Sciency Words: Belts and Zones

Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s scientific term is:

Belts and Zones

Jupiter: it’s a big ball of hydrogen. Well, mostly hydrogen. The topmost layer of clouds—the part we can see in visible light—is primarily composed of ammonia, hydrogen sulfide, and water. A dash of other as-yet-unidentified chemicals are also mixed in for color.

If you’ve ever looked at Jupiter, you’ve noticed that it has stripes. The stripes are so pronounced that you can see them even with a cheap backyard telescope, and astronomers have been observing these stripes for centuries.

1200px-PIA02863_-_Jupiter_surface_motion_animation

By a long-standing convention, the two different kinds of stripes seen on Jupiter are called belts and zones.

  • Zones: Zones are characterized by their lightly colored clouds. Winds in zones generally blow west to east, and the cloud tops rise above the clouds in the neighboring belts.
  • Belts: Belts are darker-colored, with winds blowing east to west. You may notice that Jupiter’s famous storms, such as the Great Red Spot, tend to appear where belts and zones border each other. The clouds in belts are known to sink to lower altitudes than the clouds in zones.

Jupiter isn’t the only planet with belts and zones. The Solar System’s other three gas giants show similar, though less visually distinctive, stripiness, and it’s a safe bet gas giants orbiting other stars will too.

This all seems straightforward enough, but while researching zones and belts for today’s post, I learned something that struck me as very odd. Zones, the clouds of which rise upwards, are often described as cold while belts, with their lower altitude clouds, are described as warm. Does this mean that on Jupiter, warm air sinks and cold air rises? Have the laws of thermodynamics been reversed?

Well, there’s more to these clouds than temperature alone. We also have to consider air pressure.

Just as increasing the pressure of a gas can make it hotter, decreasing the pressure can make it cooler. So rather than picturing cool air masses somehow rising, picture rising air masses cooling off due to decreasing atmospheric pressure. Such a situation can be thermodynamically stable, especially when dealing with the extreme altitudes associated with planetary atmospheres.

Still sound crazy? Well, this phenomenon isn’t unique to Jupiter. Similar changes in air pressure occur here on Earth, which is why mountaintops get so cold while the fields and valleys below stay warm.

In many ways, Jupiter is a mysterious planet. We don’t fully understand what causes its enormous cyclonic and anticyclonic storms, nor do we fully understand what’s going on in the deeper layers of the planet’s interior. We’re not even sure why the Great Red Spot looks red.

But Jupiter isn’t that mysterious. Some things which might seem odd at first glance are actually pretty easy to explain.


Today’s post is part of Jupiter month for the 2015 Mission to the Solar System. Click here for more about this series.

Article by James Pailly. Check out James’ blog for more great science articles.

  • Paulo R. Mendes

    This was (again) a very cool article.

  • Kirov

    Interesting. I didn’t realize that we still hadn’t identified all the gases in Jupiter. But since we’re on the topic of gas giants, any ideas on how microbial life might evolve in the atmosphere? For any one who’s played Elite Dangerous, gas giants with life seem to be quite common, so it caught my interest.

  • Christmas Snow

    It’s also important to mention the Coriolis effect: The faster the rotation, the more bands we can count. There is, however, a big difference between a terrestrial planet, whose atmosphere gets most of energy from the sun, and a Jovian which generates enough heat at the core to drive all that storm almost all by itself.

    A slow-rotating or tidally-locked terrestrial planet will feature an “eyeball” cyclone at the sunny side and an anti-cyclone on the dark side. How would a storm on a tidally-locked or slow-rotating Jovian behave, assuming core heat is the main drive for the storms?