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Example of a stable and an unstable equilibrium, respectively.
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Stability

Have you ever tried to balance a long stick on your hand? Hard, isn't it? That's because the stick is part of an unstable system. If the wind pushes the stick a little bit, it will keep going in that direction. If you hold the stick upside-down it's much easier to keep straight. That's because when the stick is held from above it's a stable system. If a breeze moves the stick, it will come back to its starting position.

The atmosphere can also be stable or unstable. If it's unstable, then clouds can form. The more unstable the atmosphere is, the more severe the weather can be. Clouds and storms form when pockets of air rise and cool because they expand in the lower pressure of the upper atmosphere. The air pockets become saturated and the water vapor condenses to form clouds.

These air pockets don't rise because they want to; something needs to give them a push. This is called the lifting mechanism. If there is no lifting mechanism, no storms will form regardless of how unstable the atmosphere is. The more unstable the atmosphere, the less of a lift is needed. It's similar to trying to balance a stick with a smaller and smaller cross-secion. A log is easier to balance on end than a twig and the twig is easier to tip over--it's more unstable.


Last modified May 26, 2004 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA