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Though “hot”, the air is still cooler than your body temperature. When that air hits your body, there’s a heat transfer. Your body gets a bit warmer, and the air gets a bit colder, both moving towards something in between the two temperatures. With a large amount of that air rushing over your body, the effect gets a log stronger. This is basically what the weatherman calls wind-chill.
The human body does not sense temperature. It senses heat flow. When the air is still, your body warms it up and you feel hot. When the air moves over your skin this doesn’t happen and moisture is evaporated, cooling you.
This is how fans work, and yes the air outside your car while driving.
The moving air evaporates moisture on your skin. The evaporation process actually takes some heat from your body with it. As long as the air is not significantly warmer than your body temperature, this evaporative cooling effect will be stronger than the convective hearing from the moving air.
However, I can tell you from experience that if you roll down the window in 130 degree weather and stick your arm outside while moving, it still feels hot. It has enough thermal energy that it far exceeds the evaporative cooling that is taking place and simply feels like a blast furnace.
Forced convection. Essentially, as air moves past your arm, it carries the heat away from your arm. The faster the air goes (ie, driving faster), the faster the heat is carried away.
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1. Sweat. Water absorbs a lot of energy when it goes from liquid to gas, and the wind is a great way to encourage that.
2. Moving air. Air is an insulator. Air that is next to your skin gets warmed up to your body temperature pretty quick. But that heat stays there, it doesn’t really move unless the air itself is moving. A moving car is constantly bringing fresh, cooler, air into contact with your skin.
If the air is close to your body temperature, you’re almost completely reliant on #1 to cool down. If it’s super humid, that sweat isn’t going to evaporate that well and you’re still going to feel hot.
It doesn’t depending on what your definition of “super hot” is. You don’t sense temperature, you sense temperature change. Anything above body temperature, assuming you’re dry will feel warmer. Trying sticking your hand out the window of a car while it’s 120 degrees out, it’s not cooling at all, it feels like a hair dryer.
The hairs on your skin keep a thin layer of air trapped around you. This layer of air is roughly body temperature (98.6 Fahrenheit or 37 Celsius) and quite humid.
Moving air that is either lower temperature than this, or less humid, will replace that thin layer with air that feels cooler than you. You need a certain force of wind to blow that thin layer away.
However, if the moving air is hotter or more humid, then you essentially get the opposite effect, like in a convection oven, where you are pelted with even hotter-feeling air.
The human body is basically an oven and we are constantly trying dump waste heat into the environment around us. Eventually, the air around our bodies gets locally saturated and we can’t dump more heat into it.
Now, when you hang your out the window and drive, you are constantly moving into fresh air you haven’t dumped any body heat into and this makes it feel cool.
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