Thirty-two degrees. A number, a measurement that should be universal. Yet, thirty-two degrees in Milan on a July day and thirty-two degrees in the mountains, on the same day, are two experiences that seem to belong to parallel universes. In the city, you feel like you’re swimming in a warm, sticky liquid. In the mountains, it’s a pleasant summer day.
How is this possible? The answer lies entirely in the perception of heat, or the heat index as it’s called in English. It’s not the temperature itself that matters, but everything surrounding it: humidity, altitude, wind, and even our mental state. Understanding this difference isn’t just about satisfying scientific curiosity; it’s a fundamental key to taking care of ourselves and facing the summer intelligently.
Humidity, That Unpleasant Companion
Our body is an incredibly efficient machine: when the temperature rises, we activate our internal cooling system: sweat. This is a simple and effective mechanism that works by heat exchange. Water evaporates from the skin, drawing heat away and lowering body temperature. The catch is that for it to work properly, it needs a favorable environment.
When the air is already saturated with water vapor—hey there, Po Valley!—our sweat has a terribly hard time evaporating. It just sits there, on the skin, sticky and uncomfortable. The result is that our perception of heat increases dramatically. The thermometer reads 32 degrees, but our body perceives it as 36, 39, sometimes even more.
Relative humidity is the percentage of water vapor present in the air compared to how much it can hold at the same temperature. The more humid the air, the less sweat evaporates, and the less the body cools down. It’s a vicious cycle that makes us feel like we’re running in a sauna.
A Matter of Altitude and Wind
In the mountains, the air is drier, so our sweat evaporates more easily, cooling us efficiently. But that’s not all. There’s also the altitude: the higher you go, the less dense the air is and the less heat it retains. Lower atmospheric pressure, on the other hand, affects how the body perceives temperature and dissipates heat.
Wind is also our best ally. A light breeze can make otherwise unbearable temperatures tolerable, sweeping away that layer of warm, humid air that forms around the body. It’s the difference between suffocating in the city’s oppressive heat and enjoying a sunny day at higher altitudes.
The Numbers of Perception
Here’s how perceived temperature changes with varying humidity, keeping the actual temperature constant at 32°C (90°F):
- 30% humidity: 32°C (90°F) perceived
- 50% humidity: 36°C (97°F) perceived
- 70% humidity: 43°C (109°F) perceived
- 90% humidity: 48°C (118°F) perceived
As you can see, every 10% increase in humidity raises the perceived temperature by 2-3 degrees. Once you read the perceived values beyond a certain threshold, it will be clear that above 70% humidity, you’re in a zone of potential danger, especially if you’re exerting yourself physically.
The Mental Factor
There’s also a less measurable but fundamental element: the psychological context. If you’re on vacation, in the shade of a tree, with a book in hand and the sounds of nature in the background, even 33 degrees (91°F) can seem acceptable. If, however, you’re stuck on public transport in traffic or walking on a sunny sidewalk, even 28°C (82°F) can feel like torture.
In short, your brain processes the perception of heat by combining physical and psychological signals. Expectation, environment, our state of mind: everything contributes to how you interpret that sensation of heat on your skin.
What to Do with This Awareness?
Knowing that 32 degrees isn’t always the same helps us make smarter choices. When to run, how to dress, how much to hydrate, when to stop. And perhaps even where to plan our activities, knowing that, with climate change, places that were once livable despite being hot are now becoming tropicalized (this is also why, those who can, are moving to live at higher altitudes).
The body is a perfect machine, but it has its limits. It reacts, it adapts, but you must listen to it. Recognizing that not all degrees are equal is the first step to not underestimating the heat and facing it with the right preparation.
Next time you feel like melting in the city while your friend in the mountains posts photos in short sleeves at the same temperature, don’t get annoyed with them. It’s just physics, humidity, and that complex machine that is our body. And remember: hydrate, always seek shade, and a little wind if you can find it. These are the only tricks that truly work, or at least they are the ones that don’t involve using energy or an air conditioner (which you can’t carry with you while running anyway).