Evolution of a Sunset


When was the last time you went outside and just watched the clouds move? Probably awhile, I'd guess. Seems like, as responsibilities to school, job, or family start to pile up, simple pleasures like cloud watching tend to fall by the wayside. Try it again. I promise you'll find it relaxing and enjoyable.

Clouds are never static. Their shapes evolve as time goes by. That makes for an excellent time-lapse animation, and it's what you see above. This animation consists of 6 images looping continuously. The time interval between images is about 5 minutes, so you're looking at 30 minutes of real time compressed to 30 seconds.

The entire sequence was photographed just after sunset, so there's some beautiful color changes as well. Colors created during a sunset were the topic of last week's blog. This week I want to talk more about evolving cloud shapes.

Watch the large cloud at top center. You'll see how it breaks apart and dissipates as time goes by. Part of that effect is due to Lake Pleasant, which lies just beyond those mountains on the horizon. The lake absorbs heat from the Sun all day. After the Sun sets, the air above cools much more quickly than the water. Air has 1/4 the specific heat of water, and a lower specific heat allows more rapid temperature changes.

Air in contact with the lake surface is warmed by the water. Warm air is less dense than cool air, so it floats upward into the atmosphere via the process of convection (hot air rises). It's this rising warm air that causes the changes you see.

Convection, of course, hastens the breakup of that cloud. But clouds are comprised of tiny droplets of liquid water. If those droplets are heated by warm (convecting) air, the liquid water will return to its invisible vapor form. So that's why the cloud dissipates — it is literally evaporating.

People who do hang-gliding understand the process of convection. They know how to "read" the clouds and ground-cover to locate updrafts caused by convection. If they can find a sufficiently strong updraft, they'll be able to gain altitude and prolong their flight. There are reports of hang-gliders picking up too strong an updraft. Some have been lifted to altitudes where they passed out from lack of oxygen. Good hang-gliders need to understand basic meteorology. Knowing a little science can help improve your performance in virtually any sport.

Next Monday in Sky Lights ⇒ Why the Days are Getting Shorter

Sky Lights #200!
Q&A: Why the Days are Getting Shorter