They resemble surface waves on the ocean, but exist in the atmosphere between layers of slightly different density. (realclimate.org)
It's a very distinct boundary marking different density of gases and a change in temperature from a decrease with altitude to an increase. (drtimball.com)
Memory foam mattresses are layered with different densities of foam that respond to weight and are known for comfort because they contour to the shape of your body. (rismedia.com)