There are spring and summer afternoons when very thin clouds appear overhead and turn the sky a bit white. These types of skies are physically interesting in at least two ways that are worthy of note.
First, the whiteness is a function of the fact that the cloud cover is a thin cirrus cloud. Cirrus clouds are composed of tiny ice crystals that scatter visible light without preference for any of the colors of the visible spectrum (the colors of the rainbow). This particular property is shared by snowflakes as well as by haze droplets. Individual snowflakes look clear but even a small collection of them is white since all of the light that hits the collection of snowflakes is scattered in all directions equally. The same is true of haze droplets which are most common in the summertime in southern Wisconsin.
Second, even the presence of such a thin deck of high clouds (or a thin haze layer) can substantially lower the high temperature of that afternoon. Often, record high temperatures in the summer occur on days with very little cirrus cloud and very little haze in the sky.
