English: Haze hung in the air as fall colors brightened the forests of Western Canada in late September 2017. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Aqua satellite acquired this true-color image on September 29.

The image is centered in Saskatchewan, near Prince Albert National Park, and sweeps from Lake Winnipeg, Manitoba in the east to Alberta’s Rocky Mountains in the west. The boreal forest, made up of mixed deciduous trees and evergreens, colors the northern tier and, in the south, the grasslands have taken on a coat of winter tan. The United States lies in the far south of the image.

In autumn, the leaves on deciduous trees change colors as they lose chlorophyll, the molecule that plants use to synthesize food. However, chlorophyll is not a stable compound and plants have to continuously produce it, a process that requires ample sunlight and warm temperatures. When temperatures drop and days shorten, levels of chlorophyll do as well.

Chlorophyll makes plants appear green because it absorbs red and blue sunlight as it strikes leaf surfaces. As concentrations of chlorophyll drop, the green fades, offering a chance for other leaf pigments—carotenoids and anthocyanins—to show off their colors. Carotenoids absorb blue-green and blue light, appearing yellow; anthocyanins absorb blue, blue-green, and green light, appearing red.