Our planet’s highest monthly maximums of incoming sunlight (insolation) at the top of the atmosphere occur not in tropical latitudes, but in polar ones. The top-of-the-atmosphere insolation at the North Pole peaks in June at about 520 watts per square meter. By contrast, the insolation at the equator peaks in March at about 439 watts per square meter. (You can generate a table of monthly insolation by latitude here.)
Despite the abundance of incoming sunlight in the summer, the poles don’t warm up to tropical temperatures, and this series of globes illustrates one of the reasons why: ice sheets, snow cover, and sea ice reflect a lot of the incoming sunlight right back to space. The globes show the amount of incoming sunlight (watts per square meter) reflected by Earth in June, July, and August 2009, based on measurements collected by the CERES sensor on NASA’s Terra satellite. The globes are centered on the Arctic Ocean, with North America at lower left and Asia at upper right. The most reflective areas are yellowish-green, and the least reflective areas are deep blue.
The most obvious pattern is the decreasing reflectiveness of the Northern Hemisphere as summer progresses and Arctic sea ice and the last of the seasonal snow cover in Canada and northern Russia melt. Clouds over the North Pacific and North Atlantic Oceans and bright land surfaces, such as the Sahara Desert of North Africa (bottom right edge of globes), continue to reflect a significant amount of sunlight into late summer.
The globes also illustrate the importance of the Greenland Ice Sheet (just below the center of the globes) in keeping the regional climate cool. As sea ice and snow surrounding Greenland melt, the darker land and ocean surfaces absorb more of the intense summer sunlight, causing temperatures to warm. Ice-covered Greenland remains a bright spot throughout the summer, however, reflecting far more sunlight than other land areas and ocean areas at its same latitude.