=== versions === python 3.12.0 | pystac-client 0.9.1 | planetary-computer 1.0.0 | rasterio 1.5.2 | numpy 2.5.4 | pandas 3.0.6 | matplotlib 3.11.2 === study area === bbox (-3.9, 56.98, -3.35, 57.2) -> EPSG:32630 1679 x 1241 pixels of 20 m (833 km2) DEM: 202-1307 m, median 632 m; 16% of the area is above 900 m === Sentinel-2 search === 149 Sentinel-2 L2A scenes of tile 30VVJ, 2025-10-01/2026-06-30 === Sentinel-2 scenes and snow-covered area === scenes with any data: 149; with >= 15% clear land (bands read): 65; with >= 50% clear (used): 21 sun elevation at overpass: 8.9 deg (December) to 55.7 deg (June) date sun_elev clear_frac sca snow_km2 scl_snow_agree ndsi_snow_scl_cloud 2025-10-13 11:33 24.574 0.751 0.000 0.000 1.000 0.000 2025-10-15 11:33 23.841 1.000 0.000 0.000 1.000 NaN 2025-11-22 11:34 12.327 0.679 0.871 493.158 0.750 0.223 2025-11-25 11:44 11.828 0.973 0.571 463.207 0.807 0.013 2025-11-30 11:43 10.931 0.771 0.697 447.895 0.823 0.159 2025-12-02 11:34 10.491 0.555 0.219 101.279 0.826 0.545 2025-12-24 11:35 8.931 0.811 0.264 178.180 0.741 0.005 2025-12-30 11:44 9.336 0.862 0.366 262.854 0.719 0.013 2026-02-13 11:43 18.911 0.655 0.649 354.329 0.891 0.357 2026-02-28 11:43 24.326 0.776 0.220 142.490 0.851 0.249 2026-03-20 11:43 32.224 0.764 0.081 51.438 0.915 0.348 2026-04-14 11:43 41.914 0.778 0.113 73.410 0.941 0.442 2026-04-23 11:33 44.804 0.999 0.040 33.504 0.950 0.000 2026-04-24 11:43 45.396 1.000 0.039 32.496 0.952 0.000 2026-04-26 11:33 45.781 0.640 0.034 17.897 0.959 0.181 2026-04-26 11:44 46.056 0.725 0.027 16.599 0.966 0.190 2026-04-29 11:43 47.001 1.000 0.028 23.227 0.961 0.000 2026-05-26 11:33 53.304 0.514 0.003 1.479 0.994 0.416 2026-05-26 11:44 53.605 0.627 0.004 2.148 0.993 0.284 2026-06-15 11:33 55.300 0.842 0.000 0.306 0.999 0.295 2026-06-15 11:44 55.642 0.794 0.000 0.186 0.999 0.399 === showcase scene 2025-11-25 === S2C_MSIL2A_20251125T114431_R123_T30VVJ_20251125T150420: clear 97.3%, snow-covered area 57.1% (463.2 km2), sun elevation 11.8 deg NDSI of clear pixels: 5th/50th/95th percentile -0.48 / 0.65 / 0.92 snow-covered area vs NDSI threshold: 0.10: 61.4%, 0.15: 60.8%, 0.20: 60.2%, 0.25: 59.6%, 0.30: 58.8%, 0.35: 58.0%, 0.40: 57.1%, 0.45: 56.0%, 0.50: 54.8%, 0.55: 53.2%, 0.60: 51.2%, 0.65: 48.3%, 0.70: 43.8% pixels with NDSI > 0.4 removed by the NIR > 0.11 / green > 0.10 tests: 86,914 (7.0%), of which in water (SCL 6): 10,220 === snow/cloud confusion, 2025-11-15 === SCL class of the pixels the NDSI test calls snow (%): cloud medium 36.5 dark area 18.9 snow 18.6 thin cirrus 16.0 cloud high 3.6 water 3.2 bare 2.0 unclassified 1.1 NDSI of SCL cloud (medium/high) pixels: median 0.13; 13.3% exceed 0.4 B11 (SWIR) reflectance: SCL snow median 0.071, SCL cloud median 0.300 across all scenes read: share of NDSI-snow pixels that the SCL labels cloud or shadow: median 62.3%, max 99.9% agreement of our snow mask with SCL class 11 on clear pixels (scenes used): median 95.0%, min 71.9% === MODIS MOD10A1 / MYD10A1 daily snow cover === 271 Terra and 273 Aqua days on a 75 x 54 grid of 463 m pixels days with >= 70% clear (Terra or Aqua): 45 of 273; Terra alone would give 32 monthly mean snow-covered area on clear days (%): day 2025-10 2025-11 2025-12 2026-01 2026-02 2026-03 2026-04 2026-05 2026-06 mean 2.0 80.4 38.8 63.2 47.6 27.6 8.4 0.0 0.0 count 7.0 3.0 8.0 1.0 2.0 5.0 10.0 5.0 4.0 same-day Sentinel-2 and MODIS (both >= 50%/70% clear): 15 days; SCA difference MODIS - S2: mean +1.0 points, r = 0.99 === snow cover by elevation and the snowline === date sca snowline snowline_N snowline_S 2025-10-13 0.000 NaN NaN NaN 2025-10-15 0.000 NaN NaN NaN 2025-11-22 0.871 389.0 351.0 420.0 2025-11-25 0.571 588.0 568.0 596.0 2025-11-30 0.697 475.0 460.0 459.0 2025-12-02 0.219 759.0 655.0 1029.0 2025-12-24 0.264 1079.0 785.0 1184.0 2025-12-30 0.366 821.0 578.0 943.0 2026-02-13 0.649 490.0 462.0 NaN 2026-02-28 0.220 832.0 776.0 853.0 2026-03-20 0.081 1230.0 903.0 1229.0 2026-04-14 0.113 874.0 802.0 930.0 2026-04-23 0.040 1261.0 1066.0 NaN 2026-04-24 0.039 1273.0 1067.0 NaN 2026-04-26 0.034 NaN 1099.0 NaN 2026-04-26 0.027 NaN 1187.0 NaN 2026-04-29 0.028 NaN 1165.0 NaN 2026-05-26 0.003 NaN NaN NaN 2026-05-26 0.004 NaN NaN NaN 2026-06-15 0.000 NaN NaN NaN 2026-06-15 0.000 NaN NaN NaN south-facing minus north-facing snowline: median 128 m over 9 scenes (range -1 to 400 m) MODIS snowline (100 m bands) on 19 clear days; monthly median (m): day 2025-10 2025-11 2025-12 2026-01 2026-02 2026-03 2026-04 2026-05 2026-06 snowline 921.0 398.0 991.0 449.0 637.0 834.0 876.0 NaN NaN wrote output/sentinel2_scenes.csv, modis_daily_sca.csv, snowline_sentinel2.csv and snow_20251125.tif