This dataset contains the longest continuous time series of monthly gridded Northern Hemisphere sea-level pressure data in the DSS archive. The 5-degree latitude/longitude grids, computed from the daily grids in ds010.0, begin in 1899 and cover the Northern Hemisphere from 15N to the North Pole. The dataset continues to be updated regularly as new data become available.

Each monthly grid is a simple average of all available daily grids for the month. Prior to 1955, there is one grid per day. From July 1962 on, there are two grids each day. In the interim period, the number of daily grids varies between one and two.

The grids for the period 1899-1977 were inspected and many corrections were made by Kevin Trenberth of the Laboratory of Atmospheric Research at the University of Illinois at Champaign-Urbana, and these grids are included in this dataset. For more information about these corrections, see the July 1980 issue of Monthly Weather Review.