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Originally Posted by conachair
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If you read the article san he self serving excuses and flimsy conclusions it does a good job in explaining the unreliability of source temperature data collected around the world. it doesn't deal with the mess of ocean temperature data collection. That is another amazing discussion in and of itself.
From the linked article:
Some weather station changes are pretty straight-forward. The desire for weather information at new airports around the 1940s led to station moves. Some of these stations had been set up on the roofs of post office buildings and later found themselves in an open
environment on the edge of town. Looking at the temperature records, you might see a sudden and consistent drop in temperatures by a couple of degrees.
Equipment has changed, too, like the
installation of a housing to shield thermometers from sunshine or the switch from
mercury thermometers to electronic ones. By running them side-by-side, scientists have learned that the two types of thermometers
record slightly different low and high temperatures. And because the electronic thermometers necessitated running electricity to the station, some of the stations were moved closer to buildings at the same time.
And while the impact isn’t immediately obvious, changing the time of day that the weather station data is recorded is actually a big deal. Most weather stations didn’t automatically log measurements, especially in the days of
mercury thermometers. Instead, special thermometers were designed to mark the minimum and maximum temperatures that were reached. When someone checked the station to note those measurements, they reset the markers.
Imagine that you reset the thermometer at 4:00pm on a hot summer afternoon. The maximum marker is going to immediately return to its previous temperature. Even if the following day is considerably cooler, you will return to see the same high temperature on the thermometer—yesterday’s warmth is accidentally going to be double-counted. The same goes for minimum temperatures recorded in the morning.
As far as long-term trends are concerned, luckily this doesn’t really matter, provided you always check the station at the same time of day. But if you switch from a routine of evening measurements to morning measurements, for example, you’ll suddenly be less likely to double-count high temperatures, but much more likely to double-count low temperatures. This is known as “time of observation bias.”
In most of the world, the effect of all these non-climatic factors is neutral. Changes that raised temperatures have been balanced by changes that lowered them. The US, however, is different. Here, weather stations are run by volunteers who report to the National Weather
Service. Compared to other countries, the US has more stations but less uniformity among those stations.
At times, guidelines for the volunteers in the US have changed, with new
equipment or procedures gradually spreading through the
network of stations. Around 1960, the guidelines changed from late afternoon observations to morning observations. That kicked in over time (many stations didn’t change until a new volunteer took over) (and there’s a substantial
cooling bias over that time period as a result.)
(says who - where is the evidence of this claim) In the 1980s, the National Weather
Service asked volunteers to switch to electronic thermometers, adding another
cooling bias.
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