Taking Earths Temperature
On July 4, 1776, Thomas Jefferson awoke to a cool Philadelphia summer morning: it was 20°C (68°F) at 6 am. In fact, we know the daily temperature where Jefferson was staying for almost every day of his adult life, thanks to his meticulous weather journals. Logs like these offer a glimpse of weather conditions in the recent past.
But historic weather records are spotty and cannot tell us much about global climate changes before the 1800s. Over the past 150 years, however, measurements of surface temperature have been reliably collected in most parts of the world, allowing scientists to determine the average global temperature for any one year.
Average Global Temperature, Today
Thousands of sophisticated weather stations situated around the globe now take temperature readings every day, year-round. Scientists analyze and average these readings, working first to minimize errors by identifying biased stations and adjusting for incomplete records, giving a snapshot of the planet's temperature. Currently, Earth's average surface temperature is about 15°C (59°F).
Weather journals offer a glimpse into the past. A weather journal in the exhibition gives lively accounts of monthly weather conditions in New York City exactly 200 years ago.
Meticulous weather logs, like one in the exhibition from a former observatory on the Lower East Side of New York, allow scientists to compare today's weather conditions—temperature and rainfall, for instance—to the past.
One thermometer in the exhibition—more than 200 years old—has been carefully restored and still works; the glass tube was once filled with (toxic) mercury but has been replaced with red alcohol. The dial on the bottom is a barometer, a device that measures air pressure to judge current weather conditions.