December 19 – Partly cloudy

Today’s factismal: Storm clouds can be up to fourteen miles tall on Earth.

One of the prettiest and most dangerous types of cloud on Earth is the cumulonimbus (“piled up rain cloud”). As you might guess from the name, these clouds frequently produce rainstorms and other intense meteorological phenomena, such as lightning, hail, and tornadoes. But what you might not guess is that they can also bring us blizzards and are one of the most energetic things on Earth.

Clouds are a process, not a thing to meteorologists

Clouds are a process, not a thing to meteorologists

But how do you create one of these magnificent terrors? It turns out that all you need is a little water and a little sunshine. (OK, a lot of water and a lot of sunshine.) If the water is spread out on the surface of the Earth and the Sun is shining down, then the Earth and the water get warm and the water evaporates. (The Earth just stays where it is.) The warm Earth heats up the air just above it, which is now laded with water vapor. The warm air rises, taking the water vapor along for the ride. As it rises, the air pressure decreases, which allows the air to expand. And expanding takes energy which comes from the heat in the air, causing it to cool. If the air rises far enough and cools off enough, then the air can no longer hold all of the water vapor and some of it condenses into little drops. Voilà! A cloud is born.

The colder air around the cloud has created a mass of ice crystals that refract the sunlight into a rainbow (My camera)

The colder air around the cloud has created a mass of ice crystals that refract the sunlight into a rainbow
(My camera)

This cloud is being driven up by the warmth of the air inside the cloud (My camera)

This cloud is being driven up by the warmth of the air inside the cloud
(My camera)

The remaining energy in the cloud is what creates the lightning (through friction in the cloud that is still not well-understood) and drives the tornadoes and gust fronts. And, because the energy ultimately comes from the heat in the air, you can get a feel for how much energy the cloud has by how high it goes; a taller cloud needs warmer air and so is probably more energetic (and more likely to create a thunderstorm) than a shorter one. So how tall can these clouds get? A typical cumulonimbus cloud will go up about two miles. But in the Tropics where the sunlight is more direct (and so more energetic), they can reach as high as fourteen miles! (Don’t feel too impressed – clouds on Jupiter can be several hundred miles tall.)

No, not seven miles; fourteen! (My camera)

No, not seven miles; fourteen!
(My camera)

Colder air above a cumulonimbus cloud can keep it from rising, so there's more to this story than just "more heat=taller cloud" (My camera)

Colder air above a cumulonimbus cloud can keep it from rising, so there’s more to this story than just “more heat=taller cloud”
(My camera)

Now the truly interesting thing about clouds is that they are important as a way of moving heat around in the atmosphere. And that has implications both in the short-term (for meteorologists) and in the long-term (for climatologists). So naturally, they are interested in measuring clouds. They mainly use a series of satellites that look down on the Earth to measure the cloud cover, but that means that they have to make assumptions about how what they see from above the atmosphere relates to what we see from down below the atmosphere. And scientists hate to make assumptions. So the Space Science and Engineering Center at the University of Wisconsin has developed a cool free app that will tell you when a weather satellite is overhead; if you take a picture of the sky when the satellite passes, then they can use that to “ground truth” their data. As a reward, they’ll give you a copy of the image taken by the weather satellite. To take part, head on over to:
http://www.ssec.wisc.edu/news/articles/755

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