The snowline: it’s not a line (still less an isotherm)
If you burrow in the literature of palaeoclimatology, climatic dynamics and glacial geomorphology, you can find all kinds of snowline: the transient snowline, the annual snowline, the climatic snowline, the regional snowline, the orographic snowline, and for all I know some others. Most of them are misnomers.
It is easy to see how the word “snowline” became popular. If you look at a mountain the line separating snow-covered from snow-free terrain is often the most striking feature of the view. This transient snowline is often straight and, as far as the eye can judge, horizontal. But it doesn’t have to be. It can be messy, because of outlying disconnected patches of snow and patches above the snowline that are free of snow. What is more, it can and does wander up and down over the contours, which leads to uncertainty because we often say “snowline” when we mean “snowline altitude”. On a single mountain or even a single glacier the snowline can range in altitude through hundreds of metres, so we really mean “average altitude of the snowline”.
In glaciology, we have two further complications: we don’t really want to know about the transient snowline but about the annual snowline, and we don’t even want to know that so much as the annual equilibrium line.
The annual snowline is the transient snowline just before the first snowfall of winter, or whenever the mass of the glacier reaches its minimum in the annual cycle. Year upon year, comparing annual snowlines is much safer than comparing transient snowlines from random points in the cycle of the seasons. On the other hand, observing these annual snowlines is much trickier because of the likelihood that you won’t get there in time or won’t see anything if you do (because of the weather).
The annual snowline sometimes separates the glacier into an upper part that has gained mass over the year from a lower part that has lost mass, which makes it the same as the annual equilibrium line. But not always. If meltwater from the surface refreezes when it reaches the contact of the snow with the underlying glacier ice, we distinguish it as “superimposed ice”, which represents mass gained by the glacier during the current mass-balance year. This is important for accurate book-keeping, but it also means that the snowline altitude can be tens of metres or more above the equilibrium-line altitude, with exposed superimposed ice in between.
In climatology, especially at regional and broader scales, these complications are usually set aside. This is where the word “misnomer” comes into its own, because except when they plot the snowline altitude on a graph against latitude the climatologists’ snowline (regional, climatic, orographic, whatever — let’s agree to call it the climatic snowline) is not a line at all. It is a two-dimensional surface. But we all know what the climatologists mean, and misuse of words is sometimes curiously unimportant as a barrier to the advancement of understanding.
The climatic snowline is a generalization, but a very valuable generalization, summarizing the state of the atmosphere near the Earth’s surface in a very distinctive way.
Look at the climatic snowline in the graph. Set aside the complications and inaccuracies of usage, and ignore for a moment the colour scheme and the fact that the “line” is discontinuous — pretty fat at some latitudes, missing altogether at others. What we see is the altitude at which, if there were some land, there would probably be glaciers, or glacier equilibrium lines to be precise.
One curious thing about the climatic snowline is that it is nearly always assumed to be an isotherm, often the one representing a mean annual temperature of 0° C. The colour scheme shows that this is only a good assumption if you are willing to do a lot of ignoring. For example at latitudes between 55° N and 65° N the mean temperature of the snowline during the warmest month can be as high as +8° C (the lowest snowlines) or as low as —4° C (the highest snowlines).
The graph has been lying around on my hard drive for two decades. I only dug it out because I wanted to put Snezhnika, at 44.8° N and an altitude of 2450 m, into context. You can find small but stable glaciers like Snezhnika in mountain ranges where the climatic snowline is well above the highest peaks, but they are not a reliable reflection of the big picture. There is a lot more to be said about the big picture, but it will have to wait for another occasion.
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