If you're playing hockey or mixing a drink, you might dream of perfectly clear ice. But if you're flying a plane, clear ice is less like a dream, and more like a potential nightmare. So how does that perfectly clear frozen water show up on your airplane?

When, And Where, Does Clear Ice Happen?

Clear ice usually forms in relatively warm temperatures, between 2 C and -10 C. And in addition to those warm temps, there's another key ingredient: lots of water. That water usually comes in the form of large water droplets, freezing drizzle, and freezing rain.

When you're flying in conditions with the right temp and the right amount of moisture, the possibility of picking up clear ice is pretty high. So what does it look like? Well, it's usually pretty clear. Imagine that.

You can see from the picture above, that clear ice can be pretty hard to see. That's one reason why it's so dangerous. But it's also dangerous for another reason, and that's this:

And this:

When you fly through supercooled water droplets (water droplets below 0C, but still in liquid form), they impact the surface of your plane, and then flow back, freezing into ice along the way. That's bad because as the drops freeze, they form shapes that change the overall shape of your airfoil. The worst is the formation of "horns", which are two icy protrusions that significantly disrupt the airflow over your wings and tail.

What's Heavy, Hard, and Forms Everywhere?

Yep, we're still talking about clear ice. And that's what adds the danger.

First, it's heavy. Because there aren't typically a lot of large pockets of air in clear ice, unlike rime or mixed ice, it's pretty much solid frozen water. And since water weighs in at a whopping 8.3 pounds per gallon, it doesn't take long for your aircraft's weight to dramatically increase.

Next up, clear ice is hard. Which means for aircraft with de-icing systems, like inflatable boots, it can be difficult for the ice protection system to completely crack and remove all of the ice.

Finally, and possibly the biggest danger, is that clear ice can form everywhere. Well, not literally everywhere, but the point is, as water droplets are freezing into clear ice on your airframe, they can flow past the reach of your ice protection system on your wings and tail. And if you get clear ice forming beyond the reach of your de-ice boots, that's a very bad thing.

Steering Clear Of The Ice You Can't See

Steering clear of areas where the temps are warm, and the moisture levels are high, is the best method for staying out of harm's way when it comes to clear ice.

But if you do find yourself in clear ice, descending, climbing, or diverting quickly is the best course of action to keep your airplane from accumulating ice in places you'd rather not see.