Why does air feel colder when it’s moving fast?
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Because it’s basically becomes a wind. And wind – is more air touching and taking away your heat during same time interval. Hot air on the other hand would feel hotter for this same reason – you’ll experience more air and heat exchange during same time interval.
Slightly more complex than ELI5 addition: if you’d move faster and faster, air would feel hotter and hotter. Because temperature, to put it simply, is average speed of air molecules. Cold air means that most molecules moves relatively slow, hot air – fast. But there’s no difference if it’s air molecules or you moving fast – in both cases it’d feel hot. But for most speeds which you could achieve on the ground, you won’t feel any additional heat, because air molecules at normal temperature are moving very, very fast – about 500m/s on average (meaning that some molecules do move at maybe 10m/s, and some – at few km/s). Relative to this, even 100km/h is neglectable.
Oh neat, more wind chill.
So assuming the air is cooler than a person, it will take heat away. More air, more heat removal.
There’s an imbalance in heat between person and air. The universe tends to favor balances.
Something giving off heat in a non-moving fluid will first warm the fluid around them. It’s why you can feel someone’s warmth without actually touching them.
Moving air constantly refreshes the air. Instead of a gradient of body temp-warm air-less warm air-cool air, there will always be body temp-cool air.
There is also evaporative cooling. Humans sweat to cool off. Air can only hold so much water at a certain temperature. Constantly replacing the air around a person constantly adds new air to take more liquid and therefore more energy away.
My favourite place to start is to to visualise that there’s no such thing as cold… just an absence of heat.
Heat is essentially measured as how jiggly the molecules in your skin are.
Much like blowing on a hot cup of tea, the wind facilitates the transfer of jiggly heat energy from the tea, into the air.
When there is a near-constant flow of new molecules to transfer the energy to, your skin loses heat more quickly…
In a room with no wind, your skin would heat up the surrounding air and this would serve as somewhat of a temporary shield from further heat loss.
Because you’re wearing an invisible jacket of warm moisture. Air blowing across you removes this warm jacket.
What your body senses is not the temperature, per se, but the rate at which your skin is losing (or gaining) heat. This is called “heat transfer”, and it’s what’s really important. Temperature is a big part of it, but wind and humidity have a lot to do with it too. Still air at (say) 40F will pull a certain amount of heat out of you, but the same temperature air blowing across your skin at 25 mph will pull a *lot* more heat.
Here’s another example: imagine a wooden board at 70 degrees, and a stone countertop at 70 degrees. The stone will feel much colder to the touch than the wood because stone is a much better conductor of heat, and pulls the heat out of you much more efficiently than the wood.
Air is a pretty good insulator, so when it’s not moving, it basically acts like a blanket that keeps body heat in. When it’s moving, you got more air molecules moving over and touching your skin, which allows heat to be removed faster. This is also why it feels colder in a windy winter day versus one without much wind, the moving air of the wind allows more heat to be removed from your body for a given time.
Because more particles are hitting you. Each one that does takes a tiny bit of your heat (assuming it’s colder than your skin). So the more hitting you, the more heat lost and the colder it feels.
It’s the opposite in a sauna. Blow up a breeze with a towel and it’ll feel much hotter!
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