In the lowlands of Washington State, today is sunny and clear. Not a single drop fell from the sky.
But something very different happened in the mountains, where the atmosphere became unstable, creating cumulus clouds and showers. There’s even lightning!
Let me show you and explain.
To start, here are two visible satellite images: one at 6:30 a.m. and one at 5:30 p.m. today.
Morning images have small clouds and snow visible at higher elevations
But the story is different at 5:30 pm.
The lowlands are still clear, but there are clouds in the mountains. Cumulonimbus clouds, including some highly cumulus clouds.
You can see changes to the ground from the amazing Crystal Mountain camera.
At 6:20 a.m. the sky was clear looking east.
But by 10:40 a.m., shallow cumulus clouds had formed over the mountain peaks.
By 1:20 p.m., the clouds were becoming more widespread and denser.
Photo taken at 6:10 p.m. shows a dense cumulonimbus cloud precipitating. You can see rain at the base of the clouds (called virga).
In fact, there has been measurable rainfall in some mountainous and downstream locations in the past 12 hours (see chart below).
So why do cumulus clouds and precipitation in mountainous areas (also called convection) and not on the lowlands?
Cumulonimbus clouds are associated with the atmosphere instabilityin which the atmosphere convects with up-and-down motions, not unlike what happens in your pot of hot cereal or in a lava lamp (see below).
Convection occurs when the temperature decreases with sufficient height. Usually this is due to warming near the surface but it can also occur when there is cooling aloft.
Mountains act as heat sources at high altitudes, especially after the high-altitude snowpack melts. That results in greater temperature variation with altitude in the mountains than in the lowlands. Mountain air becomes unstable, leading to cumulus clouds (see diagram below).
This effect is greatest in the afternoon when solar heating is greatest.
But wait! There’s more! During the day, there is upward movement on heated terrain slopes, leading to strongly enhanced upward movement on mountain peaks. Upward motion contributes to cloud formation and the development of instability on mountain tops.
Both of these effects occurred today, leading to more cumulus clouds and rain over the higher reaches of the Cascades and other mountains in the area.
