Many factors affect how air or water flows. Viscosity is the ultimate dissipator of energy and thus always acts to decrease turbulence. However, there are many other factors that modify turbulence and even keep it from forming in the first place. In planetary flows (atmosphere and oceans) two of these factors are rotation by the Earth and what we call stratification.

The whole Earth rotates, and that affects every fluid that rotates with it to a certain degree. Although the effect is too small to make a difference in small flows (like your toilet or a cup of coffee), it has a large effect on flows at larger scales, such as weather systems. This effect is generally called the Coriolis effect, and you can see how it affects major weather systems in this video. Effects of rotation can be estimated using the Rossby number.

Stratification, on the hand, has to do with how the density (or weight) of the fluid is distributed. If there is heavier fluid sitting on top of lighter fluid, then we say that the stratification is unstable and turbulence tends to form. If, on the other hand, lighter fluid is sitting on top of a heavy one, it will be difficult for turbulence to form even through the action of an exterior force. In geophysical flows, the Froude and Richardson numbers are two relevant parameters used to infer stratification effects. Stratification effects can lead to an interesting phenomenon called Kelvin-Helmholtz instability.

If you are interested in a more rigorous (and therefore somewhat mathy) discussion on turbulence, a good place to start is this Scholarpedia page.

Special thanks to Ken Zhao of UCLA.

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