Q: Can wind chill make things “feel” colder than absolute zero?

The original question was: Is it possible for something to “feel” colder than absolute zero?  If the forecast called for 1K (1 Kelvin) with 20mph gusts of wind, would the wind chill be below 0K?

Physicist: This is a beautiful question!

“Wind chill” is a very ad hoc, artificial measure used to describe the fact that cold air pulls heat out of us faster when it’s moving.

When air is stagnant and you’re standing still, you’ll be surrounded by a bubble of warm air (heated by body heat).  This bubble insulates us from the surrounding air temperature.  So, if it’s 40°F out, and there’s no wind, you’ll actually be losing heat as though it were warmer, because near your body it is warmer.  In fact, this is the whole point of clothes (beyond the whole modesty thing): keeping a layer of air near the body.  The insulation provided by the cloth is generally dwarfed by the insulation provided by the air it holds in.

So, when you walk outside and say “it feels like it’s about 70°F out” what you really mean is “the bubble of air around me feels like it’s about 85°F, and my experience tells me that corresponds to an ambient temperature of 70°F”.  But you’d need to be kind of a jerk to go through all that.  Instead you just say “feels like 70°F” (even though what you’re experiencing is actually warmer).

When the wind is blowing however, that layer of warm air is pushed away and we’re exposed to the actual air temperature.  What we call the wind chill temperature is the temperature it would have to be in order for you to lose body heat at the same rate, were the air to be sitting still.

Being not just squishy but warm as well, we heat up the air immediately around our bodies. This air insulates us from direct exposure to the surrounding cold air. When there's wind that bubble gets thinner and with less insulation we lose heat faster. This picture is not to scale, and is a poor representation to boot.

Wind chill is pretty subjective (people of different sizes and shapes will experience wind chill differently), and not terribly exact, but the standard gives you a decent ball park estimate.

If you were to stick your hand into liquid helium at 1K, you would again get a layer of slightly warmer liquid helium around your hand (I’m using liquid helium here because it stays liquid essentially all the way to 0).  If you were to stick your hand in flowing liquid helium, that warmer layer would be washed away and your hand would be exposed to actual 1K helium.

As strange as it sounds, it is perfectly fair to say that, with “wind chill”, the temperature is below 0K, because that’s how cold stagnant helium would have to be to match the heat loss caused by 1K helium that’s moving.

There’s nothing wrong with this statement.  Heat loss is governed by the difference in temperature between a body and its environment.  Regardless of whether or not one of the temperatures involved is below zero.