The Sound of Science - "Hoar Frost"

Jan 11, 2019

J: I’m Jeremy Benson from NIU STEM Outreach and I’m in the studio with Kate Powers, this is the Sound of Science on WNIJ.

K: Jeremy, I hear you have a question today from Maria about a spectacular weather phenomenon. 

J: That’s right, Maria wants to know why sometimes on a cold morning you wake to find everything outside covered in glittery frost and sometimes you don’t.

NIU STEM Outreach

K: That is a great question and one that I used to wonder as well. The glittery frost that Maria is talking about is called a hoar frost.

J: What kind of frost?

K: Hoar, H-O-A-R. It is from the old English word for age, the frost makes the world look like it has white hair! 

J: So people have been talking about this frost for a long time?

K: That’s right. It seems so magical so it makes sense why people have been wondering about it for centuries, but it’s not magic, it’s science. 

J: Mostly we just see regular frost or sometimes things get covered in ice, but that’s not the same as the hoar frost, right?

K: Right. Those types of ice occur from more common conditions. Hoar frosts occur on nights when the ground is colder than the freezing point of water and the air is just below freezing but still has a high relative humidity above 90%. The water in the air is in a gaseous state, and when it touches a cold ground surface (like a leaf or branch) it goes straight from a gas to a solid, not pausing in the liquid state. This process is called deposition.

J: Yeah I know that. One of my favorite science words describes the opposite process, from a solid straight to a gas, sublimation.

K: Yeah, that’s a fun one! Well, what makes deposition different from freezing is that it happens a bit more slowly and in a more ordered fashion. With the slow deposition process the water molecules have a chance to order themselves into beautiful crystals. The water molecules slowly crystallize on the surface of an object, forming delicate structures that give hoar frost that magical quality. 

J: Wow, that takes specific conditions to create a hoar frost. They really are a cool wintery surprise.

K: I recommend looking up some pictures of hoar frost, there are some spectacular images out there! And some people think watching ice freeze is boring.

J: Keep those questions coming to stemoutreach@niu.edu. This has been the Sound of Science on WNIJ. Where you learn something new every day.