Physics and Fun Frigid Facts

Why did I see what looked like steam or smoke over the water today?  Why do people throw boiling water in the air when it’s this cold?  Why do I feel a static shock when I pet the cat/dog or touch anything metal?

The answer to all of these questions is related to one simple fact…cold air "holds" less water.  The atmosphere doesn’t have hands and it isn’t a bucket, so it doesn’t really "hold" any water, but the analogy works.  The colder the air is, the drier it is — the warmer it is, the wetter it is. 

First, a quick lesson in humidity.  The temperature tells you how warm it is, the dew point tells you how humid it is.  The dew points early Sunday morning were around -25 degrees.  To put that in perspective, +25 would be more typical for winter, and dew points in the 60s would be common in the summer.  So, the air this morning was not only bitterly cold, it was desert dry.  And that is typical, because as we have already learned, cold air doesn’t contain much moisture.

So, let’s answer the first question.  Any body of water — a lake, the ocean, or even a pool — will add a little moisture to the air immediately above it.  That is especially true if the water is warmer than the air.  As the relatively "warm," moist air just above the water started to cool, it could no longer "hold" the extra moisture.  That moisture then condenses from  invisible water vapor to more easily visible water droplets, forming what looks like fog or steam. 

Second question.  You may have seen videos of people, including some of my colleagues, throwing a pot of boiling water into the air, and amazingly, the water turns to ice before hitting the ground.  Water near the boiling temperature is, by definition, already close to evaporating.  The process of evaporation happens more quickly if the air is dry.  Wind makes it go even quicker.  So by throwing a pot of boiling water into the dry, windy air, you cause that water to evaporate almost instantly.  The process of evaporation causes rapid cooling.  Side note – that’s why you feel cold when you step outside of the pool or ocean, especially on a windy day.  Back to the pot…as the boiling water evaporates almost instantly, believe it or not, it actually cools far more quickly than room temperature water would.  While most of the water that was in that pot evaporates into the air, the small water droplets that remain freeze almost instantly, usually before hitting the ground, or at least upon hitting the ground.

Finally, what’s up with that static?  If you take the cold, dry air from outside and warm it up to 65 degrees, the relative humidity drops to a mere 2%.  Remember, the warmer the air, the more water it will hold.  So, air that was not far from being saturated at -9 degrees becomes a bone dry sponge when you warm it to 65 degrees.  That means the air inside your house will soak up moisture from wherever it can get it.  Your skin becomes dry and itchy, your scalp gets flaky, your nasal passages become irritated (and more likely to get infected), and the water in the dog’s bowl seems to disappear.  The dry air also makes it easier for static electricity to form.  It gets to the point where almost everything you touch gives you a shock.  All because the air is cold and dry!