Because the under surfaces receive the radiated warmth of the earth.
These ridges and points are granulated on their under surfaces.
Both upper and under surfaces of the leaves should be covered.
The under surfaces of objects are kept warm by the radiation from the earth.
Clouds of dense smoke billowed upward, their under surfaces vividly illuminated by the ruddy reflection of the leaping flames.
Unlike most arcas, it spins a byssus, by means of which it attaches itself to the under surfaces of stones at low water.
They are crowded at the ends of the branches and hang in such a manner as to show their under surfaces with every touch of breeze.
At first the "toadstools" are all yellow but later only the under surfaces are yellow while the upper surface is orange-red.
This bird is very similar to the Black Guillemot except that the under surfaces of the wings are dark.
Craveri Murrelet is very similar to the last except that the under surfaces of the wings are dusky.