In this season, snow is seen that falls on the mountainside. Yet, all the white on distant trees need not be snow — sometimes what is seen is rime.
When snow falls from a cloud above the mountain, it spreads from the mountain top down to the melting level where it gradually diminishes. It thus presents a white mountaintop with a somewhat indistinct base.
What is one to make of a isolated white line across the mountainside? This is a result of rime from a shallow cloud.
Normally a cloud is filled with liquid droplets even when the temperature is well below 0 °C: the droplets are supercooled. They remain liquid at sub-zero temperatures. But, when the drops collide with objects, such as trees, they instantly freeze to produce rime on those trees.
A thin cloud of super-cooled water droplets had rested against a mountainside. As the droplets collided with the trees, they froze to produce a band of rime across the slope.
As the cloud extended along the whole mountain range, so did the band of rime.
And now there came both mist and snow,
And it grew wondrous cold.
…ummm..so..there is `rime and reason..
It’s that season!
What allows the water droplets to stay in liquid form until the collision and to what temperature can it remain liquid
Actually, the question is: What allows water with a temperature of less than 0 °C to freeze? One learns in school that water (necessarily) freezes when T < 0 °C, but this is false --- or stated more generously, it is only true for bulk water with impurities in it that can provide a site upon which crystallization can begin. Tiny quantities of pure water can remain liquid down to a temperature of about -48 °C. Indeed, much of the water in clouds at an altitude above the so-called freezing level in the atmosphere (the 0 °C isotherm) is actually liquid and only freezes when it encounters a freezing nucleus (dust, soot, organic matter, even bacteria). This supercooled state of cloud droplets is important to the growth of precipitation in our latitudes. It gives rise to not only rime on trees immersed in a supercooled cloud, but also the icing of aircraft flying through the cloud and the freezing rain falling from it.
Thanks Alistair, that’s an interesting response to the question, and pretty photos. I never knew rime had a name.