Fresh Snow, and some Rime.
14th March 2025
Overnight there had been a few centimetres of fresh snow above an altitude of about 500 metres. At mid mountain this gave a uniform covering of soft snow. On the summits there had been a bit of drifting. However, the quantities of the fresh snow were small, and in most places it was settling onto bare ground which mean it did not constitute much of a hazard. It was cloudy and cold on the top during the morning, but cleared up nicely during the afternoon.
A uniform covering of a few centimetres of fresh snow at an altitude of about 650 metres this morning.
At about 1050 metres, the fresh snow sitting on bare ground. This was pretty typical of the current situation, the old hard and icy snowpack is very patchy in distribution.
The view down Easy Gully. The wind was blowing up the gully causing the rocks to rime up. There was little in the way of fresh snow in the gully. The summits were in the cloud during the morning, before clearing during the afternoon.
Rounded feathers of rime on the hut at the top of the ski area today.
Delicate fingers of rime on a post at the top of the ski area. Most rime is a bit more substantial than this, such as the thicker layers on the right hand side of the image. The shapes and solidness of rime will depend on a number of factors such as the wind speed, temperature and how thick the cloud is.
When the temperature is below freezing clouds will typically consist of super cooled water droplets. When these droplets get blown onto an object they will freeze. How long they will take to freeze will depend on the temperature. A good few degrees below zero, and this freezing will be almost instant. This traps air and gives the rime a very white opaque look. As a general rule the closer to freezing, the longer the supercooled water takes to freeze, and the more the air gaps that it fills up with super cooled water before it freezes, and thus the icier the rim. At temperatures just below freezing I have seen a uniform layer of clear water ice forming on objects through the riming processes.
A lovely picture of a slightly rimed up snowflake where you can see really see the individual water droplet which have frozen on impact can be seen at https://www.sciencephoto.com/media/162016/view/rimed-snowflake
Supercooled water is a metastable state of water where it remains liquid even when cooled below its freezing point. The absence of nucleation sites (small particles or impurities that act as starting points for ice crystal formation) is the primary reason why water can become supercooled. Supercooled water droplets are common in clouds, which can remain liquid even at temperatures well below freezing.
Hoar on the other hand forms via a different process. Hoar forms through water vapour going straight to a solid (ice) form without passing through the liquid phase. This is the opposite process to sublimation where a solid goes straight to a gas. The atomic shape of water molecules combined with the physics of deposition, hoar crystals tend to be more delicate than rime crystals.
NOT TODAY! A photo from a cold period earlier in the winter of delicate surface hoar crystals which formed due to the deposition of water vapour onto a cold surface. A different process which produced a different crystal than the rime that was observed on Aonach Mor today!
Comments on this post
Got something to say? Leave a comment
