WRAL WeatherCenter Blog

Why is it on CAVU (Clear And Visiblity Unlimited) days, there is a lot of turbulence?

Posted February 17, 2007

MIKE MOSS SAYS:      Brook,     When airport or en route weather conditions are reported as CAVU or CAVOK (meaning Ceiling And Visibility Unrestricted, or OK), it doesn't necessarily mean that there will be significant turbulence. However, the fact that weather is otherwise favorable for aviation also does not guarantee the absence of significant turbulence, either. The ceiling and visibility can be fine on a very windy day, for example, when gusty surface winds would lead to strong mechanical mixing of the lower atmosphere, creating turbulence near the ground. Or, on a hot sumer afternoon, ambient winds may be rather light, but strong and uneven heating of varying surfaces by the summer sun can lead to strong "thermals," or cells of rapidly rising air interspersed with areas of descending air. These thermals and the turbulent interface between areas of upward and downward vertical velocity can lead to turbulence. It is also possible on a CAVU day to have layers of air with sharply varying density one atop the other (usually at higher altitudes) and if a strong vertical wind shear develops across this layer and the vertical temperature profile is of the appropriate stability category, then clear air turbulence may develop in that layer even in the absence of clouds or precipitation. Those three scenarios are examples of the three principle types of turbulence: mechanical, shear-induced, and convective.

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