NOAA is the National Oceanic
and Atmospheric Administration
NOAA is part of the US Commerce Department.
NOAA has a National Centers
for Environmental Information (NCEI).
NCEI publishes a temperature database
for over 7,000 worldwide "weather stations".
The NCEI database is called the
Global Historical Climatology
Network (GHCN).
The GHCN database consists of six files,
three (maximum, minimum, and average)
for ‘Unadjusted’ temperature values,
and three for ‘Adjusted’ temperature values.
The average Adjusted value
is most often used.
Unadjusted is ‘as received’ data
from agencies / centers
from around the world.
You might assume "Unadjusted" data
are ‘as measured’ raw data.
Not true !
NOAA-NCEI advises
on its GHCN web page:
“…it is entirely possible
that the source of these data
(generally called National
Meteorological Services)
may have made adjustments
to these data
prior to their inclusion
within the GHCN.”
NOAA-NCEI then makes
its own adjustments,
primarily lowering temperatures
of land stations in the earlier part
of the temperature record,
which causes some
"global warming".
In summary,
the adjusted data,
are often adjusted
more than once,
before being used
to determine the
average global temperature.
GHCN Database Problems
(1)
NOAA-NCEI’s adjustments, alone,
cause some "global warming"
not in the Unadjusted data.
And the "Unadjusted data"
received by NOAA-NCEI
may include adjustments,
made in other nations,
that also cause some
"global warming".
(2)
Weather stations that are included in the
global average temperature compilation
are not consistent over the long term.
Some weather stations
have reported temperatures
for 10 years or less.
Others have reported temperatures
in all, or nearly all, of the years
since 1900.
So, the weather stations that are
actually reporting temperature data,
are a constantly changing mix.
This important data quality factor
is ignored in NOAA’s compilation
of the global average temperature.