Answer to Map #96

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Answer: This week’s map showed the areas of the continental U.S. that receive the most sunlight. This map was created and sent to us by Luke, one of our regular submitters, who said he wanted to try his hand at making an isoline map—and well done, Luke!

Specifically, this map plots the total amount of solar radiation received in one year in each place. This kind of map would be very useful if you were trying to decide where to install some solar panels.

On this map, you can see the general pattern, which is that there is more sun in the deserts of the Southwest, with less and less sun as you move both north and east. But you can also see smaller patterns, which are usually the result of mountains. Along the Pacific coast, for example, clouds get pushed up against the Cascades and dump rain on Seattle and Portland. As a result, parts of Washington and Oregon experience dramatically less sun than the map’s general pattern would indicate. San Francisco, which is famous for its fog, also gets less sun than the surrounding areas, colored in orange. This week’s second hint referred to the fact that San Francisco has been in the news for a particularly extraordinary week of fog. The southern part of the west coast of Florida gets lots of sun, so much so that its nickname is the Florida Suncoast. And on this map, you can also see little whirls of color throughout the Rocky Mountains; following the general pattern in the temperate latitudes, there is more sun just to the east of the mountains, while the mountains themselves collect lots of clouds.

By far the most common incorrect guess this week was that the map indicated average temperatures. It’s true that the American Southwest is quite hot, but the pattern doesn’t really match up further to the north. Parts of Minnesota and the Dakotas manage to get a fair amount of clear weather in the winter while remaining bitterly cold. Another common answer was that the map has to do with cloud cover. We accepted this answer because it lines up fairly well with sunlight, though there are two issues here to bear in mind. First, if we were going to map cloud cover, it would be rather indelicate to use the hotter colors for the bits with fewer clouds. Second, clouds are not the only factor that influences sunlight. You also have to take into account latitude. Luke chose to include only the continental U.S. for various reasons of difficulty (presumably, that doing so made it vastly easier for him to map and more difficult for you to figure out), but, if he had included Alaska, you would have seen that it gets quite a bit less sunlight. Over the course of an entire year, it gets the same total number of hours of daylight (everywhere does), but nowhere close to as much solar radiation because the sun never shines directly on it.

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