photo #5857998

From high atop Pine Bluff in Riverside State Park, near Spokane, Washington, US, Ken Clason (Member 56,523) spotted fog blanketing the Spokane River Valley below. Valley fog forms overnight when the sky’s clear, the wind light, and there’s some moisture down at the ground. It is a form of radiation fog, which is so named because the heat of the day radiates away after nightfall into the starry sky fast enough to cool the low air enough to form droplets of fog. On slopes, the cooled air sinks downhill because it is denser than the air away from the ground, and so it fills valleys as it collects at the lower ground. The nighttime valley fog tends to lift as the morning progresses, and the Sun’s warmth causes winds to pick up. On the snowy peak of Mount Spokane, off in the distance, skiers are likely out on the slopes. Down in the river valley, morning takes its time to arrive with the lifting blanket of fog.