Daily and Monthly Lat-Lon Sea Level Pressure Grids (Northern Hemisphere, Jan 1899 to Current)

General Description of Daily 5-degree Lat-lon SLP Grids:

NCAR has daily sea level pressure grids from 1899 to current years from various sources as explained below. All grids are 5-degree lat-lon grids from 15N - 85N and 00E - 355E. There is also a pole value for later grids. Where possible, grid point values missing in the original grids have been interpolated. Interpolation was done along latitude bands, but not over a large number of missing points. General Description of Monthly Mean Grids:

Monthly mean grids in the same format have been computed for the entire period. Daily grids for all time periods available were used to compute the monthly means; a count of these daily grids is carried along with the mean grid. The mean grid for December 1944 is not available. The mean grids for 1945 were supplied by Scripps Institution of Oceanography. Format of Grids:

Word 1, counting bits from left to right
bits 1-6	format number ( = 10)
bits 7-17	year, e.g. 1899
bits 18-21	month, e.g. 11 = NOV
bits 22-26	day ( = 0 for monthly mean)
bits 27-31	hour ( = 31 for monthly means when more than one time period; or when time unknown as for the year 1945)
bits 32-41	level ( = 1013 for sea level)
bits 42-50	type ( = 6 for sea level pressure)
bits 51-56	source 3, for Navy data 4, for ESSPO data 10, for NOAA data 11, for MIT data 12, for U.K. data 28, for NMC data when used 29, for data reworked by Trenberth²
bits 57-60	grid type ( = 1)
Word 2
bits 1-15	pole value¹
bits 16-21	number of grids in mean ( = 0 for dailies)
bits 22-60	not used
Words 3-272
1080 binary integer data values in 15-bit bytes¹. Data values are to nearest tenth millibar and are the rightmost 14 bits of each byte. The grid of Y(I,J) is dimensioned Y(72,15). As I goes from 1 to 72, longitude goes from 00E-355E; as J goes from 1 to 15, latitude goes from 15N-85N.
Word 273
60-bit checksum of first 272 words

¹The leftmost bit of each packed 15-bit data value is used to distinguish between grid points of original data and grid point values which have been interpolated or are missing. If this bit is 0, the data value is an original value; if this bit is 1 the data value has been derived (non-zero) or is missing (zero). This applies to the pole value also. This bit is 0 (except for 15N) on grids from 1946 on because the data were interpolated from grids with a high degree of resolution. For monthly means a zero grid value indicates there were fewer than 10 daily values and the mean is considered missing.

²Dr. Kevin Trenberth, NCAR, made error and discontinuity corrections to the monthly means grids for the period, 1899-1977. These mean grids are the preferred grids, although the means grids computed from the dailies without the Trenberth corrections are also available.

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