I've moved a lot, and a city never feels like home until I can anticipate how the day's weather will feel. I don't go on instinct alone. My sense of the weather comes by combining data from the morning's report—temperature, along with humidity, wind speed, and precipitation—with accumulated experience.

Most weather reports include a metric that supposedly tells you the same thing. AccuWeather's proprietary formula is called RealFeel, and other stations use a similar metric called Feels Like. Whatever it's called, this number purports to tell you how the air outside will feel against your skin. But can the AccuWeather, the Weather Channel, or even Al Roker know you like that? What is the methodology behind this meteorologic metric?

Measuring how humans experience temperature isn't new. For instance, we all instinctively know that a stiff wind will make the air feel cooler. However, nobody had assigned a number to this feeling until the mid-1940s, when two Antarctic explorers observed that the wind actually caused water to freeze at higher than normal temperatures. Using plastic vials of water in a variety of wind and temperature combinations, they eventually published a table where you could get perceived temperature by cross referencing one reading from a thermometer and another from an anenometer. This ended up being too simplistic however, and eventually had to be amended with data that accounted for human factors like body type, clothing, and activity.

A few decades later, other researchers tried to measure perceived temperature in the other direction. Like wind and chill, humidity had a long anecdotal correlation with heat. In 1978, a researcher named George Winterling developed the heat index in an attempt to codify this correlation. Air always has some amount of water vapor. The percentage of water vapor in a given volume of air is called its humidity. The cooler air gets, the less water vapor it can hold. When the temperature drops far enough—called the dew point—the air particles become too densely packed to store any more vapor, so it starts to condense on solid surfaces. The higher the dew point, the more humid the air. Humans regulate their temperature by sweating, and we have a hard time doing so if the air is already sticky with moisture.

Wind chill and the heat index showed meteorologists two things: First, it was possible to quantify the way people experience weather. Second, they both proved that you needed more than weather data to do so. Combining the two is one of the easiest ways to determine perceived temperature, and this is basically how Feels Like temperatures are calculated. But external conditions aren't the only thing that determines how people feel temperature.

Thanks perhaps to the invention of the thermostat, the way we experience temperature is well studied. The American Society of Heating, Refrigerating, and Air Conditioning Engineers maintains the standard for thermal comfort, drawn from a broad canon of peer-reviewed research. Some of it comes from physics: The human body burns a certain amount of energy a day, which is dissipated according to the surface area of their body. This can be measured through metabolism. But this doesn't show what kind of temperature a person prefers.

Climate control research uses individuals' physical discomfort as a feedback mechanism to generate its standards for ideal temperatures. But even indoors, temperature feels different to different people. Right now, I'm slightly shivering in a hoody zipped to my chin while an editor sitting directly behind me is working comfortably in a short sleeve shirt. Obviously, things get way more complicated outside.

One of the best known weather metrics is AccuWeather's RealFeel. It uses not only wind and humidity, but also a bunch of other external data points like time of year and type of ground cover, says Mike Steinberg, a meteorologist with AccuWeather who helped develop RealFeel. But, he says, he and his colleagues built their model around the human experience, starting with the physics of heat transfer between the human body and the atmosphere. A person's metabolism determines how well they regulate temperature, and along with clothing will have the biggest influence on how they experience the weather.

"What we did was assume an average person in terms of body size and physical capability, and we also assumed they were dressed appropriately for the weather," he said.

To calibrate their model to what people actually feel, Steinberg and his colleagues ran RealFeel through a gauntlet of user surveys, and adjusted the algorithms accordingly. But even with physics, observed data, and surveys, Steinberg admits some estimation went into RealFeel. Other AccuWeather meteorologists, he says, felt like certain geographic areas and conditions altered perceived temperature in ways that couldn't be measured through data collection. Based on these meteorologists' expertise, Steinberg says he and his co-authors adjusted things locally.

To date, there have been over 100 different systems of measurement developed to measure how people experience temperature. Each has a niche where it performs better than the others, but none are perfect. Some are crude, and useful only for quick estimates, and others are more sophisticated, using multiple variables to generate a measurement for subjective experience.

Perceived temperature might not ever nail your experience, but the point is more to give a general impression of how the weather will feel to a group of people living in geographic proximity. And sometimes the only way you can measure the difference between an 80 degree day in Central Park versus the same temperature in Golden Gate Park can be measured by the spare clothes you bring to each: a dry shirt for New York, and a hooded sweatshirt for San Francisco.

Homepage image: Jeffrey Zeldman/Flickr