Planetarium

"The North Pole has sunlight 24 hours a day now and will until early autumn. But, it is only 5-6 degrees Celsius presently. If it has sunlight all the time, why isn't the Pole warmer?"


- L.P., Manchester  

Greetings!

This question reminds me that sometimes we make horrid mistakes when we attempt to explain phenomena. For instance, we will often tell people that the summer is warmer than the winter because the Sun is higher in the sky and therefore is above the horizon for a longer period. This statement implies that the duration of daylight is directly linked to warmth. However, the fact that the North Pole remains chilly despite the Sun's 24 hour a day presence above the horizon contradicts this notion.   

The North Pole isn't warm because the Sun isn't high in the sky. Its angle above the horizon is about 23.5 degrees on the solstice. As we approach autumn like helpless orphans on a runaway train careening inexorably toward a 4000 foot precipice, the Sun's North Pole altitude will gradually decrease. Throughout the summer, a North Pole observer would see the Sun gradually decline toward the horizon.      

Since the Sun at the North Pole never attains an altitude greater than 23.5 degrees,* its incoming radiation will be largely absorbed by the atmosphere. Atmospheric absorption is a key factor relating the Sun's altitude and the weather for a given location. Realize that the amount of atmosphere between an observer and outer space decreases with increasing angle. The amount of atmosphere between you and the horizon is about forty times greater than the amount between you and the zenith (point directly above you.) It is for this reason that the Tropics (known, in technical geographical parlance, as "paradise") is quite warm throughout the year. The Sun generally ascends to a high angle in that region and its heat passes through a lesser amount of the atmosphere.

Let's trudge through two examples:

Portland, ME, with an latitude of about 43 degrees. (It isn't exactly, but it's Monday and I want to make the math easy.)

The Tropic of Cancer with a latitude of 23.5 degrees.

On the summer solstice, the noon Sun attains an angle of 70.5 degrees in Portland, its maximum possible angle.

On the summer solstice, the noon Sun attains an angle of 90 degrees at the Tropic of Cancer, the maximum possible angle of the Sun at that location or any other.

On the winter solstice, the noon Sun attains an angle of 23.5 degrees in Portland, the lowest possible noon angle for the Sun. (Recall that the Sun attains its highest altitude at mean noon, or when it crosses the due south.)

On the winter solstice, the noon Sun attains an angle of 43.0 degrees in the Tropic of Cancer, the lowest possible noon angle for the Sun.**

Even on the winter solstice, the Sun still passes about halfway between due south and the zenith in the Tropic of Cancer. However, here in Portland, the Sun never ascends to a high angle around the solstice. The atmosphere is absorbing a lot of the heat to which we on the ground are most certainly entitled.

I hope this answer helped, because it just ruined my day.  ;-)


*This value isn't precise due to a slight elevation of the Sun caused by atmospheric refraction. However, 23.5 degrees is close enough.

**The celestial equator's angle above due south at any location is equal to 90 minus the observer's latitude. In Portland, the CE is 90 - 43.5 = 47.5 degrees above due south. At the Tropic of Cancer, the CE is 90 - 23.5 = 66.5 degrees above due south. On the summer solstice, the Sun is 23.5 degrees north of the Celestial Equator. On the winter solstice, the Sun is 23.5 degrees south of the Celestial Equator. So, on the summer solstice, the Sun is 47 + 23.5 = 70.5 degrees above due south at mean noon in Portland and 66.5 + 23.5 = 90 degrees above due south at the Tropic of Cancer.