Daily composites
Every selected day contributes equally. Date ranges include both endpoints. Duplicate dates in a group are removed. A comparison is the mean of Group A minus the mean of Group B, even when their lengths differ.
Anomalies & reference period
Each date is matched to its calendar-day 1991–2020 climatology from the same dataset. The composite anomaly is the mean field minus the matching mean climatology. NOAA's 365-day climatology uses the average of 28 February and 1 March for our 29 February reference. This explicit interpolation has not been verified as NOAA's own plotting convention.
Precipitation
Total precipitation includes rain and the water equivalent of snowfall. Means and anomalies use mm/day. Percentage of normal is 100 × composite mean / matching climatological mean. Reference values below 0.1 mm/day are masked; missing cells stay missing. This is a ratio of means, not a mean of daily percentages.
Temperature & zonal wind
Temperature means are in °C; anomaly differences have the same numerical size as kelvin differences. Positive zonal wind is eastward (westerly flow), negative is westward (easterly flow). m/s, mph and knots convert the same signed values.
Coverage and rendering
NCEP/NCAR Reanalysis 1 covers 1948–17 March 2026 for the supported fields. Upper-air and SLP grids are 2.5°. Surface temperature, wind and precipitation use their native Gaussian coordinates. Display shading is interpolated; it does not add physical detail. Pressure-level values below terrain may be model extrapolations.
The globe, polar and flat views use exactly the same composite grid. Missing source data are never replaced with zero. A calculation requires complete requested days for each displayed cell. This service supports recent and historical reanalysis, not a live forecast. Recent ERA5T data are provisional and may later be revised.
ERA5 preparation
Recent pressure fields use four samples at 00, 06, 12 and 18 UTC; surface means use all 24 hours. Geopotential is converted to height using standard gravity (9.80665 metres per second squared). ECMWF ARCO supplies available pressure levels and surface fields; Google ARCO supplies the additional 70, 30, 20 and 10 hPa levels, with the same four UTC sampling times. Coverage is shown separately for each field. The 0.25-degree source is sampled at 2.5-degree grid points. Precipitation totals sum hourly accumulations ending from 01 UTC to the following 00 UTC, covering the full selected UTC day. Only complete dates are published. The permanent ERA5 reference is computed from all 30 years, 1991–2020, using the same UTC samples, grid and units as the daily data. Each calendar day is averaged with equal year weighting and no temporal smoothing. A cell requires all samples in all 30 years. Anomalies are enabled as matching references become ready. For 29 February, the reference is the average of 28 February and 1 March. The recent archive retains a rolling 12 months; the reference is retained permanently.
Storage precision
ERA5 grids are packed to 16-bit values with a separate missing-data code. Each file records its measured maximum packing error. The exported composite grid reports the combined error bound in your chosen units for means and differences; precipitation percentages do not have a fixed absolute error bound. Packing does not change the selected dates or the averaging method.
Sources & attribution
NCEP/NCAR data provided by NOAA PSL, Boulder, Colorado, USA. ERA5 data: Copernicus Climate Change Service (C3S), ECMWF; CC BY attribution applies to distributed ERA5 products. Coastlines: Natural Earth, public domain. D3: ISC licence. This independent platform is not affiliated with or endorsed by NOAA or ECMWF.
NCEP/NCAR documentation ↗ · ERA5 documentation ↗