Frequent subway riders are well-acquainted with the social expectations that come along with using mass transportation. Rather than looking around at their fellow passengers, they look down at the floor or around the train. In doing so, those with a keen eye have undoubtedly spotted the mysterious thermometer sticks that dot the ceiling of subway cars. 

In a city as unpredictable as New York, these decals could serve an array of purposes. It’s actually quite fun to read the theories that try to explain them: riders, for example, have suggested that they’re actually guerrilla art installations or the work of an organization trying to start a movement against global warming. A Reddit user even suggested that they might be buttons. But what do they really mean?

They are not exclusive to one train or one line; in fact, they are found on most subway ceilings and in all train cars that have air conditioning. The HVAC (heating, ventilation and air-conditioning) target-temperature decals, as they’re known as, were installed in 2008, according to MTA spokesman Kevin Ortiz. They’re actually your new age, hot-weather guardians. MTA employees, using a laser, can scan these stickers and figure out the temperature of the subway car. They can then adjust the temperature accordingly, always trying to stay between 58 – 78 degrees Fahrenheit.

The stickers are also placed in the same general spots for each car to ensure that temperature readings from different trains can be held to the same standard. One is placed at the center of the car, and two others are placed at either end, near the heating and air-conditioning systems. This comes in handy when the temperature rises exponentially during the summer; employees can ensure that the cars will be air-conditioned to ease the already painful commuting experience. Now if only they’d invent them for subway platforms!

For more information about NYC subways, learn why metrocards have holes and slanted edges and what exactly subways signals are!