Mount Everest's photogenic weather during the post‐monsoon

Mount Everest, known locally as Sagarmatha or Qomolangma, is the world's highest (8849m), and arguably most iconic, peak. That allure draws large numbers of tourists to Nepal every year with hopes seeing climbing famed mountain. Importantly, tourist presence has wide ranging environmental (Napper et al., 2020; Aubriot 2019; Semple 2016; Faulon Sacareau, Miner 2021; Byers, 2005), cultural (Rai, 2017; 2020), societal (Pallathadka, MOFA, 2021) economic (Nyaupane, 2015; Mu, 2019) implications for Khumbu Region Nepal. Using data from a new array automatic weather stations (AWSs) installed part 2019 National Geographic Rolex Perpetual Planet Everest Expedition (Matthews 2020a,b) shows that seasonal variations in on Mt. modulates timing optimum conditions mountaineers. However, influence seasonality likelihood visitors' ability view summit Everest's (Nepalese) Base Camp not been assessed. Here, we utilize previously unpublished photos taken twice-daily by an camera at AWS (Figure 1), alongside meteorological data, examine impacts visibility this iconic peak visitors all over world. identified Matthews al. (2020a), investigate both 2020 post-monsoon seasons (1 October 30 November) using hourly measurements five AWSs, varying altitudes, along route 2a): Phortse (3810m), (5315m), II (6464m), South Col (7945m) Balcony (8430m). encompassing two post-monsoonal seasons, spanning 1 August 31 January 2021, time periods when upper slopes are least obscured clouds. In addition numerical recorded Campbell Scientific Canada CCFC Field Camera looking eastward toward was also during 1). As shown Figure 2(a), camera's viewshed left right (north south, respectively) primarily West Ridge (~7000–7200m), (8849m) Nuptse's sub-peak (~7400m). The takes photographs daily 0937 (NPT) (0352 utc) 1437 NPT (0852 utc), respectively. According summer monsoon brings significant changes Himalaya (Khadka 2021), herein, show monsoon's departure equally once more. One noticeable difference absence cloud cover season, suggested incoming shortwave radiation downward longwave measured (Figures 3a,b). mean values three-day running 3a,b), can assess trends across 2020. transition season increase accompanied decrease radiation. support images collected towards summit, which first became available 11 2019, confirm cover. Figures 4 5 each contain 32-day photo arrays depicting morning afternoon cover, Through manual assessment, these clouds enshrouding replaced clearer skies progresses. Of note, coverage tends into compared morning, especially differences between 4(e) 5(e). During post-monsoon, afternoons tend be cloudier than mornings, but diurnal variation less pronounced 4c f; 5c f). More photographic future years needed this. Depending trekkers climbers may want line sight earlier day should their goal see summit. We monitored relative humidity (RH) Phortse, Camp, II, AWSs lower four RH 3(e) (f). change drop 3e f) regional winds shift direction off Bay Bengal Arabian Sea more westerly continental trajectories (Perry 2020). higher located 6464 7945m asl, experience average around 90% 20% course 15 days, similar 80% observed 2020, staying below 50% majority season. While station missing initially close so expect it have up through / transition, after begins diverge slightly. Lower consistent time. Another quantity interest specific calculated 3g,h). stations, largest decreases short period, 8.8 4.0 6.1 2.0gkg−1, respectively, occurring same similarly RH. For years, approaches its minimum end stations. precipitation (the only currently available), plateauing cumulative start 3i,j). lack elevations alone directly indicate near clear section Nepali will benefit catch early glimpse Everest. Compared continuation winter provides much smaller still noteworthy atmospheric conditions. (f) continues reaches period. emissivity 3(c) (d) sees little change, falling 0.68 (±0.11) 0.67 (±0.14) 0.62 (±0.07) 0.61 (±0.09). Specific remains fairly throughout two-month period except small 2021 beginning January. Despite showing precipitation, 3(i) (j). emphasize our limited records cannot rule out potential anomalous big storms limiting occur any month under favourable synoptic circulation. Tropical cyclones impact region evidenced devastating Cyclone Hudhud 2014 Annapurna (Simon Wang 2015). Taken altogether, evidence suggests relatively cloud-free past 2 particularly thermal remaining resulting high identify herein tourism residents region. Tourism increasing rapidly Nepal; 45 970 1970 (Neupane 2012) 197 000 bringing US$714 million revenue (World Bank, 2021a,b), last prior COVID-19 pandemic, significantly decreased global (Weissenbach, 2021). 16.5% declared travel purpose ‘trekking mountaineering’ (MOCTCA, This explains why overall rapid number visiting Park, surrounds southern flanks, 3600 1979 (UNESCO, 35 2013 (Baral 2017) 57 289 almost 43% influx financial resources Region, creates stress local environment (Byers, 2005; Napper attempt pre-monsoon (April/May/June) 32% there determine (October/November) offers markedly better opportunity side mountain wanting – rather climb secondary window. Due burden supporting staff required prospective climbers, pressure mountain's workers could reduced if even who seeking travelled post-monsoon. research conducted partnership Society, Tribhuvan University, approval relevant agencies Government gratefully acknowledge communities Sherpa team efforts aiding setup maintenance work would possible without them. thank reviewers helpful feedback improved manuscript.