Cumulonimbus Rising v.2

You can see v.1 in my Oct 3, 2016 post. The goal is to capture the evolution of a growing cumulonimbus cloud (frequent precursors to thunderstorms). It’s an experiment in progress — v.2 is better but still has some problems.

I often see time-lapse videos of cloud buildups on TV weather intros, cropped in time and space or cluttered with weather station graphics. What I’m trying to do is capture a full view of a large storm system from start to peak. Some rainfall and lightning near the end would be cool. That’s not as easy at it sounds, even with digital cameras. I’ll share some tips with you in this post if you want to try this on your own. Note that this is not a camcorder video — my older model camcorder doesn’t have enough memory for this purpose. This is a time-lapse sequence from my Canon EOS 20D using an outboard intervalometer (newer DSLRs have a time-lapse mode built-in).

But first, let me describe what you’re seeing in the video. It shows the buildup of a large cumulonimbus cloud over the Mogollon Rim during Arizona’s monsoon season. The view is toward the northeast. Rim elevation is 1500 m (5000 ft) and the distance to that cloud is around 80 km (50 miles). Humid monsoon air is forced by prevailing winds to climb as it ascends the Rim, and eventually reaches an altitude at which the temperature equals the dew point. That altitude is briefly marked in the video by a yellow horizontal line. For more about this effect, see my May 29, 2017 post. You can see the cloud growth accelerate around the 20 second mark. This is caused by the latent heat released as water vapor condenses, further driving the updraft. Near the end, as the cloud builds to an altitude of 4500 m (15,000 ft) it encounters another layer of air blowing from the west. This shears the cloud’s top into the classic “anvil” shape common for thunderstorms. Heavy rain, lightning, and winds followed later that afternoon. 

The video was created by assembling 54 individual images into a video. Frames were shot at 60 second intervals and the video plays 60X faster than real-time (RT=0:54). I used a half-frame fade-in/fade-out transition between images to smooth the motion.

As I said, this video is better than my first attempt but there’s still room for improvement. Here’s some of the things I’ve learned so far about shooting this type of time-lapse …

  • Clean your lens. When you’re shooting the sky, any spots on your lens (or dust on your camera sensor) will stand out annoyingly against the blue. You can see several at the start of my video before the cloud covers them.
  • Use a tripod and position your camera where it will be shielded from wind (and potential rain). Without a stable mount your camera will vibrate. I use a hefty metal tripod with solid tubular legs.
  • If you aim your camera at mostly blue sky and are using autofocus, it might not want to shoot for lack of objects to focus on. You could switch to manual focus and just focus at infinity. Or you could do what I did and include some distant landscape. IMHO this also adds a better sense of scale to the video.
  • Experiment with your image interval. I used 60 seconds for this one, but you can see how when the cloud growth accelerates I get some “jumpy” transitions. This is partly a function of cloud distance, but for my next attempt I’ll try 30 second intervals. Of course, that means twice as many images. So …
  • … set your camera’s image size to a lower value. I switched from the default “large” setting (3504×2336 px) to “small” (1728×1152 px). You don’t need high resolution for cloud videos. The memory card in my camera holds 4 GB — enough to store over 3300 “small” images. At 30 second intervals, that gives me 27+ hours of recording time, so no worries. Unless …
  • … you don’t have a full charge on your camera battery. One of my earlier photo ops was foiled by a battery that died after 10 shots. My bad. I have a 2600 mAh battery which is good for 1200 shots when fully charged (if not using the flash). That limits me to around 10 hours of recording at 30 second intervals, more than enough for what I’m trying to capture. Your mileage may vary, so check your battery and memory capacities, adjust your exposure interval as needed, and always start with a full charge. 
  • Finally, be aware that it might take several attempts to capture what you want. Even during monsoon season I can’t count on a good photo op every day. More often than not the cloud you want to shoot is obscured by other clouds or gets torn apart early by other weather. As with any type of photography, patience, perseverance, and luck will eventually get you that perfect video. 

The growth of a large storm system is fascinating to watch, more so if you understand the meteorology at work. It’s a classic demonstration of fluid mechanics, heat transfer, and phase transitions. It’s also just fun to watch, since most times our view of this process consists of intermittent “snapshots” — we notice the cloud is growing, but we don’t have the time or inclination to watch for an extended period. Videos like this provide that opportunity.

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