Two Latitude Questions
I did a lot of reading to answer my two questions and unfortunately did keep track of my sources. Good thing I don’t work for a newspaper.
September 2 - Sitting forty miles south of the Arctic Circle looking out at stunted trees we realized we had questions.
1) What exactly is the "Arctic Circle"? Or maybe the question is, How did they decide where to put the line for the arctic circle?
2) How does latitude and elevation relate to tree-line?
The Arctic Circle is the lowest latitude where the center of the sun doesn’t go below the horizon on the longest day of the year, “midnight sun,” and where the center of the sun never rises above the horizon on the shortest day of the year, “polar night.”
Because the elevation of the terrain varies, the sun can go behind a hill from one vantage point but stay visible in a valley. I was curious how this variation was accounted for. What I found was the tides cause the earth to wobble on its axis making the exact latitude of the arctic circle vary around the planet. The latitude of the arctic circle is also moving north at approximately 48 feet per year due to cyclic shifting of the earth on its axis, Milankovitch Cycle. The cycle causes the tilt to vary between 22.1 to 24.5 degrees from magnetic north over a 20,500 year period. We are currently at 23.5 degrees. That didn’t exactly answer my question of how elevation effects where people say the circle is. Maybe they assume average sea level at a given location.
When we were in Iceland at the end of November 2018 it felt like twilight lasted all day. That is because the sun doesn’t rise very far above or set much below the horizon but it might rise and set at, for example 10:20 am and 4:04 pm.
Sunset over geysers around 4 pm in Iceland on November 22, 2018
The answer to the second question is even more complicated. For simplicity, I am only going to address the northern hemisphere. We know that as you move away from the equator tree-line gets lower. We thought there might be a formula. We found a graph and a formula!
Around the equator, up to 30N, the average elevation of tree-line is 11,500 to 13,500 feet. As you move north, the elevation lowers about 430 feet for every degree up to 50. Farther north, up to 70 degrees, the tree-line falls 245 feet for every degree north of the equator. Above 70N there aren't many trees.
Of course tree-line is never a straight line because of localized conditions. The most important factor seems to be the length of the growing season. If the soil is too cold, tree growth will be stunted. Then there are contributing factors. Mountains with poor soils will farther limit tree growth. Other locations might retain varying levels of moisture due to wind, position relative to the sun, or snow pack.
Each tree species has its own maximum elevation
The “formula” above seems to only account for temperature. With every 1,000 ft gain in elevation, the temperature is said to decrease by 3-5 degrees F depending on the moisture content of the air. But that rule of thumb is localized. Consider the angle of the sun through the atmosphere at the equator and at the arctic circle. At the equator, plants will receive more sunlight and heat than at the arctic circle. Therefore the formula takes the reduction in sunlight and temperature into account with angle of the sun to the earth’s surface.
The two sides of this valley have different tree lines.
Yes, it’s complicated.
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