In Season Spotter Questions we ask you about snow. Is there a lot or a little? Is it on the trees or just on the ground? These might seem like strange questions to ask. After all, snow doesn’t tell us that much about phenology. Trees and shrubs can have leaves or not have them when there is snow. Flowers can appear before all the snow has melted in the spring.
The main reason we ask about snow is that it affects our automated processing of image greenness — sometimes in weird ways. You’ll recall that we calculate something called GCC, which is a measure of the greenness of an image. When there are lots of green leaves in an image, we get a high GCC value.
What happens when there aren’t any green leaves in the image? Well, it depends on what the landscape looks like. If the landscape looks yellow or brown because it’s full of dead, dry vegetation it will have a particular GCC value. And if it is covered by white snow, it will have a different GCC value.
Snow is typically white, and so for snowy pictures GCC tends to be about 0.33, because there are equal amounts of red, green, and blue in “white”. But dead vegetation can have a GCC higher than this. Or, it can have a GCC a bit lower. And that’s where the trouble comes in. In some locations, snow on the ground can actually make an image look “greener” than when the vegetation is dead and there is no snow.
As a result, our GCC time series sometimes looks like this for snowy sites:
If we were just looking at the GCC time series, we might mistakenly think that this site has two green periods — one big one in the summer and a smaller one in the winter. Patterns like this can be seen in tropical and subtropical regions where rainfall controls vegetation greenness rather than temperature. But it would be very odd to see two green periods each year in Canada!
Another problematic thing that snow does is that it adds a lot of noise to some GCC time series. In many places in the U.S., it snows and then the snow melts. Then it snows and the snow melts. And so sometimes there’s a lot of snow in the images, sometimes no snow, and sometimes just patches of snow. And this goes on all winter long. As a result, the GCC values jump around a lot.
At this site at Niwot Ridge in Colorado, we can see that there’s a lot more variablity in our GCC time series in the winter than in the summer: