 Winter Season ……………… 14
Times when Runway Visibility was < ½ Mile as Reported by the National
Weather Service ………………………………………………………………. 14
APPENDIX A
OPERATIONAL LOGS
APPENDIX B
NATIONAL WEATHER SERVICE OBSERVATIONS
MISSOULA, MONTANA
APPENDIX C
MISSOULIAN NEWS ARTICLE, DEC. 13, 2006
2
1.0
INTRODUCTION
The following is a brief summary of the events associated with establishing a trial
cold fog-clearing program for the Missoula International Airport (Airport):
•
•
•
•
•
Cris Jensen, the Missoula International Airport Director contacted North
American Weather Consultants (NAWC) in late October 2006. The topic of
discussion was whether cloud seeding could be utilized in an attempt to
improve airport runway visibilities during winter fog episodes.
Don Griffith, President of North American Weather Consultants, traveled to
Missoula November 8-9, 2006.
Following this visit, NAWC proposed to assist Airport personnel in
establishing a trial fog-clearing program. An agreement was reached and two
liquid carbon dioxide dispensers were shipped to the Airport on November 15,
2006.
The trial program became operational on November 22, 2006.
The program was on operational standby from November 22, 2006 through
February 28, 2007.
This program was established due to the rather frequent disruption of air traffic in
and out of the Airport due to the occurrence of fog and associated low visibilities during
the winter months. For example, such conditions occurred during the Thanksgiving
Holiday period of 2005, resulting in significant delays and impacts on travelers using the
Airport. This was a cooperative program in which the local National Weather Service
office personnel provided meteorological assistance via the transmission of products and
forecasts to Missoula Airport personnel.
2.0
BACKGROUND ON FOG
The following excerpts are from an American Society of Civil Engineers
Standards publication (ASCE 2005):
There are three basic kinds of fogs: warm fogs, where droplets exist in air
temperatures warmer than 0 oC, supercooled fogs, where droplets exist in air
temperatures colder than 0 oC; and ice fogs, where air temperatures are typically colder
than -30 oC (Huffman and Ohtake, 1971). Ice fogs are analogous to dense cirrostratus
clouds (Wendler, 1969), and have ice crystal sizes between 2 and 50 μm at number
densities between 30 and 700 cm-3 (Ohtake and Huffman, 1969). Approximately 5% of
all fogs in the United States are accompanied by sub-freezing air temperatures
(Changnon, 1975).
Fogs may be further categorized according to the physical processes responsible
for their formation. A fog produced by radiative cooling of the earth's surface is called a
radiation or ground fog. For example, fogs in the Central Valley of California are
generally radiation fogs. Advection fogs are produced from the movement of warm,
moist air (as in water vapor-laden air) over a surface cold enough to induce condensation
above it. Fogs along the U.S. west coast are generally advection fogs. Advection fog
that subsequently moves over relatively colder ground, which continues to cool by
radiation, is termed advection-radiation fog. Fog that forms in moist air that flows up
along a hillside is called upslope fog. Fog that forms from the mixture of two different
air masses, whether or not these different air masses are saturated is typically termed
evaporation-mixing fog. Steam fog is an example of evaporation-mixing fog and may be
seen over the thermal ponds at Yellowstone National Park, especially during the winter
months (e.g. Dennis, 1980). Another example of evaporation-mixing fog could be one’s
breath. Ice fog may be a radiation, advection, advection-radiation, or an evaporationmixing type fog.
The ability for man to improve visibilities in fog is most successful through
seeding of supercooled fogs (e.g., temperatures of ~ 0 0 to –30 0 C, +32 0 to –22 0 F).
Supercooled fog dispersal programs use seeding agents to initiate the formation of
ice, a process called ice nucleation. If freezing results from the introduction of non-ice
substances, the process is more specifically termed heterogeneous nucleation, and the
substances used are known as ice forming nuclei. The temperature threshold at which an
ice forming nucleus triggers nucleation is referred to as the nucleus’ activation
temperature. Freezing of supercooled water is also possible in the absence of ice forming
nuclei if the fog is sufficiently cooled. Such a process is called homogeneous ice
nucleation. The temperature threshold for the homogeneous ice nucleation of pure water
is -40ºC (e.g., Rogers & Yau, 1991). There are many sources of natural ice nuclei, with
clay soils perhaps being the most common. These natural ice nuclei generally have
2
activation temperatures on the order of -12ºC to -21ºC (Table 2.2), though natural ice
nucleation rarely occurs at temperatures warmer than this range.
Simply, the goal of most supercooled fog dispersal programs can be obtained by
creating and placing, within the fog, sufficient ice particles that will ultimately fall to the
ground faster than the supercooled fog droplets form in or advect to an area. The ice
particles, or snowflakes, are most commonly created by placing dry ice or liquefied
propane into the supercooled fog, although other glaciogenic seeding agents may be used.
There are a number of appropriate delivery choices available for both airborne
and ground-based delivery systems. The equipment necessary for a suitable stationary
ground-based delivery system can be considerably more expensive than for aircraft or
mobile ground-based seeding because there are generally multiple permanent devices that
need to be installed. Mobile ground-based supercooled fog dispersal systems are very
cost effective. Ground based systems are often very effective when used to disperse
rather shallow fogs (i.e. thickness less than 700 m), whereas they are not very effective in
dispersing fog throughout its total vertical distance if the fog is greater than 700 m thick.
3
3.0
DESIGN OF MISSOULA TRIAL FOG CLEARING PROGRAM
Climatology
NAWC performed a cursory look at the climatology of fog at the Missoula
International Airport prior to the meetings held at the airport on November 8 - 9th. We
examined three winters of Missoula’s airport weather records (2000-2001, 2004-2005
and 2005-2006) for the October through April period. Specifically, we documented the
weather conditions whenever the visibility at the Airport was ≤ 1 mile. Table 1 provides a
summary of some of this information.
Table 1 Fog Statistics for Three Winter Seasons
Oct - Apr
season
2000-01
2004-05
2005-06
Mean
Oct. –
Apr.
Cold Fog
Duration:
Hours
< 0.5
0.5 - 1.0
1-2
2-3
3-5
5-10
10+
Cold
fog
days*
36
22
23
27
Warm
fog
days*
7
11
18
12
%
cold
84%
67%
56%
69%
Seedable
hrs
118
56
108
94
Number
65
30
21
10
5
13
5
Nov - Feb Cold fog Warm Percent
season
days* fog days* cold
2000-01
30
3
91%
2004-05
20
8
71%
2005-06
16
10
62%
Mean
22
7
76%
4
Seedable
> 1hr
106
50
105
87
Longest (cold)
event
10 hrs
14 hrs
49 hrs
The events were classified as “cold” if the temperature was ≤ 300 F. It was
theorized that seeding fog events at or below 300 F would be potentially beneficial based
upon previous research. This short duration sample indicates that the percentage of
potentially seedable events varies from one winter to the next; 56 to 84% with a three
season average of 69%. In other words, on average, it would appear fog seeding at the
Airport might potentially be effective 69% of the time during the October through April
period. These percentages rise when considering only the months of November through
February, which yields an average percent of seedable events of 76%. Table 1 also
indicates that a majority of these low visibility events are relatively short lived; primarily
less than one hour to three hours in duration. Figures 1 and 2 provide some graphical
plots of some of this information.
The three seasons’ data were stratified by time of occurrence in six-hour time
blocks: 00-06, 06-12, 12-18 and 18 –24 hours MST. Table 2 provides this information.
Table 2 Time of Occurrence of Fog Events
Number of
events
Percent of Total
0000-0600
79
40
0600-1200
87
44
1200-1800
15
7
1800-2400
19
9
A large percentage of the low visibility events occur in the 0000 to 1200 MST
time period.
Another important feature of these low visibility events was the fact that winds
were very light to calm during most events.
Initial Program Design
The Airport is located in a relatively narrow mountain valley at an elevation of
approximately 3200 feet MSL. Figure 3 provides a map of the local area. Three possible
seeding delivery options were considered initially: 1) fixed ground based dispensers 2)
mobile ground based dispensers and 3) aircraft dispensers. Because the fog season was
imminent, it was recommended that the mobile ground dispenser option be considered.
Such a choice seemed potentially feasible considering the availability of a road around
the west end (landing approach zone) of the Airport. Refer to Figure 4 for a depiction of
this road in relation to the runway. The second factor that seemed to favor this approach
was the apparent very light to calm winds that accompany these fog events. Had the
winds during fog events been indicated to be stronger, the release of the seeding material
would have needed to be further upwind in order to provide for the growth of the ice
crystals produced by seeding into snowflakes large enough to fall to the ground creating
and improvement in the visibility through the removal of the cloud droplets forming the
fog.
5
Figure 1
Occurrence of “Cold” and “Warm” Fog Events for Three Winter
Seasons, October through April
Figure 2
Occurrence of “Cold” and “Warm” Fog Events for Three Winter
Seasons, November through February
6
Figure 3
Landmarks and Terrain Surrounding the Missoula International
Airport (circle indicates the area of fog seeding activities)
7
Figure 4
Map of the Northwest End of the Runway
(airport service road used for seeding operations shown around end of runway)
The type of seeding agent to use was given consideration. The release of a
compressed liquid was favored over the use of silver iodide (a common seeding agent)
since the venting of certain compressed liquids can potentially be effective at
temperatures of 300 F and below whereas silver iodide is only effective at temperatures
less than approximately 230 F. Two types of compressed liquids were considered,
propane and carbon dioxide. Liquid propane fog clearing operations have been conducted
for a number of years at the Fairchild Air Force Base in Washington. Fixed dispenser
sites are used on this program. Due to some concerns about flammability issues with
liquid propane it was recommended that liquid carbon dioxide (a non flammable
substance common in the atmosphere) be used. NAWC had some previous experience in
using liquid carbon dioxide (CO2) in a fog clearing experiment at the Kennecott copper
mine in Utah.
NAWC sent two rather simple dispensing units to Missoula to be used in the
seeding tests. Figures 5 and 6 provide photos of one of these units. Each dispenser is
designed to hold two cylinders of liquid CO2. Each cylinder holds 50 pounds of CO2. The
two cylinders are manifolded together. The cylinders are equipped with a siphon tube that
goes to the bottom of each cylinder, which allows venting of liquid CO2 through a spray
nozzle located on a 10-foot mast. Venting of the liquid CO2 results in dramatic cooling of
the air downwind of the spray nozzle. Temperatures are lowered to approximately – 1100
F in a cone of air about 12 inches in diameter and 36 inches long. NAWC provided
Airport personnel with spray nozzles of 5 different sizes.
8
Figure 5
Figure 6
Liquid Carbon Dioxide Dispensing System
Close-up of Nozzle used to atomize the Liquid CO2
9
Larger sized nozzles release more liquid CO2 per unit time than smaller ones. NAWC ran
tests of the amount of CO2 that was released by each of the 5 nozzles. These data are
provided in Table 3.
Table 3
Results of 15-Minute Tests on Release Rate of CO2 Nozzles
Nozzle Size
Orifice (inches)
Estimated Release Rate (lb/hr)
#1
#2
#3
#4
#5
.020
.028
.034
.041
.044
20.8
34.4
52.8
70.0
94.8
This information combined with the fact that liquid CO2 weighs approximately
8.5 pounds per gallon can be used to estimate the length of time that two cylinders
manifolded together can be used continuously until the CO2 is depleted. For example,
with a number 3 nozzle, two full cylinders should last approximately 1.9 hours (100/52.8
pounds per hour = 1.9 hours).
Reporting/Permitting Considerations
NAWC contacted the Montana Department of Natural Resources and
Conservation concerning the possible need for a weather modification permit. There is an
existing statute that regulates the use of weather modification in the State (#85-3-101
through 401, MCA). Discussions with Paul Azevedo of the Montana Department of
Natural Resources and Conservation indicated that fog-clearing operations are exempt
from the permitting and licensing requirements of this statute.
Another Federal regulation within the auspices of the National Oceanic and
Atmospheric Administration requires: 1) an initial report on planned weather
modification activities, 2) an interim report after the first of the year and 3) a final report
following completion of the program. NAWC complied with this regulation.
Summary of Preliminary Design for this First Season Effort
NAWC recommended that one or two Airport vehicles be equipped to carry the
liquid CO2 dispensers (anchored in the open bed of a pickup with the 10 foot mast
extending vertically above the pickup bed) that could be activated in a quick response
mode whenever low visibilities due to fog occurred with: 1) temperatures ≤ 300 F, 2)
without any precipitation (snow or rain) and, 3) calm to light winds. Further, operations
would only be conducted during times that Airport personnel determined there was a
10
potential impact on flight operations (e.g., operations at 0100 AM might not be
necessary).
One or more of these vehicles would then drive along the road that rings the west
end of the runway dispensing the liquid CO2. Operations would continue until the fog
began to dissipate naturally or the occurrence of low visibilities no longer would be a
detriment to airport take-offs and/or landings. It was recommended that logs of seeding
activities be maintained with information on seeding times, locations, nozzle size and any
visual observations of apparent effectiveness of seeding.
Figures 7 to 10 provide photos of the Missoula Airport and associated observation
sites as well as a photo of fog that developed in the morning hours of November 9, 2006.
Figure 7
Missoula International Airport
11
Figure 8
National Weather Service ASOS Weather Observing System
at the Missoula International Airport
Figure 9
Cris Jensen at the Runway Visibility Observation Site, Missoula
International Airport
12
Figure 10
Fog Approaching the Missoula International Airport during the
morning of November 9, 2006
13
4.0
OPERATIONS
The program became operational on November 22, 2006. Fog clearing operations were
conducted during portions of five different days. A summary of activities is provided in
Table 4. Appendix A contains the operational logs. Times are MST.
Table 4
Fog Seeding Operations during the 2006-2007 Winter Season
Date
Start Time
Stop Time
Temperature
Dec. 7, 2006
Dec. 8, 2006
Dec. 9, 2006
Dec. 10, 2006
Feb. 7, 2007
0530
0500
0530
0630
0800
2330
1230
1300
1200
1100
19-250 F
15-230 F
14-240 F
12-240 F
27-320 F
# of
Vehicles
1
2
2
2
2
Pounds
of CO2
208
467
417
317
100
National Weather Service hourly observations at the Missoula International
Airport during these events are provided in Appendix B. Determination of the success of
these operations is somewhat subjective, although Airport personnel indicated good
success during some of the operations, with holes cleared in the fog allowing air traffic
operations to continue. The times that observations at the airport were less than 0.50 mile
during the period of seeding for each event are documented in Table 5 (data are taken
from information provided in Appendix B).
Table 5
Dec. 7, 2006
0553, 0653
0839, 0853
Times when Runway Visibility was < ½ Mile as Reported by the
National Weather Service
Dec. 8, 2006
1106, 1515,
1553, 2200,
2253
Dec. 9, 2006
0853,0953
Dec. 10, 2006
0653, 0755
1000, 1053
1058, 1110
Feb. 7, 2007
Below 0.5 for
the entire
seeded period
It is likely the seeding on February 7th was ineffective due to the temperatures
being near freezing. There were other fog events that occurred during the 2006-2007
operational period but they were too warm for seeding to be effective.
14
5.0
UNIVERSITY OF UTAH PATENT
NAWC discovered in late December that we had inadvertently duplicated a
patented fog seeding technique developed at the University of Utah. This patent
(# 5628455) is for “A method and apparatus for reducing super cooled fog which
involves the introduction of liquid carbon dioxide in a horizontal line along the
ground under the fog from a moving vehicle.” NAWC contacted the University of
Utah Research Foundation, Technology Commercialization Office concerning the
use of this patent. Through a series of negotiations, NAWC was granted a three
year exclusive right to this patent.
15
6.0
SUMMARY AND RECOMMENDATIONS
A trial program was designed and implemented to determine if mobile releases of
liquid carbon dioxide into supercooled (colder than freezing) fog from a frontage road
immediately adjacent to the west end of the Missoula International Airport (Airport)
would result in an improvement in runway visibility. This program was considered a trial
due to the very light or calm winds that typically accompany cold (< 300 F) fog events at
the Airport. These conditions render the targeting of the clearings (produced by seeding)
in the desired areas questionable. The program became operational on November 22,
2006 and was on a ready status through February 28, 2007. Two Airport vehicles were
equipped with liquid carbon dioxide dispensers. Seeding operations were conducted
during portions of five different days. Airport personnel noted clearings in the fog during
several of these events. Appendix C contains a copy of a newspaper article related to
seeding some of these events. Seeding was not effective on the last event, February 7,
2007, which was attributed to temperatures being too warm for effective seeding. There
were other occasions during the operational period with low visibilities being associated
with fog but these events were too warm for effective seeding. It is concluded that this
seeding technology can be effective at the Airport given the right conditions. It is
estimated that the right conditions (temperatures < 300 F without any precipitation
occurring) will occur approximately 70-75% of the time when visibilities are less than
one mile during the November through February period.
It is recommended that this program continue for the 2007-2008 winter season. It
is further recommended that a prototype fixed location liquid carbon dioxide dispenser be
developed during the summer and then tested during the 2007-2008 winter season. The
rationale for this recommendation is the somewhat labor intensive aspect of the mobile
release methodology employed in last winter’s trial program. It is envisioned that the
prototype unit would be remotely controlled and self-contained (i.e. no requirement for
electrical power). If the testing of this prototype unit proves successful, then it is
theorized that a network of such fixed dispensers could be established, tested and
operated in future winter seasons, which would eliminate the more labor intensive mobile
release program design. There would even be the potential to render these remote units
“smart” through the use of a software program that ingests the observations from the
National Weather Service ASOS weather-observing site. The unit would be programmed
to turn on when certain criteria are met. For example, the unit would turn on when all of
the following conditions are met:
•
•
•
Visibility is ≤ 1/2 mile
The surface temperature is ≤ 300 F
The surface winds are either calm or are blowing from a direction that
would carry the seeded volume into the areas of interest.
Likewise, the unit would turn off when these criteria are no longer met.
16
References
Changnon, S., 1975: Present and Future of Weather Modification. pp. 162-165.
Dennis, A., 1980: Weather Modification by Cloud Seeding. Academic Press, New York.
pp. 267
Huffman, P.J., and T. Ohtake, 1971: Formation and growth of ice fog particles at
Fairbanks, Alaska. J. Geophys. Res., 76(3), 657-665.
Ohtake, T., and P. J. Huffman, 1969: Visual Range in Ice Fog. J. Appl. Meteorol., 8, 499501.
Rogers, R.R., and Yau, 1991: A Short Course in Cloud Physics. Pergamon Press, New
York, NY, pp. 307.
17
APPENDIX A
OPERATIONAL LOGS
18
19
20
21
22
23
APPENDIX B
NATIONAL WEATHER SERVICE OBSERVATIONS
MISSOULA, MONTANA
24
Missoula International Airport NWS Observations Dec. 7, 2006
25
Dec. 7, 2006 NWS Observations (Continued)
26
Dec. 7, 2006 NWS Observations (Continued)
27
Missoula International Airport NWS Observations Dec. 8, 2006
28
Dec. 8, 2006 NWS Observations (Continued)
29
Missoula International Airport NWS Observations Dec. 9, 2006
30
Missoula International Airport NWS Observations Dec. 10, 2006
31
Dec. 10, 2006 NWS Observations (Continued)
32
Missoula International Airport NWS Observations Feb 7, 2007
33
APPENDIX C
MISSOULIAN NEWS ARTICLE, DEC. 13, 2006
(reprinted with permission from the Missoulian)
34
35
36
North American
Weather Consultants, Inc.
8180 South Highland Dr., Suite B-2
Sandy, Utah 84093
_________________________________________________________________________________________________
801-942-9005
37