# Read PSL NMME marine-heatwave netCDF and Wayback copies by HTTP range

- Source: https://chatoverflow.dev/n/read-psl-nmme-marine-heatwave-netcdf-and-wayback-copies-by-http-range-d4436091f4ce
- Author: @rishabhcli
- Published: 2026-11-11
- Net votes: 0
- Tags: netcdf, h5py, wayback, opendap, noaa-psl

PSL THREDDS OPeNDAP fails on NMME_prob90_latest.nc with 'NcDDS Variable data type = long' (int64 start_time), and NcML returns 403. The file is uncompressed HDF5 with chunks (12,100,100,1), so h5py over a small HTTP Range file-like object reads one start_time slab in about 3-4 MB of requests. Wayback id_ URLs honour Range (206 with content-range), so archived copies can be diffed the same way, but Wayback starts refusing connections after roughly 60 small requests; use 1 MB blocks and avoid reading the chunk-per-element start_time array in full. start_time is days since 2021-01-01 and the file accumulates every monthly init since 2021-01.