Sunday, December 11, 2011

Altitude, latitude, snow-line and tree-line

There is hardly ever a conversation with a European friend of mine that doesn't go without my party asking me about the elevation at Park City. I say, “7,000 feet” and they think “Uh, that's very high, must be a really harsh climate where you live...” and then I need to go into the important relationship between latitude and altitude, the snow-line and the tree-line. Most of them hear what I'm trying to say, but most of them don't fully grasp it. This is the reason why I thought it was time to tackle that "existential" subject...

Let's first talk about how that relationship affects the so-called “snow line.”  The climatic snow line is the point above which snow and ice cover the ground throughout the year. Of course, in winter, the actual snow line maybe significantly lower. At or near the equator, it is typically situated at approximately 15,000 feet above sea level. As one moves towards the Tropic of Cancer and Tropic of Capricorn, the parameter at first increases: in the Himalayas the permanent snow line can be as high as 18,700 feet, while on the Tropic of Capricorn no permanent snow exists at all in the Andes because of their extreme aridity: Monte Pissis in Argentina is the highest mountain in the world to be free of glaciers to its summit.
Beyond the Tropics, the snow line becomes progressively lower as the latitude increases, to just below 10,000 feet in the Alps and falling all the way to sea level itself at the ice caps near the poles. In addition, the relative location to the nearest coastline can influence the altitude of the snow line. Areas near a coast, like for instance the Olympic Peninsula in the State of Washington might have a lower snow line than areas of the same altitude and latitude situated in a landmass interior due to more winter snowfall and because the average summer temperature of the surrounding lowlands would be warmer away from the sea.

This applies even in the tropics, since areas far from the sea will have larger diurnal temperature ranges and potentially less moisture, as observed with Kilimanjaro and presently glacier-free Mount Meru. A higher altitude is therefore necessary to lower the temperature further against the surroundings and keep the snow from melting.

The tree-line is also equally affected by this altitude-latitude relationship. Again, the “tree-line” is the edge of the habitat at which trees are capable of growing. Above that, they can't grow because of inappropriate environmental conditions (cold temperatures or lack of moisture). The tree-line generally appears well-defined from a distance, but closer-up, it's more of a gradual transition; trees generally grow shorter towards the inhospitable climate until they simply stop growing.  Just like the snow-line, the tree-line is affected by local variables, like the type of slope, rain shadow and proximity to either geographical pole. In addition, in some tropical or island localities, where species diversity is limited, lower tree-lines can be observed than one would normally expect.
Conversely, Kerguelen Island, Île Saint-Paul, South Georgia, South Orkney, and other Sub-Antarctic islands are all so windy and have such a cold summer climate that none have any indigenous tree species. The Antarctic Peninsula is the northernmost point in Antarctica and also the one that offers the mildest weather. It is only located 670 mi from the Cape Horn, but no trees live on Antarctica.  Averaging over many locations and local micro-climates, the tree-line rises 246 ft when moving 1 degree south from 70 to 50°N, and 430 ft per degree from 50 to 30°N. Between 30°N and 20°S, the treeline is roughly constant, between 11,500 and 13,000 ft.

Now, you can realize that altitude and latitude are really inseparable in gauging what altitude truly represents in terms of vegetation, weather and... most liveable conditions!


Wink Lorch said...

Thank you for this, which I did indeed stumble upon. I'm writing an article on high altitude vineyards and was looking for an easy explanation of the important relationship between altitude and latitude - yours came up, thank you! As a fellow lover of winter sports and mountains, it doesn't surprise me that it was explained by a keen mountain person!

dixit sharma said...

Thanks a lot. Was searching for exact explanation for higher snowline in tropics than at equator.