Beach frost

This is a good time of the year to admire the distribution of frost. When snow covers the ground, any frost on top of it is much more difficult to distinguish.

Frost formed when the surface temperature dropped below 0°C.

Frost forms when water vapour (in the atmosphere) diffuses toward a colder surface and condenses onto it as crystals. In that way it is just like dew, but frost occurs when the surface temperature is low enough for ice to form. Usually, the distribution of either frost or dew is locally uneven: some here, none there. This unevenness results from slight variations in the surface temperature: slightly below 0°C here, slightly above, there.

There are some common situations that result in these small variations in surface temperature: variations in soil (subsurface) conductivity; variations in exposure to sunlight; variations in exposure to the nighttime sky. For now, I will ignore the first two (maybe get some pictures showing them later), and just deal with relative exposure to the nighttime sky.

In this first picture, the frost on the grass avoids the trees, both under and beside them. It is not that the frost fell from the sky, as snow does, but that the tree is warmer than the sky. Everything emits (infrared) radiation: the ground does, the tree does, the sky does—and for that matter, a person does. The amount of radiation being emitted (that is energy being lost) depends upon the object’s temperature: the hotter an object, the greater its radiative output. So, imagine being a bug on the grass well away from those trees. You are both emitting radiation (losing energy), and you are receiving other radiation (gaining energy) from your surroundings—principally from the sky. The sky is much colder than you, so you receive less energy than you lose and your temperature drops below 0°C and frost forms. But, unlike the sky, the tree is roughly the same temperature as the ground. So near the tree, a bug will be receiving nearly as much radiative energy (from the tree) as it loses. The temperature does not drop as much and frost does not form.

The situation on the beach is similar even though the variations in terrain resulting from endless footprints are quite small. Here the frost hugs the ridges and avoids the valleys. Again, imagine being a bug on a ridge. The only radiation you receive to compensate for your own loss comes from the much colder sky, so your temperature drops. If you were a bug in the valleys, a portion of the sky would have been replaced in your view by valley walls—walls that are at about the same temperature as you are and a good deal warmer than the sky. On the ridges, there is a net radiative energy loss, the temperature drops and frost forms; in the valleys, the net radiative loss is smaller and the temperature does not drop low enough for frost. The valleys are just that little bit warmer than the ridges.

While both of the above pictures were taken this morning, I thought it would be fun to add a view of the same thing happening on a summer beach, but this time with dew. Again, the ridges are cold and dew forms; the valleys are warmer and it does not. Of course, another interesting comparison is the fact that the colder ridges, with frost, look lighter, while the colder ridges, with dew, look darker. That is a story for another time.

These are interesting things to watch for during early morning walks.