Dena Hess, age 12, of Wichita, Kansas, for her question:

What causes snow to turn yellowish?

In a few weeks the snowy season will be here again. Some morning we will wake up to see the whole world covered with a soft blanket of whitest white. Way up in the lonely woods of the northwest, the snow will stay white all through the winter. The first layers will be white when new snowfalls cover them with more whiteness. This is because the air in these woodsy regions is clean and free from dusty and sooty dirt. This is true of most forested regions where the soil is covered with plant life and matted with their roots. The blowing breezes can find little or no dusty debris to swish around and drop onto the blanket of clean white snow.

But lots of our land is covered with cities or with wide open stretches of bare or almost bare ground. Tons of sooty smoke pour into the air from our factories and dwellings. These yellowish, brownish and grayish fragments later fall down on top of the white snows. Stretches of bare land are dusty. The breezy winds whisk away the drab dusty fragments and dump them also on the freshly fallen snows. The clean white snows of winter are discolored by all sorts of dirty debris from the air.