Today, I noticed an interesting variant on a familiar pattern while walking along the beach: snow distribution sculpted by infrared radiation. An unexpected dusting of snow overnight had set the stage for the patterns.
Most people spend little time thinking about infrared radiation (IR), yet it is all around us. Everything emits in the IR by an amount (almost entirely) determined by its temperature. Rocks emit, trees emit, clouds emit, people emit. The warmer the object, the more energy is emitted.
Similarly, everything absorbs IR radiation from the things in its line of sight. So, an object loses energy based on its temperature, but gains based on what it receives from its surroundings. Whether an object’s temperature rises or falls depends on whether it gains more than it loses, or vice versa.
Consider the ground at night (so, we are not dealing with sunlight). If the ground temperature is, say 10C and the (effective) sky temperature is -20C, the ground will emit more IR radiation than it absorbs from the sky and so it will will cool: the ground temperature will drop.
That is assuming the ground has a clear (hemispheric) view of the sky, either because the surface is flat, or we are on the top of a ridge. But, what if we are dealing with the ground in a valley? The valley bottom will see only a portion of the colder sky, but also a portion of the warmer valley walls. It will end up warmer than the ridges owing to it also receiving from the warmer valley walls.
The first two pictures were posted three-and-a-half years ago as beach frost. They serve to set the stage for the last picture which was taken today.
That overnight, the footprint ridges have become cooler than the valleys is clear from the distribution of dew.
The same is true of frost: it forms preferentially on the colder ridges. Of course, unlike dew which darkens the sand, the frost makes the ridges appear lighter.
In the previous two pictures, the variation of temperature from ridge to valley determined where condensation took place. In the picture, below, it determines where melting takes place. A dusting of snow had covered the whole beach, but it melted first in the warmer valleys caused by footprints.
Hi Alistair
Thanks for this. I haven’t seen this phenomenon explained or illustrated nearly so well before.
Lichens (what I study) respond to microclimate at fairly high resolution, so lichen populations sometimes form distributional patterns not unlike the one you show here. Other patterns too of course.
Congratulations on ten years of excellence!
Trevor
Trevor, you have given me something new to watch for.
congrats on those 10 years.
i specially like all that scientific stuff.
your site is such a breath of fresh air among all that crap of singing dogs and burping babies.
peter bartl
Peter, I also show dogs and babies, but, my dogs follow the Sun (1, 2), and my babies walk logs (1, 2).