The National Weather Service is forecasting a high of 74°F here in Anchorage for the Independence Day holiday as of this writing. To most people in the Contiguous U.S. (Lower 48), this probably sounds like an altogether pleasant temperature. At high latitudes, temperatures in the 70°s and 80°s (temperatures in the 90°s are very uncommon) feel much warmer than one would expect. Summer visitors to Alaska frequently react with amazement at how warm a temperature of 75°F feels on their skin compared to wherever they are from. Quiz an out-of-state visitor on the temperature when it is sunny and 75°F and few, if any people will respond with a guess of less than 80°F-85°F. What is going on?
Over the course of an entire year, locations in the Lower 48 receive far more solar energy (solar radiation) than their counterparts in Alaska. This is simply a function of the angle of solar rays striking the Earth. At high latitudes, solar radiation is spread across a much larger area. This means any given location in the North has far less solar input compared to areas in the Lower 48.
Instead of talking about the solar radiation over the course of a year, let's talk about solar radiation during the high sun time of year, specifically the month of June. For this analysis, we use the solar radiation data collected at U.S. Climate Reference Network (USCRN) stations around the U.S. These stations are generally 5-15 years old and are expected to collect 50 years of baseline climate data uncontaminated by effects related to urbanization or other land surface changes.
The map above shows the 90th percentile daily solar radiation (Megajoules per square meter) for 120 USCRN stations with at least five years of data. In Alaska, most of the USCRN dots on the map are either green or yellow. This is equivalent to stations in most of the eastern half of the Lower 48. In fact, the Barrow 4 ENE station at the northern tip of Alaska, 300 miles north of the Arctic Circle, receives more June solar energy on these top 10% of days than all stations in Louisiana, Mississippi, Alabama, Georgia, South Carolina, and Florida. Remember, this only looks at the sunniest of sunny days (90th percentile; or 1 out of 10 June days). Since Alaska is far cloudier than any portion of the Lower 48, the average solar radiation for all June days is notably lower in Alaska than nearly the entire Lower 48.
Wait, what? How does Alaska receive more solar energy than Florida? There is one key reason. This is a 24-hour total. In northern Alaska during the month of June, the sun never sets. Therefore, solar energy is collected all day and all "night." Contrast this with Florida. In mid-June, the central part of Florida has 14 hours of daylight and 10 hours of darkness (or twilight). Also, few June days are completely cloud-free in the eastern Lower 48. Billowy cumulus clouds are efficient solar reflectors and solar absorbers. Finally, atmospheric moisture (water vapor) absorbs a percentage of incoming solar radiation.
If we identify the highest hourly solar energy values for all June days and plot the 90th percentile, the map looks quite a bit different. Every station in Alaska is in the lowest category (note: units for hourly solar radiation are watts per square meter since it represents an instantaneous measurement, as opposed to a summed total). That means the clearest of clear skies with maximum solar energy are about ~15% less intense in Alaska compared to the eastern Lower 48. Again, this is not surprising given that a low sun angle is easily demonstrated to be less intense.
Now, back to the original question, why does a 75°F temperature on a sunny day in Alaska feel so much warmer than a 75°F day in the eastern Lower 48 at this time of year?
- In most portions of Alaska, 75°F is an above normal temperature. Wherever you are, an above normal temperature in the summer feels warm.
- The low sun angle exposes most of your body to direct sunlight. An observer in Louisiana wearing a light-colored, wide-brimmed hat at midday can block most of the direct solar radiation from reaching their torso. Since the sun is nearly overhead, the hat blocks most of the energy. In contrast, the sun never rises more than 47° above the horizon north of the Arctic Circle. This means the sun's rays are shining on whichever half of your body is facing the sun whether you have a hat on or not. That direct sunlight raises the apparent temperature by 10°F-20°F.
- Many Alaskans do not own shorts or light colored clothing . Cold climate attire is typically dark in color and insulating. This raises the effective temperature for the typical observer.
A similar situation exists with housing and "warm" temperatures. On a 75°F day in Anchorage, the temperature inside a house without trees for shade will easily reach 85°F-90°F unless windows are opened and fans are deployed (note: very few houses in Alaska have air conditioning). This may seem odd at first, but a little bit of science easily explains this phenomenon. First, houses in Alaska are built to keep heat inside. This means once the temperature rises, the house does a great job retaining the warmth. Second, and most importantly, the low sun angle allows a large quantity of solar radiation to penetrate windows and reach deep inside a house. Approximately 2.1 times the solar energy enters a house through a window when the sun is 45° above the horizon compared to 70° above the horizon at any given moment. With 19+ hours of daylight, the accumulated solar input travelling through the windows can dramatically increase the temperature inside a typical house.
So when your favorite Alaska climatologist complains about 75°F temperatures this summer, go easy on him or her. It really can be uncomfortable.
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