Albedo their reflectivities. Fresh 'black' ice exhibits Fresnel reflection. Snow on top of this sea ice increases the albedo to 0.9. Cloud albedo has substantial influence over atmospheric temperatures. Different types of clouds exhibit different reflectivity, theoretically ranging in albedo from a minimum of near 0 to a maximum approaching 0.8. "On any given day, about half of Earth is covered by clouds, which reflect more sunlight than land and water. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth." Albedo and climate in some areas are affected by artificial clouds, such as
Incompressible surface ∂"D". If "D" were to bound a disk inside "S" (which is always the case if "S" is incompressible, for example), then compressing "S" along "D" would result in a disjoint union of a sphere and a surface homeomorphic to "S". The resulting surface without the sphere might or might not be isotopic to "S". There is also an algebraic version of incompressibility. Suppose formula_4 is a proper embedding of a compact surface in a 3-manifold. Then "S" is "π"-injective (or algebraically incompressible) if the induced map on fundamental groups is injective. In general, every "π"-injective surface is incompressible, but