Our only thermometers pre-1950 were tubes of mercury.
Two-thirds of the Earth is ocean, and we had almost no air temperatures over the water.
Arctic regions were essentially unmeasured.
For the land, thermometers were few and far between.
National Weather Bureau readings in the United States were sometimes made on the top of multi-story buildings in cities.
There were few weather reporting stations in rural areas.
There were few readings from high mountain elevations.