The first surface measurements in Europe were in the 1850s.
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The US has had some sensors since 1880.
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There are about 6,000 land surface measurement stations used by the World Meteorological Organization (WMO).
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The US, Europe (excluding France, Spain and Norway), and eastern China are well covered.
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There are few sensors in Greenland, northern Canada, central Africa, Australia, on mountains, in deserts, in forests and on any land area where humans don't go (humans live on less than 2% of earth's surface).
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The US Commerce Department's NOAA (National Oceanic and Atmospheric Administration) says it uses only 1,500 of the 6,000 available weather stations. Their explanation for ignoring 75% of them makes no sense to me.
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Weather buoys at sea are the most commonly used method of measuring temperature.
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NOAA says there are almost 1,300 buoys in the world‘s oceans.
The Gulf of Mexico and US west coast have many sensors.
The Pacific Ocean has uneven coverage.
There is too little coverage of the Atlantic Ocean, Indian Ocean, and around the Poles.
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In summary surface measurement instruments are not even close to being evenly distributed around the world.
The UK and the US are well covered.
The Indian Ocean, Australia, the North Atlantic, Canada and North Pole have few sensors, on land or at sea.
Earth has a surface area of about 500 million square kilometers.
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Accurate surface analysis would require five million sensors -- 1,786 times more than the 2,800 actually being used.
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Surface measurements should have been discarded in 1979 when weather satellites were launched.
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But "global warmunists" love surface measurements, because their bureaucrats can "fill in the blanks" (areas with no sensors) with whatever numbers they want.
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And "infill", they do -- wild guessing is used for a majority of the temperature data required for Global Climate Models.
The pro-warming bias in the "infilled" data is one reason why surface measurements show more warming than satellite data.
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Even worse, past surface measurements have been repeatedly "adjusted", almost always in a way that increases global warming.
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The hot 1930s, for example, have been "cooled" by repeated "adjustments".
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Cooling the past makes current temperatures seem warmer, in comparison.
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These "adjustments" account for up to half the claimed global warming since 1880.
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But never forget:
Even after all the pro-global warming "adjustments" and "infilling", the claimed warming since 1880 is only about one and one-half degrees F. = totally harmless !