Rolling Clouds Later, as an adult, I fl ew halfway around the world to see an even stranger cloud. I waited for days in a dusty Australian town, hoping to see it.
I waited for 10 days without seeing the
curious cloud. Finally, aſt er 11 days, a slight breeze began to blow. I could feel a change coming. Far out at sea, two winds collided, sending a giant wave of air toward the land. If you could see it, the wave would look like a giant ocean wave rolling across the sea. T is wave picks up moist air as it rolls over
the sea. Vapor rides up the front of the wave, cooling and condensing into a giant cloud shaped like a tube. It rolls over the ocean and fl at land, where there is nothing to stop it or slow it. I watched as this cloud roll toward me at
50 kilometers per hour. As it passed over my head, it momentarily blocked the moon and stars from view. It then rolled into the distance. T is rare cloud is called the Morning Glory.
Bubbling Clouds Knowing how air, land, and water interact led me to the right place and right time to see a Morning Glory cloud. Other clouds are more mysterious. We don’t know exactly how they form, so it takes luck to see them. I got lucky driving through a storm in
France one day. Above me, the storm clouds started to change. T e clouds sagged in spots, forming spheres that dangled like bubbles. Soon, these bubbles fi lled the sky. I felt like I’d driven into an alien world. T ese clouds don’t follow the same rules as
other clouds. T ey’re made of ice crystals, even though they don’t always form at high altitudes. T ese clouds form when cool, dry air from
high in the atmosphere moves downward, freezing the moist air below. Scientists don’t yet know why this happens. It just goes to show that we don’t know everything about clouds.
22 NATIONAL GEOGRAPHIC EXTREME EXPLORER
A tube-shaped cloud rolls in. It’s like the Morning Glory cloud I saw in Australia.
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