Snow Line, the line, usually in mountains, above which there is permanent snow. Below the line, temperatures are sufficiently high during the warmest months to melt the covering of snow. Virtually nowhere is the snow line in a fixed position, for the altitude at which it occurs varies with the depth of the annual snowfall, exposure to the sun, steepness of the slope, and the direction and velocity of the wind. In general, the snow line decreases in elevation from the equatorial regions, where it lies as high as 20,000 feet (6,000 m), to sea level near the poles.
Depending on who you ask, urban sprawl is either the best thing that ever happened to growing families -- or the downfall of civilization and the environment as we know it. Learn about the history and consequences of this American phenomenon.
World, the earth with its inhabitants and their activities. Earth and world have somewhat the same meaning, but as used here earth refers to the planet while world refers to the earth as lived on and used by humankind.