#132 - Special Conditions Needed for Rime to Form

Rime time! . . .
I took these photos on the evening of January 22, 2024 at the outdoor basketball courts near Hays-Lodgepole High School. The high school is in Hays on the Ft. Belknap Reservation. That night the air in Hays met all the requirements needed to form an unusual kind of frost called "rime" (aka rime ice). The white, feathery ice crystals form when supercooled water droplets in fog freeze instantly upon contact with a surface that is also colder than the freezing point. There has to be a breeze too - The crystals build up in the direction of the wind is coming from, creating beautiful but heavy accumulations on objects like trees, wires, buildings . . . or basketball nets!

Recipe for Rime:

1. The microscopic fog droplets must be supercooled - colder than the freezing point, but still liquid. Not all fog is made of droplets that are supercooled.

2. There has to be a breeze pushing the air toward a surface.

3. The surface that the rime builds up on must also be below freezing. The supercooled droplets freeze on contact with cold surfaces, growing as the breeze pushes more droplets toward the surface.

Below: Rime crystals point into the wind on this chain-link fence.

Dew Point . . .
Normal frost and dew form when the air temperature drops below the dew point. Dew points vary depending on the amount of humidity (water vapor) in the air. If there is a lot of vapor in the air, the dew point will be higher. Dew points in Montana are typically much lower than places like Florida. So for example, if the dew point is 37 degrees and the temperature drops to 35 degrees that night, dew will form as water condenses on surfaces. However, if the dew point is 29 degrees and the temperature drops to 27 that night, it will be frost that forms (because the dew point was below the freezing temperature). Frost forms as vapor changes directly to solid - called deposition. When the Sun comes up the next morning the temperature may rise above the dew point, evaporating the dew or changing the frost back to vapor (called sublimation).

Term: sublimation, supercooled

Below: Here is the freezing fog that was responsible for the rime. Students call these outdoor courts "Four Courts" or "4C" for short.