On this picture you can see two different kind of clouds, the bright orange ones are those in the troposphere, the part of the atmosphere we live in and where all clouds usually form; they are orange and dark because the sun was low behind the horizon.
But one cloud is white. Why? Because it's much higer as it is in the stratosphere (80,000ft in this instance), so high that it can pick up the direct sunlight even if it's not up yet! They only form in polar regions and are called ... Polar Stratospheric Clouds !
Those clouds, made of Nitric acid and water, play a key role on the destruction of the ozone layer in polar regions.