Table 2. U.S. Weather Bureau Instructions to Marine Meteorological Observers.

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                       EXAMPLES OF CHANGES IN CODES AND OBSERVING PRACTICES
                                Wind      Wind              Sea        Mercury
EDITION       YEAR   Clouds   Direction  Speed   Weather    Temp.    Barometer
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Instructions  Late  Proportion Mean    Beaufort  Beaufort  Bucket  No correct-
attached to   1800s of clear   magnetic   force   weather   temp.      ions as
forms               sky 10ths  direction             code                read* 

First         1906  Amount of  True wind    "         "       "           " 
                    clouds in  direction
                    10ths      32-point

Second        1908      "         "         "         "       "           "

Third         1910      "         "         "         "       "           "

Fourth        1925      "         "         "         "       "    No correct-
                                                                       ions as 
                                                                        read**

Fifth         1929      "         "         "         "       "           " 

Sixth         1938      "         "         "  00-99 pres.   Bucket/      "
                                               weather code  injection 

Seventh       1941      "         "         "         "       "           "

Provisional   1949   Eighths   36-point   Knots       "       "    Corrected to
                                                                      sea level
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* Indications are that this was probably the practice, however, it is not
clearly evident from reading the instructions.
** Corrections to be added for radio transmissions.

NOTE: Updates after 1941 include Circular M (editions 8-12) in 1950, 1954,
1959, 1963, 1964, and NWSOH No. 1 editions in 1971, 1982, and 1992.  A new
update will be required for the November 2, 1994 approved WMO code changes.
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