It appears many people are intrigued with the cool, vertically billowing clouds we’ve seen the last few days in the afternoon (I know they have captured my attention!). The day starts out sunny and then bam…. by early afternoon we have been getting puffy clouds. By mid-to-late afternoon then a few showers fall from these clouds.
Well, in order to understand the science behind this week’s cloud formation let’s first look at the large scale weather set up and then trickle down to what the atmosphere above us has been looking like since the weekend to generate such cool clouds.
A low pressure in the upper-levels is helping to drag the jet stream, or zone of high velocity winds in the upper-levels, to the south of the region. This is allowing cooler air from Canada to seep south into the Mid-Atlantic.
Just like spokes going around a wheel, there are weak disturbances that are much smaller in size rotating around the main low pressure (circled in the upper-level image below). As these disturbances rotate around the low like the spokes on a wheel would move around a bicycle tire, it sparks showers and storms during the day.
(Story Image: Upper-level chart from Tuesday morning showing winds aloft, the upper-level low in the Northeast and the jet stream winds)
Now let’s back up a bit and talk about how and why the clouds form and hence the showers and storms. Note the image below which basically shows the temperature, dewpoint and wind profile through the entire atmosphere column at Dulles International on Wednesday evening. First off, note how the line farthest to the right (highlighted in the image below) bends pretty far to the left as it goes up. This indicates temperatures fall off quickly as you go up in the atmosphere. The freezing level as noted on the sounding is only 8,232 feet up in the atmosphere.
(Story Image: Tuesday evening sounding from Dulles International showing temperature, dewpoint and winds through the vertical column of the atmosphere)
Here’s what all this boils down to: In the morning before and around sunrise in this pattern, the skies are blue and the sun shines brightly. Then, as the sun heats the ground, since the sun angle is so high in the sky in early June the ground warms quickly. Since warm air is less dense, it rises. It will continue to rise as long as the air surrounding it is cooler.
Well, the air sure is much colder aloft, so the air rises high into the atmosphere. It naturally cools down though and then condenses (temperature of the rising air cools to its dew point) and forms clouds during the day. As the sun gets higher in the sky during the midday, it warms the ground even more and this air rises faster from the surface and helps form the “billowing” cumulonimbus clouds that look like cauliflower.
This is what we mean when we say the atmosphere is “unstable.” Warm surface temperatures combined with rapid cooling aloft through the atmosphere leads to cloud formation and then sometimes showers and storms. Well, sometimes the clouds need a “kicker,” or “trigger” to generate rain! Over the last few days, the minor spokes rotating around the main upper-level low combined with the sun’s daytime heating have been just enough to touch off scattered showers and storms.
As a matter of fact, one just north of Lake Ontario this evening will rotate around the main low and arrive in the Mid-Atlantic Thursday during the warmest part of the day, helping to bring more cool-looking towering cumulonimbus clouds and even a few showers and storms.
Keep an eye to the sky and be sure to take the umbrella if you have any outdoor afternoon plans!
As the sun gets lower in the sky around dinnertime and then finally sets, the ground cools off quickly because the air is so dry (noted by the dew points in the 50s instead of the typical summertime 60s and 70s). As the air cools, we lose that upward momentum driven by the warm surface temperatures and the clouds flatten out a bit (they don’t look as vertically oriented like the image below).
(Sunset over Washington, D.C., as seen from the WJLA-TV WeatherBug camera on Tuesday evening)
Finally, overnight the ground cools off enough that there is no rising warm air at all to help build and sustain clouds so that allows skies to clear! Now, want to see a cool time lapse from this evening, captured by the WeatherBug camera here at WJLA-TV.... see the youtube video below! Enjoy!