ICOADS Web information page (Tuesday, 11-Feb-2014 11:53:18 MST):

Release 2.0 Delayed-mode (ICOADS.DM) Archive Overview (1784-1997)


Background

The underlying Release 2.0 data were from three intermediate releases, described on the following webpages:
Release 1a (1980-97)
Release 1b (1950-79)
Release 1c (1784-1949)
Collectively, these were referred to (starting in 2002) as Release 2.0.

Figures 1a through 4d below illustrate the enhancements in coverage gained in Release 2.0, in comparison to the original COADS Release 1 data. These show total numbers of observations per decade for 2° latitude x 2° longitude boxes for the indicated variables:
Fig. 1a | Fig. 1b | Fig. 1c | Fig. 1d: sea surface temperature (SST)
Fig. 2a | Fig. 2b | Fig. 2c | Fig. 2d: wind speed (WSPD)
Fig. 3a | Fig. 3b | Fig. 3c | Fig. 3d: sea level pressure (SLP)
Fig. 4a | Fig. 4b | Fig. 4c | Fig. 4d: relative humidity (RHUM)
Figure letters indicate the decadal coverage:
1800-09,...,1840-49 (a figures)
1850-59,...,1890-99 (b figures)
1900-09,...,1940-49 (c figures)
1950-59,...,1990-97 (d figures)
The Release 1 data (left-hand column on each figure) are from the "trimmed" version, and the Release 2.0 data (right-hand column) are from the "enhanced" version. Decades prior to 1854 and after 1979 are blank for Release 1 because it only covered 1854-1979, whereas statistics for Release 2.0 covered 1800-1997.

A wide range of colors was used to show different levels of detail during earlier and later time periods. Colors were assigned to ranges of total observations per decade, each equivalent to an average maximum number of observations per year or month (note that all observations within a given decade could fall within one month):
Color Decadal range Average maximum
blank 0 0
grey 1-10 1 per year
navy 11-120 1 per month
blue 121-600 5 per month
cyan 601-1200 10 per month
green 1201-1800 15 per month
chartreuse 1801-2400 20 per month
yellow 2401-3000 25 per month
orange 3001-3600 30 per month
red 3601 or more >30 per month
For the abbreviated decades 1854-59 and 1990-97, the decadal ranges were adjusted so that the colors would have the same meaning under the maximum column.

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Updated: Feb 11, 2014 11:53:18 MST
http://icoads.noaa.gov/r2.html