These data sets contain gridded temperature anomalies for three parameters (mean, 
minimum, and maximum temperatures) from the GHCN V2 monthly temperature data sets. 
GHCN homogeneity adjusted data was the primary source for developing the gridded
fields.  In grid boxes without homogeneity adjusted data, GHCN raw data was used to provide
additional coverage when possible.  Each month of data consists of 2592 gridded data points
produced on a 5 X 5 degree basis for the entire globe (72 longitude X 36 latitude grid boxes).
   
Gridded data for every month from January 1880 to the most recent month is available.
The data are temperature anomalies in degrees Celcius.  Each gridded value was multiplied by 100 and
written to file as an integer.  Missing values are represented by the value -9999.
   
The data are formatted by year, month, latitude and longitude.  There are twelve longitude grid
values per line, so there are 6 lines (72/12 = 6) for each of the 36 latitude bands.  Longitude
values are written from 180 W to 180 E, and latitude values from 90 N to 90 S.  Data for each
month is preceded by a label containing the month and year of the gridded data.
   
     for year = begyr to endyr
      for month = 1 to 12
        format(2i5) month,year
        for ylat = 1 to 36 (85-90N,80-85N,...,80-85S,85-90S)
          format(12i5) 180-175W,175-170W,...,130-125W,125-120W
          format(12i5) 120-115W,175-170W,...,70-65W,65-60W
          format(12i5) 60-55W,55-50W,...,10-5W,5-0W
          format(12i5) 0-5E,5-10E,...,50-55E,55-60E
          format(12i5) 60-65E,65-70E,...,110-115E,115-120E
          format(12i5) 120-125E,125-130E,...,170-175E,175-180E
   
Each file has been compressed using 'gzip'.  They can be uncompressed with 'WinZip' for those
using Windows 95 or with 'gzip' from most UNIX platforms.  The FORTRAN utility program
'read_gridded.f' can be downloaded to assist in extracting data of interest.  This program allows
the user to extract non-missing values for selected months and write the data to an ascii output
file.  The latitude and longitude of the center of each corresponding grid box accompanies each
gridded value in the output file.
   
We calculated these anomalies with respect to the period 1961 - 1990 using the First Difference
Method, an approach developed to maximize the use of available station records (see, e.g.,
Peterson et al., 1998, 'The First Difference Method: Maximizing Station Density for the
Calculation of Long-term Global Temperature Change', Journal of Geophysical Research).  The
First Difference Method involves calculating a series of calender-month differences in
temperature between successive years of station data (FDyr = Tyr - Tyr-1).  For example, when
creating a station's first difference series for mean February temperature, we subtract the
station's February 1880 temperature from the station's Februrary 1881 temperature to create a
February 1881 first difference value.  First difference values for subsequent years are calculated
in the same fashion by subtracting the station's preceding year temperature for all available years
of station data.
   
For each year and month we sum the 'first difference' value of all stations located within the
appropriate 5 X 5 degree box and divide by the total number of stations in the box to get an
unweighted first difference value for each grid box.  We then calculate a cumulative sum of these
gridded first difference values for all years from 1880 to 1998 to produce a time series for each grid box.
The cumulative sum is calculated for each grid box and each month of gridded first difference
data independently through time.  Each grid box time series is then adjusted to create anomalies with respect to the period 1961 - 1990.

We developed this gridded data set to produce the most accurate time series possible.
However, this required that we treat months and grid boxes independently through time.  The use
of this data is most appropriate for analyzing the change in temperature within a particular grid
box, or set of grid boxes, over a span of years.  If one is more interested in analyzing temperature
changes within individual years, e.g., the change in temperature between February and March,
1908, or between two regions in 1908, we recommend that the GHCN station data be used
directly.

To access anonymous FTP at NCDC use the following information: 

Machine Address : ftp.ncdc.noaa.gov
Login Name      : anonymous 
Password        : your email address
Directory       : /pub/data/ghcn/v2/
Enter           : bin
Enter           : get 'filename'