Research articleAtmos. Chem. Phys., 22, 7995–8008, 2022https://doi.org/10.5194/acp-22-7995-2022 Author(s) 2022. This work is distributed underthe Creative Commons Attribution 4.0 License.The formation and composition of theMount Everest plume in winterEdward E. Hindman1 and Scott Lindstrom21 Department2 SpaceEarth and Atmospheric Sciences, The City College of New York, New York, 10031, USAScience and Engineering Center, University of Wisconsin, Madison, Wisconsin, 53706, USACorrespondence: Edward E. Hindman ([email protected])Received: 21 November 2021 – Discussion started: 6 January 2022Revised: 15 March 2022 – Accepted: 12 May 2022 – Published: 21 June 2022Abstract. Mount Everest’s summit pyramid is the highest obstacle on earth to the wintertime jet-stream winds.Downwind, in its wake, a visible plume can form. The meteorology and composition of the plume are unknown. Accordingly, daily from 1 November 2020 through 31 March 2021 (151 d), we observed real-time imagesfrom a geosynchronous meteorological satellite to identify the days plumes formed. The corresponding surfaceand upper-air meteorological data were collected. The massif was visible on 143 d (95 %), plumes formed on63 d (44 %) and lasted an average of 12 h. We used the upper-air data with a basic meteorological model to showthe plumes formed when sufficiently moist air was drawn into the wake. We conclude the plumes were composedinitially of either cloud droplets or ice particles depending on the temperature. The plumes were not composedof resuspended snow. One plume was observed to glaciate downwind. We estimated snowfall from the plumesmay be significant.1IntroductionMount Everest’s summit is the highest elevation on earth at8848 m and its summit pyramid (Fig. 1a and b) is the largestobstacle to the upper-air winds. With sufficient flow, a turbulent wake forms downwind of the pyramid and a visibleplume can form in the wake as seen in Fig. 2. The meteorology and composition of the plume have been studied, buthave not been determined conclusively. This study is a firststep to determine the plume’s meteorology and composition.We studied the plume in winter as have all previous investigators. The previous studies, to our knowledge, are as follows.A January 2004 plume was investigated by Moore (2004)(Fig. 2, top and middle). He concluded the plume was composed of resuspended snow blown from the peak. He arguedthat because the atmosphere was too dry the plume could nothave been a banner cloud (Douglas, 1928), i.e., a collectionof cloud droplets. A plume photographed by Venables (1989)looks almost identical to Moore’s plume (Fig. 2, bottom).Venables, who was on his way to climb Everest’s east face(obscured in the image by the plume), referred to the plumeas “the usual plume of cloud and snow, blasted off the summit by the prevailing westerlies”.Plumes from the Everest massif were observed in November and December 1992 by Hindman and Engber (1995) asshown in Fig. 3 and captured in a video by Hindman inNovember 1995 (see Movie 1 in the Supplement). As canbe seen in the figure and in the video, the plumes were notpresent in the morning but were present in the afternoon.The video illustrates that the plumes formed like clouds andflowed and undulated like clouds. Based on this behavior,plus investigations of the Everest airflow by Hindman andWick (1990), Hindman and Engber reported these plumeswere banner clouds.Movie 1 captures the formation and evolution of a plume:The movie began at 09:40 LST (local solar time) showing thesummits of Everest (poking over Nuptse) and Lhotse (to theright) were plume free. At about 10:50 LST, a plume beganin the wake of Lhotse. Clouds began to form on the valleyslopes about 12:00 LST. The plume reached full developmentat about 14:00 LST. At that time, the plume began to be intermittently obscured by clouds filling the valley. The moviePublished by Copernicus Publications on behalf of the European Geosciences Union.
7996E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winterFigure 1. (a) The Mount Everest and Lhotse summit pyramids are outlined. The bases of the pyramids are at an elevation of approximately7900 m. The summits are, respectively, 8848 and 8501 m in elevation. The map is from the November 1988 issue of the National Geographicmagazine. (b) The Everest summit pyramid at sunrise in May 2010 as viewed from near the summit of Lhotse (from https://www.coryrichards.com/ (last access: 3 June 2022), and Anker et al., 2013). (c) The Mount Everest region with the major summits and locations identified (HEVis the Hotel Everest View; the chart is from skyvector.com).ended at 16:30 LST because the Hotel Everest View (HEV)was enveloped by the clouds that had completely filled thevalley.Numerical simulations by Reinert and Wirth (2009), Voigtand Wirth (2013), and Prestel and Wirth (2016) demonstratethat banner clouds form in the lee of steep mountain peaks asa result of dynamically forced lee upslope flow. This resultconfirms the flows postulated by Hindman and Wick (1990)that were inspired by Douglas (1928). The simulations showthe speed of the lee upslope flow is much smaller than thespeed of the wind impacting the peak. Thus, we think the leeupslope flow may be too weak to resuspend snow.Schween et al. (2007) show still images and animations,all with the same view, from the summit of the Zugspitze inthe Bavarian Alps. Because of the best possible spatial andtemporal resolution, they were able to show the formation ofbanner clouds and snow blown off an adjacent peak.Atmos. Chem. Phys., 22, 7995–8008, 2022Here we use the best possible spatial and temporal resolution images available to us from a geostationary meteorological satellite to observe the formation of plumes in thelee of the Everest massif. When we saw a plume form inthe morning, and if our calculations predicted cloud formation through condensation of moisture in the airstream upwelling in the immediate lee of the massif, the plume waslikely a banner cloud. The composition of the cloud was inferred from its temperature.2ProceduresTo our knowledge there is no systematic imaging of the Everest massif from either Nepal or Tibet (China). (Note: Anonymous reviewer, personal communication, 2022, informed usof a live-stream of the massif from the HEV (https://www.youtube.com/watch?v O7ozpVbakZg, last access: 13 June2022). The stream was not useful for this study because ithttps://doi.org/10.5194/acp-22-7995-2022
E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winter7997Figure 2. Top: The Everest plume studied by Moore (2004) imaged from the International Space Station (ISS) on 28 January 2004 at10:01 UTC (16:01 LST, Local Solar Time). Middle: The plume 3 min later from the ISS, not reported by Moore. Bottom: The Everest plumepublished in Venables (1989) photographed from the Pang La in Tibet on 6 March 1988 at about 06:00 UTC (12:00 LST). The major peaksin the images are labeled and their summit elevations are given.began in January 2022). Therefore, daily we observed inreal time, every 10 min, images of the Everest region during winter 2020–2021 (1 November through 31 March).Observed were Band 3 (visible) and Band 13 (infrared)from the Himawari-8 (H-8) Japanese geosynchronous meteorological satellite at img.php?area ha2, last access: 3 June 2022).https://doi.org/10.5194/acp-22-7995-2022The spatial resolution of the H-8 images is sufficient toresolve the plumes, not as they form, but after they reach alength of a couple of kilometers. The following is our reasoning. The sub-satellite point is at 0 N, 104.7 E and the summit of Everest is at 27.99 N, 86.93 E. At the sub-satellitepoint, the satellite zenith angle is 0 (nadir) and the spatial resolution is 0.5 km for images in the visible band andAtmos. Chem. Phys., 22, 7995–8008, 2022
7998E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winterFigure 3. The plumes studied by Hindman and Engber (1995) photographed from the Nepal side of the Everest massif during Hindman’strek to Everest’s base.2.0 km for images in the infrared band. Careful examinationof pixel edges suggests that the 0.5 and 2 km nadir resolutions increase to, respectively, about 1 and 4 km in the vicinity of Everest. Moore (2004) estimated the plume he studied, shown in Fig. 2, to be 15 km long. Also comparing theplumes in Fig. 3 with the map in Fig. 1a, it can be seen thatthe plumes were kilometers in length. Thus, had the H-8 beenin orbit in 1992 and 2004, these plumes would have been observed.The images from the H-8 website were displayed daily inthe “Hi-res Asia 2” window and observed in the both the“still” and “animation” modes. The images could be magnified 300 on the FireFox browser and the site providedanimations up to 23 h before being overwritten. The formingplumes were observed as moving elements against a mostlystationary background. Once they reached a couple of kilometers in length, the lengthening of the plumes, shown inMovie 1, was observed.To permit the reader to observe the formation and development of the plumes, we present movies made from theevery-10-min H-8 images. All of the H-8 images presentedhere are oriented such that the vertical points toward truenorth; Fig. 1c is a map of the region. The map provides adistance scale and identifies the locations of the major peaks,Atmos. Chem. Phys., 22, 7995–8008, 2022the HEV, Phortse, Tengboche, and the Arun Valley. The timesand dates for all the H-8 images are displayed on the imagesand the movies themselves. The images and movies wereproduced following procedures in the Data Availability section.Daily, we collected meteorological data correspondingto the H-8 images: atmospheric profiles (vertical distribution of temperature, moisture (dewpoint), and wind)from the National Oceanic and Atmospheric Administration (NOAA) (https://www.ready.noaa.gov/index.php, lastaccess: 3 June 2022) at the location of Phortse, Nepal(27.84 N, 84.75 E, Fig. 1c); constant-pressure analyses of the region from the College of DuPage (http://www.weather.cod.edu/forecast/, last access: 3 June 2022);surface measurements from the automatic weather station (AWS) at Phortse tual-planet/everest/weather-data/, last access:3 June 2022). The AWS is described by Perry et al. (2021).Both Everest and its neighbor to the south, Lhotse, presentsignificant obstacles to the typically west-to-east air flow(Fig. 1a). Hence, both peaks produce wakes and, as seen inFig. 2 (top), both produce plumes. Cloud formation was investigated in the dynamically forced lee upslope flow in thesehttps://doi.org/10.5194/acp-22-7995-2022
E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winterwakes. The lifted condensation level (LCL) of the upslopeflow was calculated with the following procedure.The atmospheric profiles were displayed using the American Skew-T adiabatic diagram. The profiles were graphically analyzed to determine the LCL: the temperature anddew point values at the 400 mb level, the approximate pressure level at the base of the Everest pyramid, were raised,respectively, dry-adiabatically and with moisture constant tothe level where saturation was achieved. If the LCL wasachieved before reaching the 300 mb level, the approximatepressure level at Everest’s summit, a plume was expected toform. If the LCL was not achieved before reaching 300 mb,a plume was not expected to form; the unsaturated parcelwould be swept downwind by the high-speed summit winds.We checked the LCL values using http://www.csgnetwork.com/lclcalc.html (last access: 3 June 2022).The composition of a forming plume was inferred fromthe temperature at the LCL. Baker and Lawson (2006) reportthe composition of mountain wave clouds, an analogue tothe Everest plume. They found the clouds could contain iceparticles at temperatures colder than about 35 C. Thus, ifan LCL temperature was warmer than 35 C, initially liquid droplets are expected to have formed. Conversely, if anLCL temperature was at or colder than 35 C, initially icecrystals are expected to have formed. A mixed-phase plume(coexisting droplets and crystals) is expected near 35 C.We looked for the following events in the daily H-8 imagesto identify the conditions in which plumes formed and theconditions in which plumes did not form:1. A day with no visible plume and no measured snowfall at Phortse either that day or the previous 2 d. Thissequence will illustrate the H-8 view of the cloud-freeEverest region and the corresponding non-plume atmospheric conditions.2. A day with a visible plume and no snowfall either thatday or the previous 2 d at Phortse. This sequence willillustrate the atmospheric conditions for plume formation.3. A day with a visible plume with no snowfall measuredat Phortse that day but snowfall measured the previous 3 d, an event similar to Moore’s (2004) study. If themodel does not predict a plume, we conclude the plumewas composed of resuspended snow. If a plume was predicted, we conclude the plume was a banner cloud.We recorded the days the Everest massif was observed toproduce a plume, the formation time of the plume, the plumeduration and how many plume events were predicted by theLCL model. Cases where a plume was observed but notpredicted were investigated because they might have beenplumes of resuspended snow.We studied images from a geosynchronous meteorological satellite of the Moore (2004) plume event to determinewhether the plume behaved similarly to Event ResultsEvent 1No plumes were observed (Fig. 4a–c) and no snowfall wasmeasured at the AWS on 25, 26, and 27 January 2021. Sharpedge shadows cast by the Cho Oyu and Everest summits canbe seen in these afternoon images indicating no plumes werepresent. The shadows are more easily seen in Movie 2 for 27January 2021. The movie begins just before sunrise and endsjust after sunset, 00:40 to 11:50 UTC (06:40 to 17:50 LST).The Everest massif is in the center of the images. Scrollingslowly through the video, the long shadows in the morning cast by the massif can be seen shrinking and no plumescan be seen streaming from the summits. The shadows reappear in the afternoon. Further, the movie illustrates the snowcovered, cloud-free east face of Everest illuminated by therising morning sun.We computed the LCL values, as illustrated in Fig. 4, onthe atmospheric profiles corresponding to the images. Thevalues are given in Table 1. It can be seen that the valueswere all above the level of the Everest summit. The 400 mblevels were too dry. The temperature-minus-dew point (T Td ) values were all 31 C or larger. This result is consistentwith the observation of no plumes.It can be seen from the profiles and in Table 1 that thewinds at the summit were from the west at about 100 knots(51 m s 1 ) all 3 d.3.2Event 2A plume was observed on 21 December 2020 (Fig. 5c) butno snowfall was measured at the AWS between 19 and 21December. As observed in Event 1, sharp-edge shadows castby the Cho Oyu and Everest summits in the images from 19and 20 December (Fig. 5a and b) indicate no plumes werepresent. On 21 December 2020, plumes are seen streamingfrom these summits; the ovals in the image are elongated tobracket the plumes. Convective clouds are seen to the southof the peaks. These features are more easily observed inMovie 3 for 21 December 2020. The movie begins just before sunrise and ends just after sunset, 00:40 to 11:50 UTC(06:40 to 17:50 LST). Scrolling through the movie illustratesthe late-morning onset of the plumes and convective clouds.The LCL values computed on the profiles in Fig. 5 aregiven in Table 1. The values were above the level of the Everest summit on 19 and 20 December 2020, consistent withthe observation of no plumes. The 400 mb level T Td values were all 21 C or larger. The LCL value was below thesummit level on 21 December, consistent with the observedplumes. That 400 mb level T Td value was 4 C, quite moist.The 27 C temperature at the LCL shows the plumes werelikely liquid clouds.Atmos. Chem. Phys., 22, 7995–8008, 2022
8000E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winterFigure 4. The images and profiles (a), (b), and (c) are for 25, 26, and 27 January 2021, at 15:00 LST or 09:00 UTC. The major peaks arecircled and the location of Phortse is labeled. The lifting condensation level (LCL) values are determined graphically on the correspondingatmospheric profiles from Phortse and are listed in Table 1. The graphical procedures are described in the text. The pressures at the base andsummit of the Everest pyramid, respectively, are approximately 400 and 300 mb.It can be seen from the profiles and in Table 1 that thewinds at the summit were from the west-north-west between77 and 103 knots (39 and 53 m s 1 ) for the 3 d.3.3Event 3A plume was observed on 8 February 2021 (Fig. 6c–e) andsnowfall was measured at the AWS on 5 and 6 Februarybut none on 7 and 8 February 2021 (images from 5 through7 February are not presented in Fig. 6 because the regionwas obscured by clouds from a passing western disturbance;Lang and Barros, 2004). As can be seen in Fig. 6a and b, on8 February shadows from the summits appear in the 07:30and 09:00 LST images, indicating no plumes. Cho Oyu andEverest are producing plumes in the 12:00 and 15:00 LSTimages (Fig. 6c and d). These plumes along with Lhotse’sand Makalu’s plume are seen as the bright objects in the17:30 LST image (Fig. 6e). The corresponding 17:30 LST inAtmos. Chem. Phys., 22, 7995–8008, 2022frared image did not resolve the plumes nor did the overnightinfrared images. However, the visible image the next morning (Fig. 6f), on 9 February at 07:30 LST, is almost identicalto the previous morning’s image (Fig. 6a). Thus, the plumesdissipated overnight. No plumes were present either morning.Features in Fig. 6 are more easily viewed in Movie 4for 8 February 2021. The movie begins just before sunriseand ends just after sunset, 00:50 to 12:10 UTC (06:50 to18:10 LST). Slowing the video using the scroll bar, the animation illustrates the development of the plumes in the afternoon and their final illumination at sunset. At sunset, themovie reveals four plumes, one streaming from Cho Oyu’ssummit, a second from Everest’s summit, a third from thesummit of nearby Lhotse, and the fourth from Makalu. Themovie illustrates the plume from Lhotse was much largerthan the plume from Everest.https://doi.org/10.5194/acp-22-7995-2022
E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winter8001Table 1. Air parcels lifted from the 400 mb level, approximate pressure at the base of the Everest summit pyramid, to their condensationlevels (LCL) using gdas1 profiles for Phortse, Nepal (27.84 N, 84.75 E). The approximate pressure at Everest’s summit is 300 mb.Date25 January 202126 January 202127 January 202119 December 202020 December 202021 December 20208 February 20218 February 20218 February 20218 February 20218 February 20218 February 20218 February 20219 February 20219 February 2021Time(LST)Time(UTC)T Td at 400 mb( C)LCL(mb)T at LCL( C)T at300 mb ( C)Plumeexpected?Plumeobserved?300 mb winds(deg./kn./m s 1 300310320330320310270 47 48 50 42 42 27 43 40 40 39 35 34 34 35 43 38 27 32 37 37 38 41 39 40 40 38 37 37 36 /41 * No The infrared images could not resolve a plume.The LCL values, shown in Table 1, were above the level ofEverest’s summit ( 300 mb) at 00:00 and 03:00 UTC (06:00and 09:00 LST) consistent with the observation of no plumes.The LCL values were at and below the summit level between06:00 and 12:00 UTC (12:00 and 18:00 LST) consistent withthe observed plumes. The values remained below the summitlevel overnight. The next day the 24:00 UTC (06:00 LST)value is above the summit level consistent with the observation of no plumes.It can be seen from Table 1 that the winds at the summitwere from the northwest between 55 and 86 knots (28 and44 m s 1 ) on 8 and 9 February. These winds were caused bythe jet stream that moved through the Everest region during 8and 9 February, as shown by the sequence of images in Fig. 7.The red sinuous region defines the jet stream. Additionally,it can be seen in the sequence that the trough of the westerndisturbance, in which the jet stream was embedded, was eastof the Everest region and was moving slowly eastward.3.4Plume statisticsTable 2 displays a summary of our daily observations of theH-8 imagery and the 400 mb LCL values calculated from thecorresponding atmospheric profiles. It can be seen from thetable that Everest was almost always visible, 143 of the 151 d(95 %). On the days Everest was visible, plumes were observed to form on 63 d (44 %). Of these plumes, 59 (94 %)were predicted to form and four (6 %) were not predicted.Were those four plumes composed of resuspended snow?The four plumes were observed on 5 December 2020,29 January 2021, 3 and 11 February 2021. The 400 mbLCL values for the plumes ranged from 295 to 249 mb, allabove the 300 mb level of the Everest summit. The plumesformed between 12:00 and 14:00 LST and dissipated 0 LST. The plumes were not visible at sunrise. Therefore, these plumes were not composed of resuspended snow.Thus, none of the 63 plumes we observed were composedof resuspended snow. However, plumes of resuspended snowmay have been smaller than the H-8 detection limit of a couple of kilometers.Twice-daily images of the Everest summit coincident witha portion of our H-8 observations became available fromGrey et al. (2022) while this study was in peer-review. Theimages were taken from 16 December 2020 through 16 January 2021 (32 days) at 10:00 and 17:00 LST. We studiedthe images to determine the number of days the summit wasvisible and the number of days plumes occurred. The summitwas visible on 28 d (88 %) while the corresponding H-8 observations revealed the massif was visible on 32 d (100 %).The summit produced 18 morning plumes and 11 afternoonplumes. The corresponding H-8 observations detected eightof the morning plumes and four of the afternoon plumes. Thiscomparison shows a number of Everest plumes did not reachthe requisite length (a couple of kilometers) to be detected inthe real-time H-8 images.We observed plumes we suspect were composed primarily of snow formed in situ, as shown in Movie 5. The moviewas constructed from the real-time H-8 infrared images asdescribed in the Data Availability section. Note, in the movieNT is Nepal time, which is approximately LST. The majorsummits are labeled and are seen as white, stationary objects.On 21 December 2020, plumes are seen to form in the morning downwind of the Everest massif and Cho Oyu (also, theseplumes are shown in Movie 3). The plumes dissipated 4 dlater on 25 December 2020 early in the morning. The plumesfluctuated in length and can be seen to stream well into Tibet.The 400 mb LCL values were between 393 and 356 mb, indi-Atmos. Chem. Phys., 22, 7995–8008, 2022
8002E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winterFigure 5. The images and profiles (a), (b), and (c) are for 19, 20, and 21 December 2020, at 09:00 UTC (15:00 LST). The major peaks arecircled and the location of Phortse is labeled. The LCL values are determined graphically on the corresponding atmospheric profiles fromPhortse and are listed in Table 1. The graphical procedures are described in the text. The pressures at the base and summit of the Everestpyramid, respectively, are approximately 400 and 300 mb.Table 2. Results from the observations of Himawari-8 imagery and the lifted condensation level (LCL) calculations from the correspondingatmospheric profiles (roman text denotes summed values, italics denotes average ruaryMarchNumberof daysobservedEverestvisiblePlumeobservedAverage plumeformation time(hour LST)Average plumeduration(hours)Average LCLtemperature( C)Average300 mb winds(deg./m s 1 2771571717107999814141111 32 31 31 35 363912 33264/3859495 %44 %94 %6%Atmos. Chem. Phys., 22, 7995–8008, 2022https://doi.org/10.5194/acp-22-7995-2022
E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in winter8003Figure 6. The visible images (a), (b), (c), (d), and (e) are for 8 February 2021 and (f) is for 9 February 2021 at LST (UTC 6h). Thelocations of the major peaks are circled. The corresponding LCL values are listed in Table 1.Figure 7. The 00:00 UTC Global Forecast System forecast for 8 February 2021. Left: 00:00 UTC (06:00 LST); center: 12:00 UTC(18:00 LST); right: 24:00 UTC (06:00 LST 9 February). Shown are the 250 mb isotachs (knots) in the color scale, geopotential heights (gpm),and the location of Everest. Collected from the College of DuPage NEXLAB website https://weather.cod.edu/forecast/legacy/ (last access: 3June s. Chem. Phys., 22, 7995–8008, 2022
8004E. E. Hindman and S. Lindstrom: The formation and composition of the Mount Everest plume in wintercating extremely moist conditions, although no precipitationwas measured at Phortse.3.5The Moore plumeMoore (2004) studied plumes that streamed from theEverest–Lhotse–Nuptse massif late in the afternoon of 28January 2004 (Fig. 2, top and middle). The plumes wereimaged from the International Space Station (ISS). To determine whether the plumes were present that morning andthe next, we analyzed all available images from the Geosynchronous Orbiting Environmental Satellite-9 (GOES-9). TheGOES-9 was lent by the USA to Japan after the failed launchof MTSAT-1.The GOES-9 images are shown in Fig. 8. The earlymorning image at 07:25 LST on 28 January (Fig. 8a) showssharp-edge shadows from Everest and Makalu. Had theplumes been present, the shadows would have been fuzzyand diffuse. The plumes were not visible until lit by the lateafternoon sun as seen in the 16:13 and 16:49 LST images(Fig. 8c and d). This illumination of the plumes at sunset alsooccurred for the plumes presented in Fig. 6e and Movie 4,Event 3.The GOES images for the afternoon of 28 January showa cloud layer moved toward the Everest region from thewest. The layer is visible in the 16:13 and 16:49 LST images (Fig. 8c and d). In the 16:49 LST image, the layer casta shadow on the lower clouds. Moisture ahead of this layermay have formed the afternoon plumes imaged from the ISS.Based on this interpretation of the GOES images, we conclude the plume Moore studied was not present in the morning and formed in the afternoon.Overnight, the cloud layer moved into the Everest regionbecause at dawn on 29 January, the plumes produced by themajor summits are seen to protrude above the overcast (07:25and 09:02 LST images, Fig. 8e and f). The protruding plumesare difficult to identify in the figures. Thus, we searched thearchives for images of finer spatial resolution from polarorbiting satellites.Finer detail of these plumes was found in the Terra/MODerate resolution Imaging Spectroradiometer (MODIS) visibleimage of 09:10 LST on 29 January 2004 (Fig. 9). The spatialresolution of this MODIS image is 0.38 km per pixel: the distance between Everest and Lhotse summits is 3 km (Fig. 1a)and 8 pixels covers that distance. Unfortunately, the MODISvisible image on 28 January was not useful because it was onthe limb and pixelated, smearing the features. The MODIS0.85 µm wavelength image is good for cloud detection (compared to 0.65 µm on GOES) because atmospheric scatteringis less at 0.86 µm and contrasts are better maintained.The MODIS image shows distinct plumes in the wakesof the major peaks. The Everest plume casts a shadow onthe lower cloud layer indicating that it rises above that layer.The shadow shows the plume has a sharp edge, the edge of aliquid cloud. A short distance downwind, the plume mergesAtmos. Chem. Phys., 22, 7995–8008, 2022with the plume from Lhotse and becomes fuzzy, suggestingglaciation. The regions of the plumes containing primarilycloud droplets are the most reflective hence the brightest andthe whitest. The regions of the plume containing primarilymuch larger ice crystals are less reflective and appear dimmerand grayer. The fuzzy plume traveled across the Arun Valley.It is possible crystals fell as snow that may have reached thesurface.44.1DiscussionMeteorologyThe plume observations and the corresponding meteorological analyses are summarized in Tables 1 and 2. The LCLvalues show plumes were observed when the 400 mb LCLwas below the 300 mb level of the summit of Everest. This result shows that moisture condensed in the dynamically forcedrising air in the Everest wake to produce the plumes. Moisture likely was transported vertically in morning convection(Hindman and Upadhyay, 2002) and entrained by the wakeproducing the afternoon plumes. Some of the moisture couldhave come from sublimation of snow. Stigter et al. (2018)measured cumulative sublimation and evaporation from aglacier in the Nepalese Himalayas to be 21 % of the totalannual snowfall. Finally, the morning moisture transport andafternoon appearance of the plumes are consistent with thefindings of Wirth et al. (2012, Fig. 5b) for banner clouds produced by the Zugspitze.All the plumes
that banner clouds form in the lee of steep mountain peaks as a result of dynamically forced lee upslope ﬂow. This result conﬁrms the ﬂows postulated by Hindman and Wick (1990) that were inspired by Douglas (1928). The simulations show the speed of the lee upslope ﬂow is much smaller than the speed of the wind impacting the peak.