by Gadgetgeek » Fri 11 Jan, 2019 1:14 pm
-20, also known as "slightly chilly' (you know I had to) can start to look different. Frost forms slower and larger at warmer temps, so any frost or snow is going to be more fine grained, although its still "snowflakes" as most people would conceptualize them. Colder, around -30 any snow is much more like sand since there is little water left in the air. Out in the prairie where I grew up there is very little humidity at -20, where as somewhere close to the great lakes will still have massive amounts of humidity a the thermal mass of the lake dumps water into the air. Ice fog, or haze looks different than warm fog, but that could be due to when it occurs. We think of fog as being an early morning or evening thing, where as ice fog can be in the middle of the day. Imagine being inside a photographers white-box. Ice fog can feel very unnatural because you have no depth if its light, something that fog normally has unless its really thick. If there is just a slight haze, in the heat if the sky starts to look dirty, or murky, in the cold it will look watery, pale, you may even see the refraction rings from the sun's light.
Frost in particular will change over time, so if you had a drastic temp shift and the temp went from just over 0 to -20 over a couple days, everything would have thick fuzzy frost, but as time went on that would be slowly sublimated away by the sun, so after some time it would have dissipated, presuming that a wind didn't disturb it. New snow has more of a velvet look, and as it ages it gets more of a gloss, or can become more matte, like sand, depending on how the wind interacts with it, and the day/night temp shifts.
At -20 with no wind there is also not the same sort of convection, so smoke will sit and hang without dissipating as fast. In a forest you'll see it just start to haze around the branches as it doesn't really escape, in an open yard from a chimney it corms more of a column, and as it gets even colder, it might start to settle down towards the ground after cooling. Its also possible to see the thermal layers where the smoke might get trapped in a yard ringed by trees. I've heard stories of people who could tell the temp by looking at smoke coming out of the chimney, but like all tall tales, the thermometer might have been out that window. In general thought air stays cleaner as dust gets stuck to the snow.
Also for reference, -20 for someone who's grown up in it is really not that bad. It was well within the realm of working outside all day without exceptional planning or equipment, about like working outside at +28, obviously you can't be too dumb, but its also not that big of a deal for those conditioned to it. The frostbite limit is around 30 minutes, but we are talking, sitting still, no way to keep warming, and only exposed skin. Activity level matters, not hard to start sweating at -20.
Sound really changes when it gets cold. Sound travels a lot farther as the air gets more dense, but because you don't have the noise of wildlife it gets quieter as well. Some sounds will travel farther than others, its possible to hear say the blade of a saw mill and not the engine or even the wood being cut, just the musical note resonating from the blade. Because you hear the high frequency sounds more things sound "sharper" where as heat and humidity can make things duller and more muffled. This get more extreme the colder it gets. Snow starts to creak, and you can hear much more noise from trees. I've heard that in the right conditions sounds can be heard due to the sound bouncing back down off a thermal layer in the atmosphere, but I don't know I've experienced that. The fact that most of the trees will not have leaves, also changes how things sound.