There’s a site with population density maps here:
Someone pointed me at it some time ago and I’ve lost track of who and where, so if it was you, take a bow.
They require a login to get the maps. I generally studiously avoid doing anything that leaves identity information laying around or gives me Yet Another Account to manage or Yet Another Password to track or (HORRORS!) more email… but I did find one image that wasn’t locked down that is good enough for my purposes From the “do you want to download a detailed map of this” image:
I’d been wondering why there was a stripe of thermometers just under the Sahara. Looks like that’s where there’s a stripe of people.
There are exceptions. The USA has thermometers all over even out in the big empty. Australia HAD a bunch of thermometers (the blue) but now have only a few in GHCN (the red) China, too, had a lot, mostly where there are the people, but now a lot fewer. India has a lot of people, but not a lot of thermometers. Still, the general trend is that places empty of people have poor coverage.
It is my opinion that this fact, by the very nature of it, causes a fundamental bias in the temperature data. We’ve increased population a lot since 1850 and all of them burning a LOT of gasoline, kerosene, Diesel, coal along with electrical appliances. You can’t dump a few TW of waste heat into the places people live and have them not be a bit warmer than the big empty places. I would be very surprised if there were not a 1/2 C to a few C just from that in the unadjusted data. (Then we’ve seen evidence that the UHI Urban Heat Island adjustment that is done is not right and sometimes backwards).
But at least I know why there is a line of thermometers under the Sahara, and why Canada had so many on the US Border during the baseline period (as that’s where all the Canadians go to get as close to warm as possible ;-)
Does make for an interesting question though. How do you get historical data to compare to current data from places without many people?