Morning mists often spread over the large plain of Alsace. They occur in spring and winter due to the degraded oceanic climate of the region. Radiation fogs are the most common. They need calm weather and clear skies at night to pick up the morning moisture. Sandy soils are also conducive to drops in temperatures maintained by forests. When the air is saturated, the water vapor condenses into mist droplets. Though it usually scatters with the first sun rays, it can sometimes thicken and hold pollutions down on ground level.