Spring is our local atmospheric time most unstableusually seeps in an up-and-down motion like hot cereal in a pan on your stove.
The visual manifestation of such unstable movements is cumulative convectionfrom relatively mild cumulus clouds in fair weather to towering cumulus clouds.
On Tuesday, the atmosphere became unstable and cumulus clouds appeared.
In the afternoon, relatively shallow cumulus clouds had formed by 2 p.m. (see below from the Seattle PanoCam on the Space Needle). No precipitation is visible below them.
By 5:00 p.m., after the surface had heated up in the afternoon, vertical turrets began to form, indicating greater vertical growth and instability.
The 6 p.m. image shows even more vertical growth and the formation of two whorls (the flat, elongated objects at the top of the image.)
30 minutes later, the anvils had explored their full size. Very impressive! It was raining beneath the big cumulus clouds and there was thunder and lightning
Cumulonimbus clouds like these are a sign of longitudinal instability due to large longitudinal temperature changes.
Stability 101
Most of the time the atmosphere is stable. You push some air up and it goes down again.
But if the temperature drops with altitude quickly enough, the situation will change. In this case, a displaced mass of air can accelerate upward.
The explanation for this behavior is something we describe in detail in classes in my department, but the key point is:
As temperatures cool rapidly with height, the air mass will easily become warmer and less dense than the surrounding air. And such warmer/less dense air masses tend to rise and rise.
Think of a hot air balloon! (see below)
In the Pacific Northwest, temperature variations with altitude are greatest in the spring. That’s why this is the best time of year to see dramatic cumulus clouds.
But why spring, you ask?
In the spring, after a long winter, temperatures aloft are quite cold…in fact, the coldest of the year. To show this, below is the temperature at about 10,000 ft (pressure 700 hPa) at Forks, WA, on the Northwest coast.
The lowest temperature is in March and April. The beginning of May is quite cold.
While temperatures aloft are cold, solar heating is now becoming very strong, especially on not so cloudy days. Below is a graph of solar radiation reaching the surface in Seattle over the past year. Becoming quite strong lately. Keep your sunscreen handy.
The sun on May 2 is as fierce as the beginning of August!
So, with strong warming at the surface and cooling temperatures aloft, there are often very large temperature variations with altitude in early spring, which produce a lot of cumulus clouds…. from shallow clouds to deep storm clouds.
Just to drive the point home, here is a graph of the observed temperature change with altitude (called the lapse rate) at Forks/Quillayute between the surface and 3 km (about 10,000 ft). Very high at this time of year.
Finally, you can see the development of cumulus clouds during the day as temperature changes increase with altitude. Let’s start with satellite imagery visible over eastern Washington around 8 a.m. Wednesday. There are no clouds over the Columbia Basin.
By 2:00 p.m., with abundant solar heat at the surface, the temperature variation with altitude increased significantly and instability developed, leading to many cumulus clouds.
