Surface air temperature
Surface air temperature, as measured by land-based thermometers, tells us how cold or hot it is outside. Weather forecasters have records of temperature measurements going back a century and a half, and show both cooling and warming trends. Cities seem warm up much more rapidly than rural or remote areas. Researchers call this the "urban heat island effect": the larger the city, the larger the increase. Counties in California with more than 1,000,000 people have warmed up an average of 3.1 F over the last 100 years, which is four times as much as the 0.7 F increase for counties under 100,000 in population.
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