I am always interested in time-lapse photography, and I got an unexpected chance to capture some thunderstorms yesterday! I was just milling around at home, and decided to set up my camera when I saw some developing cumulus clouds to the east. Five hours later, I had gotten a really great series of transient thunderstorms!
The first 20 seconds of the video are pretty boring, but then the thunderstorms really start to get going. It was hard to guess where they would form, so I kept moving the camera in the middle of the video. Right at the end, there is a faint rainbow along the right side of the video (but it’s mostly off-screen to the right).
Technical weather discussion begins here (continue reading at your own risk):
What is really happening in this time lapse video? As long as the summertime airmass is sufficiently unstable, meaning that air forced upward will want to continue rising on its own (given several limiting conditions/factors, such as density differences), thunderstorms are likely to form in the mid-afternoon. These types of storms are very common, and are not usually severe (generally no tornadoes, hail, or damaging winds), although they can still be hazardous to people doing outdoor activities. Given a lot of sunshine and warm temperatures in the low 90s like we had yesterday, this was an ideal day for this to occur. During the first 20 seconds of the video, plumes of air heated by the early summer sun are rising, condensing into tiny liquid water droplets, and forming puffy cumulus clouds. Interestingly, all the air condenses at about the same level in the atmosphere. This is called the Lifting Condensation Level, or LCL, and is found at the flat base of the cumulus clouds (notice that this level remains constant through the video).
However, there is another important level in the atmosphere called the LFC, or Level of Free Convection. Why is this important? Any plumes of warm air that bubble up below the LFC tend to not have enough energy to keep rising and form thunderstorms. They sink back down. However, clouds that do rise above the LFC tend to keep rising on their own. All the cumulus clouds that form and decay during the first 20 seconds of the video don’t rise above the LFC. However, the cloud to the far right around the 22-second mark has enough energy and momentum to surpass the LFC and keep rising. It forms a thunderstorm. A number of cumulus clouds later in the video rise above the LFC as well, forming other thunderstorms.
But will the air parcels in the thunderstorm keep rising forever? No. At a certain level, air parcels in a thunderstorm will lose momentum and stop rising. Due to differences in density caused by the difference between the temperature of the rising air parcels compared with the temperature of the surrounding environment, thunderstorms will not grow above a certain height called the Equilibrium Level (EL). Here, air parcels have neither an inclination to rise or sink, and are content to stay at the top of the thunderstorm. They slowly spread out like a pancake, in an area called the anvil of a thunderstorm (looks like an actual anvil), which can be seen in the thunderstorm at right around 0:30.
Why do thunderstorms form in the first place? This is a matter of instability. When the sun is out, it is only able to warm the Earth’s surface (and, by extension, a small layer of atmosphere near the surface). This leaves air aloft, 1000 feet above the surface and higher, a good deal cooler than the warm surface. Surface air parcels want to rise up and cool off in this situation (general definition of instability). One of the most common ways this happens is through thunderstorm formation. Due to complicated issues of thermodynamics (the transfer of heat energy), I won’t elaborate too much. What I will say is that air parcels that rise up in a storm and form rain are able to take heat from the surface and redistribute it into the upper atmosphere, decreasing the temperature imbalance formed by the sun. Paradoxical. You can (and rightfully should) blame the sun for stormy afternoons. It is the culprit.
In summary, what you see in the time lapse video are plumes of air that are too warm to stay put at the surface. They rise and form thunderstorms, trying the bring the atmosphere back into balance. Weather is the resulting dance of the atmosphere, trying to regain balance from imbalances caused by the sun. Neat stuff!