[The Clouds of Glory Say:]
Even when it is clear
He thickens clouds;
He spreads out His rain clouds.
During the forty years in the wilderness, Yisrael was protected and given light by the Clouds of Glory. The Talmud teaches that they had no need for sunlight because the holiness of the clouds gave them all the light they needed. Also, when the earth needs rain, G-d thickens the clouds with moisture and gladdens the dry earth.
[Tanach, Rabbi Scherman, Perek Shira]