How do you take an entire country’s temperature?

July was the hottest month on record in the United States, according to the National Oceanic and Atmospheric Administration, the agency tasked with taking the nation’s temperature. How do you calculate an average temperature for an entire country over the course of a month?

With 1,218 thermometers. NOAA uses two different kinds of weather stations to take the nation’s temperature. Modern stations wirelessly relay weather data without human intervention, other than occasional checks to make sure bears haven’t attacked the thermometers and birds haven’t nested in the rain gauges. NOAA also continues to rely on old-fashioned, manual stations scattered around the country. These are just ventilated boxes that block out the sunlight while allowing the air through. They contain two thermometers. An alcohol thermometer measures the daily minimum temperature. A bar located inside the liquid is sucked down as the temperature drops and remains there even when the temperature rises again. A mercury thermometer measures the daily maximum. It has a bubble in the liquid that causes the mercury to stick at the highest temperature recorded during the day. A person visits the station once a day to record the daily high and low, then resets the thermometers. When NOAA announces the average temperature for a month, it’s an average of the daily highs and lows from those 1,218 automated and manual weather stations, subject to a few statistical corrections.

To compare data from year to year, NOAA has to account for changes in its equipment and the locations of its weather stations. For example, many of the nation’s thermometers used to be located on rooftops in urban and suburban environments. In the 1950s and 1960s, weather monitors decided to move the stations to less populated areas to minimize the effects of pavement and human activity on the readings. Airports were ideal locations because the areas around most airports were sparsely populated in the mid-20^th century. (Weather measurements are also, of course, important for air-traffic control.) While the move improved accuracy, it created an inconsistency in the data. Today, NOAA corrects for this by subtracting a precisely calculated number of degrees from the old, urban temperature data.

In some cases, cities have grown up around their thermometers. As any New Yorker, Washingtonian, or Los Angeleno knows, airports aren’t quite as bucolic as they were 60 years ago, and NOAA has to subtract a calculated amount from modern readings to account for the urban heat island around some stations. Slight adjustments are also made for poor technique in the past. Some older thermometers were placed on the tops of buildings with black roofs, which absorbed the sun and caused the thermometer to record an artificially high temperature.

The United States has the best-monitored climate in the world, by far. NOAA relies on 7,280 thermometers to measure temperature worldwide, and 2,300 of those are in the United States. (More U.S.-based thermometers are in the global data set than in the national average data set because NOAA’s selection criteria for the national average are more stringent.) That means nearly 32 percent of the official land-based thermometers are located in the United States, even though the United States represents just over 6 percent of global land mass. Only 4,400 of the global thermometers have more than 25 years of historical data, and a disproportionate share of those are in the United States. NOAA uses a series of computational strategies to fill in the international gaps and blends land and marine data (collected by sensors on ships and buoys, among other places) to calculate global temperature changes.