First, as a subterfuge to gain more than one or two readers, this priceless photo taken recently on our porch of our dog, Cody, doing a “play bow” for a free range cow that’s drinking out of his kiddie pool. Tell your friends…
Full title:
A Review and Enhancement of the “Comment” by B. Geerts and P. Adhikari on “Great Expectations: A Review of the Colorado River Basin Pilot Project—The Nation’s Most Expensive Randomized Orographic Cloud-Seeding Experiment” by A. L. Rangno and D. S. Schultza
aCentre for Atmospheric Science, Department of Earth and Environmental Sciences, and Centre for Crisis Studies and Mitigation, University of Manchester, Manchester, United Kingdom
This post is linked to in a short published “Reply” in the AMS’ journal, Weather, Climate and Society. The full article is here:
https://journals.ametsoc.org/view/journals/wcas/17/3/WCAS-D-24-0076.1.xml?rskey=OsFQ7b&result=24
Corresponding author and sole writer of the short and this “comprehensive” Reply: Arthur L. Rangno, art.rangno@gmail.com
Geerts and Adhikari (2025), hereafter “GA25,” have provided graphics of long-term cloud and precipitation characteristics for the San Juan Mountains where the Colorado River Basin Pilot Project (CRBPP) took place.
The comments of GA25 are a subset of a major computer driven analysis of SLW over the entire western United States by Adhikari et al. 2025, an article that could be considered one of the most important in support of cloud seeding activities ever published due to its scope and because of calculations indicating appreciable supercooled liquid water (SLW) over so many western mountain ranges. Absent a critique of GA25 as here, a subset of Adhikari et al. (2025[1]), the latter’s results are likely to be taken prima facie as showing widespread cloud seeding potential. In doing so, Adhikari et al.’s (2025) results, if flawed, could trigger widespread, but ineffectual seeding operations. This “Reply” falls under the rubric described by Changnon and Lambright (1990): “Honest controversy is essential for progress in science” and here is some, in long form.
GA25 point that their findings are dependent on the cloud model they employed. In this Reply, we will check the accuracy of that model against in situ observations made in support of the CRBPP. Too, we know from the checkered history of cloud seeding that reports of cloud seeding potential deserve extra scrutiny. Too often we have been misled by exaggerated claims, even from academic sources, such as those that led to the CRBPP, and most recently by those that led to disappointing randomized seeding results in Wyoming (Rasmussen et al. 2018) and Israel (Benjamini et al. 2023). In essence, “the beat goes on.”
This magnitude of this Reply, too, is inspired by the recent editorial in Weather and Forecasting by Bunkers et al. 2023 that encourages such exchanges and as such, that they should provide interesting new information for the journal reader. This has been done.
This “enhanced” Reply is organized in the following way: A brief review of the findings reported from the CRBPP that are relevant to GA25; an evaluation of the claim by GA25 that ice particle concentrations are solely a function of cloud top temperature; the CRBPP precipitation climatology is contrasted with that in GA25, and lastly; a rudimentary evaluation of the existence of a non-precipitating, relatively deep, low-based cloud that is required to explain all of the prior successes that the CRBPP was based. There is also a brief description of CRBPP spring storms because those differ appreciably in their onset from the RS25 diagram (4b) that was more typical of fall and winter storms.
- Was the seeding potential indicated by model computed radiometer values when 500 hPa temperatures were >-23°C realized during the CRBPP?
The great expectations from seeding when the 500 hPa temperature was >-23°C were not realized in the CRBPP (Rangno 1979, Fig. 18). In the only test of the Grant et al. 1969 criterion under which large seeding increases in snowfall (50-200%) in the Wolf Creek Pass experiment, Rangno (1979) found that in the CRBPP there was no viable indication of increases in snowfall on such days. However, if present, the appreciable SLW calculated by GA25 was not generally available to CRBPP ground seeding operations in the first two months of the DJFM period due to frequent deep stable layers (e. g., Marwitz et al. 1976, Rangno 1979, Marwitz 1980).
In contrast to early and mid-winter storms, there was virtually no impediment to vertical dispersion of ground released plumes of silver iodide during the latter part of the CRBPP operations, March through mid-May. An examination of this period by GA25 and the result of random seeding during this period could make an interesting follow up study as a test of the “appreciable” SLW hypothesis when the 500 hPa temperature is >-23°C.
Too, springtime storms onset differently than in the fall and winter archetype shown in RS25, 4b. In the spring, storms were generally not preceded by vast, lowering layers of high and mid-level clouds having high and cold cloud tops as were the fall and wintertime storms (e.g., Medenwaldt and Rangno 1973, Fig. 4.2). Rather, they onset with increasing cloud forms such as higher based Cumulus and Stratocumulus whose bases lowered while tops rose, and ice and virga began to appear in them as an upper trough approached (Medenwaldt and Rangno 1973, 1974, Hjermstad et al. 1975).
2. Airborne measurements in support of the CRBPP differ substantially from those deduced in model calculations by GA25 on when seeding potential exists
GA25’s findings inadvertently raise questions about the usefulness of airborne studies and also the validity of model calculations in GA25 that indicated when appreciable SLW is supposed to be present.
Why?
The University of Wyoming research team flew ~45 h in support of the CRBPP during its final season (Marwitz et al. 1976, Marwitz 1980, Cooper and Marwitz 1980, Cooper and Saunders 1980, Cooper and Vali 1981). They asserted from their measurements that the seeding potential was near the end of storms when relatively shallow orographic clouds, under cooling aloft, became more convective and therefore, contained more SLW than in the early stages of storms. In the early, warm, stable stage of the storms, Cooper and Saunders (1980) wrote: “During early storm stages the precipitation developed primarily by diffusional growth of ice crystals.” They did not encounter “appreciable” SLW as the model used by GA25 calculated under those conditions. Rather, they found that in the early stage of storms, the SLW content averaged “<0.1 g m-3.” The lack of rimed ice particles supports the view of limited if any SLW during this stage. Quoting from Marwitz et al. (1976, p52): “The stable stage of the San Juan storms is therefore unseedable, because there is negligible liquid water present and because the seeding material cannot the cloud level from the ground-based generators.” The stable stage referred to by Marwitz et al. was when the initial precipitation bearing cloud shields arrived in the San Juan mountains and when 500 hPa temperatures were almost always >-23°C, and usually with much colder cloud tops. In the latter cold phase of the CRBPP storms, the Wyoming team reported that SLW averaged 0.5 to 1 g m-3 in relatively shallow orographic clouds with moderate top temperatures or 5-10 times greater SLW than in the early stage of storms.
Also, the early stage of storms was characterized by chaotic surface and low-level winds (Marwitz 1980[1]), a condition that hampered targeting from ground seeding releases while the cold stage was much less so.
For these several reasons, the Wyoming team deemed the early warm portion of storms unsuitable for cloud seeding.
Thus, the Wyoming team’s conclusions on what storm stage is best for cloud seeding are diametrically opposed to those of GA25. GA25 concluded that the warm, early portion of storms marked by 500 hPa temperatures >-23°C, had the most cloud seeding potential due to appreciable model calculated SLW, twice as much, GA25 stated, as their calculations indicated for the cold stage of storms when hPa 500 temperatures were <-23°C.
——footnote concerning the Wyoming analyses of surface winds——-
[1] Marwitz (1980) and Marwitz et al. (1976) produced maps showing a “convergence” zone during the early portions of storms. Marwitz was contrasting CRBPP surface reporting stations with elevation differences of about 2000 feet (650 m) to arrive at the ersatz idea of a convergence zone. No convergence zone is seen in the streamline analyses by the E. G. & G, Inc., forecasting team (e.g., Medenwaldt and Rangno 1973) who realized the effect of elevation differences from the reporting stations could lead to the misperception of a surface-based convergence zone. Marwitz was alerted to this problem by the writer.
——end of footnote——-
The differences between GA25 and the Wyoming team’s results could be laid to several factors inherent in airborne research, in a defense of GA25’s calculations. Limited airborne sample volumes, limited flight durations on a relatively few storm days, and the inability in IFR situations to sample in the lowest elevations above the ground, all of which could have contributed to why the Wyoming researchers concluded that the cold stage of the storms was the one with seeding potential, not the warm one. Thus, even with as much contrast as they reported in seeding potential, the Wyoming team’s results must be considered, “suggestive,” not conclusive. More in situ and radiometer measurements are critically needed to validate the GA25’s model calculations due to their importance regarding future cloud seeding operations.
In a further comment regarding the seeding potential indicated by GA25, Vardiman and Hartzell (1976) reported that rimed particles were usually present at the top of Wolf Creek Pass regardless of the temperature at 500 hPa. Their report suggested that some SLW is escaping the precipitation process on the west slope of Wolf Creek Pass and may represent some seeding potential that was not realized during the CRBPP. Their findings, moreover, agree with those of Hindman (1986) concerning SLW at mountain tops in the Colorado Rockies.
3. The factors that affect model calculations of appreciable radiometer SLW in storms with 500 hPa temperatures >-23°C
Spuriously low ice particle concentrations output by the model used by GA25 (Thompson and Eidhammer 2014), would lead to appreciable SLW not consumed by ice particles. What is the concentration of ice particles that the model produces over and upwind of the San Juans with cloud tops at -20°C? How do they compare to the concentrations encountered by the Wyoming team? This critical information is not displayed or known by GA25; nor is it displayed in Adhikari et al. 2025, the latter the progenitor study of which GA25 is a subset. A request by this writer for benchmark Thompson and Eidhammer (2014) model calculated ice particle concentrations at -10°C, -20°C, and -30°C for comparison purposes with the Wyoming CRBPP measurements and this in other field programs could not be accommodated by GA25, nor from Adhikari et al. 2025 (B. Geerts, personal communication, November 2025).
As was learned from the many models evaluated for the prediction of orographic precipitation by Liu et al. 2011, accurate model microphysics are critical in the assessment of seeding potential.
One definite problem in the Thompson and Eidhammer model is that the onset of ice in “maritime” clouds is at -13°C, a far lower temperature than has been observed in numerous maritime cloud studies over the preceding 50 years. Ice onsets routinely in such clouds at temperatures as high as -4°C (e.g., Mossop et al. 1967, 1968, Ono 1971, Hobbs 1969, 1974, Hobbs and Rangno 1985, 1990, 1998, Rangno and Hobbs 1991, 2005). Due to this oversight, model calculated SLW will be too high in maritime and near maritime locations in the parent article by Adhikari et al. 2025 along the West Coast of the United States because ice will not onset in model clouds until top temperatures are -13°C.
But what about ice formation in the CRBPP ?
Figure 1, in effect a reality check, shows an example of a shallow orographic cloud’s rapid evolution into a glaciated mass after the formation of all SLW upwind edge. This photo demonstrates how orographic clouds can rapidly convert to ice clouds and is for the purpose of showing how important it is for a model to do what nature does. Figure 1, too, is a virtual replication of the orographic cloud described by Cooper and Vali (1981) for an orographic cloud in the San Juan mountains with a top at -20°C, about the same as the cloud top temperature in the photo. Too, the Cooper and Vali studied cap cloud evolved into an ice cloud just like this one, except for having a thin SLW top. Cooper and Vali measured a maximum of 100 per liter concentration of ice particles downwind from the cap cloud’s leading SLW edge. The model used by GA25 and by inference, Adhikari et al. 2025, should reflect this evisceration of SLW across barriers in clouds with similar top temperatures of about -20°C if model ice particle production is reasonably accurate.
Figure 1. A cross-section lifecycle of a shallow orographic cloud passing over the La Plata Mountains northwest of Durango, CO, during the CRBPP, that illustrates how liquid water at the upstream edge of the cloud (leftmost) is consumed by the rapid formation of high concentrations of ice particles as the cloud air moves downwind (center and right). Ice concentrations are estimated by the author to be in the 10s to 100 per liter by the density of the ice cloud. Photo by author taken at 1200 MST, 31 March 1974, a CRBPP experimental seeded day (Medenwaldt and Rangno 1974; Elliott et al. 1976) with southwesterly flow from the surface through 500 hPa. Cloud tops were estimated in real time by this writer to lie be between 16,000 and 17,000 feet Above Sea Level which would have made top temperatures of about -20°C. The 500 hPa temperature was -21.4°C at 1045 MST decreasing to -24.0°C by 1410 MST. The La Plata Mountains are upwind of the CRBPP target.
Similar comparisons to measurements from airborne research besides those made during the CRBPP can be examined at the several other barriers examined by Adhikari et al. (2025) that would help to establish the critical reliability of “model ice concentrations” (e.g., those in the Cascades by Hobbs 1974, 1975, Hobbs and Atkinson 1976).
4. Do ice particle concentrations increase as cloud tops get colder as stated by GA25?
Yes and no. GA25 make the general statement that “natural ice crystals become increasingly appreciable at lower temperatures.” This is not the end of the story. We know from the CRBPP airborne studies that no ice formed in those rare San Juan Mountain clouds with tops >-10°C (Cooper and Saunders 1980, Fig. 11b). At temperatures between -10°C and -19°C in measurements near cloud tops, Cooper and Saunders did find from a “very limited sample,” indeed and as asserted by GA25, that ice crystal concentrations increased linearly with lower crystal origin temperatures from 2 to about 50 per liter.
Disruptions of a relationship between cloud top temperatures and initial ice particle concentrations arise after the formation of the initial ice crystals, usually near cloud top. The ensuing deterioration of that relationship is due to fragmentation of crystals via collisions with other ice crystals(Vardiman 1978), the breakup of delicate crystal forms when growing to larger sizes, or due to collisions with cloud droplets during riming (Dye and Hobbs 1968), crystal collisions with graupel, or “unknown mechanisms.” Quoting Cooper and Saunders (1980) from ice crystal data collected at levels below cloud top in the CRBPP: “The observed ice crystal concentrations were far above the corresponding ice nucleus measurements, and the discrepancy could not be attributed to known ice multiplication processes.”
Furthermore, the assertion of a relationship between cloud top temperature and ice nucleus concentrations expected to be activated at cloud top has been refuted on many occasions (e.g., Hobbs 1969, Auer et al. 1969, Vardiman and Hartzell 1976[1], Grant et al. 1982, Rangno and Hobbs 1986, Korolev et al. 2003).
Grant et al. 1982 in an inventory of orographic clouds wrote: “In general, the largest values of crystal concentrations are observed within the lowest levels of the clouds. These observations are not consistent with the concept of ice nucleation occurring to a greater extent at the lowest temperatures (i.e., cloud top) followed by crystal fallout.” The Grant et al. statement was extremely important because it was Grant’s 1968 findings that ice particle concentrations were a function of cloud top temperature that helped lead to the CRBPP and during it, caused the change to cloud top temperatures from that at 500 hPa for two seasons before it was abandoned.
As an early example of the lack of a relationship between cloud top temperatures and ice concentrations was reported by Borovikov (1968) who wrote: “Should the phase of a cloud depend on temperature only the frequencies of liquid phase of clouds of different forms might be the same at any temperature. However, our data does not prove this idea.” (Italic “data” inserted by this writer in place of “Figure 5” for clarity.)
- Simple cloud observations also refute the idea of increasing ice particle concentrations with lowering cloud top temperatures. Altostratus and Nimbostratus have been observed to have SLW (Altocumulus-like) tops of -25°C to < -30°C (Cunningham 1957, Hobbs and Rangno 1985, Rauber and Tokay 1991, Korolev et al. 2003). This once unexpected situation has been termed, “the upside-down storm” because the very coldest portion of the cloud system is SLW and the warmer portions are all or mostly ice crystals. A vignette of the “upside down” storm can be seen in Altocumulus clouds trailing virga as here:
Observations of SLW-topped stratiform clouds with very low top temperatures are a direct refutation of the idea that “natural ice crystals become increasingly appreciable at lower temperatures” as GA25 assert. No water-topped layer clouds at the low temperatures cited above could exist if the GA25 assertion was true, and by inference, if the model they used was reliable in predicting ice particle concentrations. Furthermore, the ice particle concentrations falling out of such clouds are not in the tens to thousands per liter as they might be as derived from computer model calculations of ice concentrations from clouds with temperatures as low as those above, but are modest (e.g., Hobbs and Rangno 1985). If these upside-down situations cannot be replicated in the model used by Adhikari et al. 2025 and GA24, what does it mean for their model and cloud seeding potential? More or less? Is seeding a liquid layer at cloud top viable?
Finally, porous, all ice clouds such as transparent versions of Altostratus (i.e., translucidus, see photo) and Nimbostratus (the latter occasionally produced inconsequential snowfall in Durango) are clouds low in ice particle concentrations even with extremely low cloud top temperatures located at Cirrus levels.
In sum, there is a general lack of correlation of ice particle concentrations and cloud top temperatures except in the very early stages of ice formation in some clouds before secondary ice processes kick in and, in essence, seed clouds.
——footnote——
[1] Most of the cloud top temperatures used by Vardiman and Hartzell (1976) in forming their statement were from the La Plata County airport rawinsondes, and the reliability of those over Wolf Creek Pass is questionable.
—-end of footnote——
5. On CRBPP storm synoptics as described by GA25
GA25 state that “warm” periods (500 hPa temperatures >-23°C) represent those under “upper-level ridging and pre-frontal conditions.” This is mainly true in the late winter and spring. The characteristics of fall and early winter storms depart from this description (e.g., Rangno 1972[1]). Every hour of precipitation during all the phases of the five storms described by Rangno, pre-trough to post-trough passage, occurred with 500 hPa temperatures >-23°C. This was not an unusual sequence; storms in the early portion of the CRBPP snow accumulating season, mid-October through December) because they are generally warmer throughout than those in late winter and spring. In the spring, a trough at hPa 500 in the mean occupies the interior of the western US (Crutcher and Meserv 1970[2]).
6. When does the heaviest snow fall in the San Juans?
Figure 4 in GA25 indicates that most snow falls when the 500 hPa temperatures are <-23°C during their study period of DJFM and that the heaviest rate of precipitation also falls during this condition. This contrasts with several studies on when the heaviest precipitation occurs. Here it is presumed that the heaviest daily precipitation events also contain the heaviest rates of precipitation. Grant et al. (1974) stated: “A peak in the daily snowfall appears in the -21°C to -24°C class interval while the running mean indicates a peak around -20°C to -21°C.” The Grant et al. data were based on the unseeded daily amounts in the first two winters of the Wolf Creek Pass Experiment (Figure 40, p69). Hjermstad (1970, p53) expanding upon Grant’s findings analyzed eight Nov-Apr seasons of 24 h, non-seeded days data, reaching virtually the same conclusions; snow water equivalent amounts were nearly identical in the 500 hPa categories of -16°C to -20°C, as they were in the -21°C to -25°C. There was no decrease as in amounts as 500 hPa temperatures increased as suggested by the graphics of GS25. Moreover, amounts decreased at 500 hPa temperatures <-25°C.
Rangno (1979), and Hobbs and Rangno (1979) in long term studies of several Rockies NOAA cooperative stations for November through April, also found that the heaviest average daily precipitation fell at 500 hPa temperatures >-23°C.
The above studies used interpolated rawinsonde data from nearby NWS stations. GA25 do not disclose how they obtained 500 hPa temperatures for hourly SNOTEL data over the San Juan Mountains. Thus, differing methodologies may have influenced these different conclusions such as not using the actual NWS rawinsonde data.
These studies beg the question, too, that if there was a wider examination of the snow accumulating season in the San Juan Mountains, such as that planned for the CRBPP (mid-October through mid-May), would a different conclusion have been found by GA25 regarding when the heaviest snowfalls occur?
——footnotes——–
[1] The first official report finding that cloud top temperatures and those at 500 hPa were not correlated. An appendix quoting this finding is provided at the end of this “Reply.”
[2] For this reason, March is the coldest month aloft in general in the West and wettest snow water equivalent month at Wolf Creek Pass, CO, over its period of record, 1958-2001.
—–end of footnotes——-
7. GA25 observed that the CRBPP targeted less frequent warm storms rather than the more frequent ones that occur under low 500 hPa temperatures.
This was what was intended for the CRBPP because the less frequent storms with >-23°C 500 hPa temperatures were reported to be those when seeded, apparently produced extremely large (50-200%) increases in snow via cloud seeding in the Wolf Creek Pass experiment. Those with <-23°C 500 hPa temperatures had no apparent seeding potential (i.e., Grant et al. 1969, 1974).
8. Was the supercooled, non-precipitating cloud responsible for all the Wolf Creek Pass and Climax cloud seeding successes real?
Determining the climatologically representative hours of non-precipitating, relatively thick, low-based cloud SLW over a potential target represents a baseline of cloud seeding potential. The most viable element of the work that the CRBPP was based on was the statement by Chappell (1970): “seed clouds, not precipitation.” Chappell wrote that because all the successes in the Colorado State University cloud seeding experiments that led to the CRBPP were due to making non-precipitating clouds apparently precipitate virtually like natural storms (e.g., Chappell et al. 1971). In retrospect, it was a red flag.
The idea of seeding such a non-precipitating supercooled cloud with no guidance on when it might occur had a major effect in the CRBPP first season’s operations. With no guidance, it was thought that non-precipitating clouds might accompany “close calls” when a trough was expected to pass by with only a small chance of precipitation. No one knew. With a hardwired time to draw decisions in the first year by 9 AM, and the experimental day ending at 11 AM on the next, and the criterion being precipitation occurring “anywhere” in the target, decisions were drawn “fast and loose.” Moreover, due to the delayed start of the first season’s operations until December 1970, forecasters were urged by the sponsor of the CRBPP to not miss an opportunity for a random draw. The result was many more zero precipitation days in the first season’s experimental days compared to later seasons when more experience accrued, and a policy of having a more substantial chance of precipitation for the draw of an experimental day was implemented by new E. G. & G., Inc., management.
Preceding the CRBPP, there had been extensive studies of the precipitation climatology of the Wolf Creek Pass and Climax regions (e.g., Grant et al. 1969). However, there had not been a cloud climatology study, one that would have revealed that a non-precipitating, thick, SLW-loaded, low-based orographic cloud was largely fictitious and would have exposed the spurious nature of the reported increases in snowfall due to seeding in the Colorado State University experiments on which the CRBPP depended.
In intensive, visual observations for the full seasons of the CRBPP beginning with the 1972/73 operating season by this writer attempted to document all instances of low, relatively thick, non-precipitating clouds. The documentation of such a cloud was sought during the third season because the outcome of the CRBPP depended solely on such a cloud and the CRBPP was now failing after but two seasons in producing evidence that cloud seeding was increasing snowfall on seeded days. And because such a cloud did not appear to exist.
Low cloudiness that was at least 2,000 feet (about 650 m) in estimated depth that did not form ice and was not visually precipitating was kept track of during all daylight hours of all days of the CRBPP for three full seasons. Examples of the cloud classifications are shown in Medenwaldt and Rangno (1973, 1974) and in Hjermstad et al. (1975). These observations with cloud thickness estimates also appear on those days chosen as experimental days. The result of those several thousand observations was that such cloudiness constituted but 8-12 % (about 80-100 h in each season), and generally occurred in short durations of an hour or two and in limited coverage.
If these relatively shallow non-precipitating clouds were in the same proportion at night, and were not spotty in coverage, and they could be made to precipitate at 0.01 inches per hour it would represent, in a perfect seeding scenario where no opportunities are missed, a water equivalent potential of ~2 inches each season with no randomization and 1 inch in a 50-50 randomized experiment.
In contrast, the 50-50 randomized CRBPP was expected to produce 7.7 inches of extra water equivalent precipitation per season seeding non-precipitating clouds based on Grant et al. 1969, as documented in RS25. Nevertheless, while much less than expected, there appears to be some realizable potential for cloud seeding to increase snowfall in the San Juans.
Those thousands of visual observations, primitive as they were, were supported by the University of Wyoming’s findings years later that ice onset in CRBPP clouds with top temperatures as high as -10°C to -12°C leaving little room for non-precipitating supercooled clouds over the San Juan mountains. These inferences are not valid for lower elevation barriers than the San Juans.
REFERENCES
Adhikari, P., B. Geerts, S. Tessendorf, L. Xue, and T. Schneider, 2025: Climatology of cold-season supercooled liquid water and glaciogenic cloud seeding potential in the western United States, according to a 4 km resolution climate reconstruction. Journal of Appl. Meteor. and Climatology, 64, 1215-1231. https://doi.org/10.1175/JAMC-D-24-0246.1
Auer, A. H., D. L. Veal, and J. D. Marwitz, 1969: Observations of ice crystals and ice nuclei observations in stable cap clouds. J. Atmos. Sci., 26, 1342-1343. https://doi.org/10.1175/1520-0469(1969)026%3C1342:OOICAI%3E2.0.CO;2
Borovikov, A. M., 1968: Preprints, Cloud Phys. Conf., Toronto, Amer. Meteor. Soc., Boston, 290-294.
Bunkers, M., W. Ashley, S. Bieda, K. Calhoun, B. Kirtman, K. Kosiba, K. Mahoney, L. McMurdie, C. Potvin, Z. Pu, and E. Ritchie, 2023: Editorial, Wea. Forecasting, 38, 633-636. https://doi.org/10.1175/WAF-D-23-0051.1
Chappell, C. F., 1970: Modification of cold orographic clouds. Atmos. Sci. Paper No. 173, Dept. of Atmos. Sci., Colorado State University, Fort Collins, 196pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
________., L. O. Grant, and P. W. Mielke, Jr., 1971: Cloud seeding effects on precipitation intensity and duration of wintertime orographic clouds. J. Appl. Meteor., 10, 1006-1010. https://doi.org/10.1175/1520-0450(1971)010%3C1006:CSEOPI%3E2.0.CO;2
Cooper, W. A., and J. D. Marwitz, 1980: Winter storms over the San Juan mountains. Part III. Seeding potential. J. Appl. Meteor., 19, 942-949. https://doi.org/10.1175/1520-0450(1980)019%3C0942:WSOTSJ%3E2.0.CO;2
__________, and C. P. R. Saunders, 1980: winter storms over the San Juan mountains. Part II. Microphysical processes. J. Appl. Meteor., 19, 927-941. https://doi.org/10.1175/1520-0450(1980)019%3C0927:WSOTSJ%3E2.0.CO;2
__________, and G. Vali, 1981: The origin of ice in mountain cap clouds. J. Atmos. Sci., 38, 1244-1259. https://doi.org/10.1175/1520-0469(1981)038%3C1244:TOOIIM%3E2.0.CO;2
Crutcher, H. I., and J. Meserv, 1970: Selected-level heights, temperatures and dew point temperatures for the Northern Hemisphere. Naval Wea. Service, Report NAVAIR 50-1C-52 (revised)
Cunningham, R. M., 1957: A discussion of generating cell observations with respect to the existence of freezing or sublimation nuclei. In Artificial Stimulation of Rain, H. Weickmann, Ed. PerGA25mon Press, NY., 267-270.
Elliott, R. D., Shaffer, R. W., Court, A., and J. F. Hannaford, 1976: Colorado River Basin Pilot Project Comprehensive Evaluation Report. Final Report to the Bureau of Reclamation, Aerometric Research, Inc., Goleta, CA. 641pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
Geerts, B., and P. Adhikari, 2025: Comment on “Great Expectations: A Review of the Colorado River Basin Pilot Project—The Nation’s Most Expensive Randomized Orographic Cloud-Seeding Experiment.” Wea. Climate and Soc., in press.
Grant, L. O., 1968: The role of ice nuclei in the formation of precipitation. Proc. Intern. Conf. Cloud Phys., Toronto, Amer. Meteor. Soc., 305-310.
________, Chappell, C. F., Crow, L. W., Mielke, P. W., Jr., Rasmussen, J. L., Shobe, W. E., Stockwell, H., and R. A. Wykstra, 1969: An operational adaptation program of weather modification for the Colorado River basin. Interim report to the Bureau of Reclamation, Department of Atmospheric Sciences, Colorado State University, Fort Collins, 98pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
_________, Chappell, C. F., Crow, L. W., Fritsch, J. M., and Mielke, P. W. Jr., 1974: Weather modification: a pilot project. Final Report to the Bureau of Reclamation, Contract 14-06-D-6467, Colorado State University, 98pp plus appendices. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
_________, DeMott, P. J., and R. M. Rauber, 1982: An inventory of ice crystal concentrations in a series of stable orographic storms. Preprints, Conf. Cloud Phys., Chicago, Amer. Meteor. Soc., Boston, MA. 584-587.
Hindman, E. E., 1986: Characteristics of supercooled liquid water in clouds at mountaintop sites in the Colorado Rockies. J. Appl. Meteor. 25, 1270-1279. https://doi.org/10.1175/1520-0450(1986)025%3C0180:AAWBOA%3E2.0.CO;2
Hjermstad, L. M., 1970: The influence of meteorological parameters on the distribution of precipitation across central Colorado mountains. Atmospheric Science Paper 163, Atmos. Sci. Dept., Colorado State University, 86pp.(Available from the Western Waters Digital Library, Special Collections, University of Washington).
________, A. L. Rangno, and R. A. Medenwaldt, 1975: Colorado River Basin Pilot Project Comprehensive Atmospheric Data Report, 1972-1973 Season. Report to the Bureau of Reclamation, E. G. and G., Inc., Durango, CO. 961pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
Hobbs, P. V., 1969: Ice multiplication in clouds. J. Atmos. Sci., 26, 315-318.https://doi.org/10.1175/1520-0469(1969)026%3C0315:IMIC%3E2.0.CO;2
__________, 1974: High concentrations of ice particles in a layer cloud. Nature, 251, 694-696. https://doi.org/10.1038/251694b0
__________, 1975: The nature of winter clouds and precipitation in the Cascade mountains and their modification by artificial seeding. Part I. Natural conditions. J. Appl. Meteor., 14, 783-804. https://doi.org/10.1175/1520-0450(1975)014%3C0783:TNOWCA%3E2.0.CO;2
__________, and D. G. Atkinson, 1976: The concentrations of ice particles in orographic clouds and cyclonic storms over the Cascade mountains. J. Atmos. Sci., 33, 1362-1374. https://doi.org/10.1175/1520-0469(1976)033%3C1362:TCOIPI%3E2.0.CO;2
__________, and A. L. Rangno, 1979: Comments on the Climax randomized cloud seeding experiments. J. Appl. Meteor., 18, 1233-1237. https://doi.org/10.1175/1520-0450(1979)018%3C1233:COTCAW%3E2.0.CO;2
_________, and ________, 1985: Ice particle concentrations in clouds. J. Atmos. Sci., 36, 2523-2549. https://doi.org/10.1175/1520-0469(1985)042%3C2523:IPCIC%3E2.0.CO;2
__________, and _______, 1990: Rapid development of ice particle concentrations in small polar maritime cumuliform clouds. J. Atmos. Sci., 47, 2710-2722. https://doi.org/10.1175/1520-0469(1990)047%3C2710:RDOHIP%3E2.0.CO;2
__________, and _______, 1998: Microstructures of low and middle-level clouds over the Beaufort Sea. Quart. J. Roy. Meteor. Soc., 124, 2035-2071. https://doi.org/10.1002/qj.49712455012
_________, L. F. Radke, J. R. Fleming, and D. G. Atkinson, 1975: Airborne ice nucleus and cloud microstructure measurements in naturally and artificially seeded situations over the San Juan mountains in Colorado. Research Report X, Cloud Physics Group, Atmos. Sci. Dept., University of Washington, Seattle, 98195-1640. Available at: https://carg.atmos.washington.edu/sys/research/archive/colorado_seeding.pdf
Korolev, A., G. A. Isaac, and J. Hallett, 2003: Ice particle habits in stratiform clouds. Quart. J. Roy. Meteor. Soc., 129, 19-38. https://doi.org/10.1256/qj.01.203
Liu, C., K. Ikeda., G. Thompson, R. Rasmussen, J. Dudhia, 2011: High-resolution simulations of wintertime precipitation in the Colorado headwaters region: sensitivity to physics parameterizations. M. Wea. Rev., 139, 3533-3553. https://doi.org/10.1175/MWR-D-11-00009.1
Marwitz, J., 1980: Winter storms over the San Juan mountains. Part I. Dynamical processes. J. Appl. Meteor., 19, 913-926. https://doi.org/10.1175/1520-0450(1980)019%3C0913:WSOTSJ%3E2.0.CO;2
_________., Cooper, W. A., and C. P. R. Saunders, 1976: Structure and seedability of San Juan storms. Final Report to the Bureau of Reclamation, University of Wyoming, Laramie, WY, 324 pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
Medenwaldt, R. A., and A. L. Rangno, 1973: Colorado River Basin Pilot Project Comprehensive Atmospheric Data Report, 1972-1973 Season. Report to the Bureau of Reclamation, E. G. and G., Inc., Durango, CO. 965pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
__________, and A. L. Rangno, 1974: Colorado River Basin Pilot Project Comprehensive Atmospheric Data Report, 1972-1973 Season. Report to the Bureau of Reclamation, E. G. and G., Inc., Durango, CO. 1074pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
Mossop, S. C., Ono, A., and Heffernan, J. K., 1967: Studies of ice crystals in natural clouds. J. Res. Atmos., 1, 45-64.
_______, Rushkin, R. E., and J. K. Heffernan, 1968: Glaciation of a cumulus at approximately -4° C. J. Atmos. Sci., 25, 889-899. https://doi.org/10.1175/1520-0469(1968)025%3C0889:GOACAA%3E2.0.CO;2
Ono, A., 1971: Some aspects of the natural glaciation process in relatively warm maritime clouds. Memorial Volume of the late Prof. Syono. A special issue of the J. Meteor. Soc. Japan, 49, 845-858.
Rangno, A. L., 1972: Case study on some characteristics of the specially monitored storm episodes within the Colorado River Basin Pilot Project. Special Project Report to the Bureau of Reclamation, 105pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
__________, 1979: A reanalysis of the Wolf Creek Pass cloud seeding experiment. J. Appl. Meteor., 18, 579–605. https://doi.org/10.1175/1520-0450(1979)018%3C0579:AROTWC%3E2.0.CO;2
__________, and P. V. Hobbs, 1986: Deficits in ice particle concentrations in stratiform clouds with tops <-30°C. Preprints, 23rd Conference on Radar Meteorology and the Conference on Cloud Physics, Snowmass, CO, Amer. Meteor. Soc., Boston, 20-23.
__________, and __________, 1991: Ice particle concentrations in small, maritime polar cumuliform clouds. Quart J. Roy. Meteorol. Soc., 118, 105-126. https://doi.org/10.1002/qj.49712051705
_________, and P. V. Hobbs: Further analyses of the Climax cloud-seeding experiments. J. Appl. Meteor., 32, 1837-1847, 1993. https://doi.org/10.1175/1520-0450(1993)032%3C1837:FAOTCC%3E2.0.CO;2
__________, and __________, 2005: Microstructures and precipitation development in cumulus and small cumulonimbus clouds over the warm pool of the tropical Pacific Ocean. Quart. J. Roy. Meteorol. Soc., 131, 639-673. https://doi.org/10.1256/qj.04.13
__________, and D. M. Schultz, 2025: Great Expectations: A Review of the Colorado River Basin Pilot Project—The Nation’s Most Expensive Randomized Orographic Cloud-Seeding Experiment. Wea. Climate Soc., https://doi.org/10.1175/WCAS-D-24-0076.1
Rauber, R. M., and A. Tokay, 1991: An explanation for the existence of SLW at the top of cold clouds. J. Atmos. Sci., 48, 1005-1023. https://doi.org/10.1175/1520-0469(1991)048%3C1005:AEFTEO%3E2.0.CO;2
Thompson, G., T. Eidhammer, 2014: A study of aerosol impacts on clouds and precipitation development on a large winter cyclone. J. Atmos. Sci., 71, 3626-3658. https://doi.org/10.1175/JAS-D-13-0305.1
Vardiman, L., 1978: The generation of secondary ice particles in clouds by crystal-crystal collisions. J. Atmos. Sci., 35, 2168-2180. https://doi.org/10.1175/1520-0469(1978)035%3C2168:TGOSIP%3E2.0.CO;2
___________, and C. L. Hartzell, 1976: Investigation of precipitating ice crystals from natural and seeded winter orographic clouds. Final Report to the Bureau of Reclamation, Western Scientific Services, Inc., 129 pp. (Available from the Western Waters Digital Library, Special Collections, University of Washington).
APPENDIX
From the Summary and Conclusions section of Rangno (1972):
“A. Cloud system top variation
The meteorological evidence accumulated in these five samples is.in essential agreement with the data compiled during the 1970 -71 operating season with respect to cloud system tops, namely, that they are highly variable within relatively short times {even within the same storm) and that the 500 mb temperature exhibits little skill in predicting cloud top temperatures. For example, as the sounding data for these storm periods demonstrates, some of the coldest cloud tops are observed with some of the warmest 500 mb temperatures and vice versa when the clouds are shallow. In fact, it is difficult to escape the conclusion from these soundings that the 500 mb temperature is the least likely temperature of the cloud tops at any particular moment, and that the 500 mb surface merely forms the fulcrum point about which the tops continually oscillate suggesting a bimodal distribution.”
