Van Allen belts

Vector born disease

Disease produced by a biological vector: an organism that transmits disease by conveying pathogens from one host to another.


Process for determining the accuracy of a weather forecast, achieved by comparing the predicted weather with the observed one during the forecast period.

Vernal equinox

Equinox at which the sun crosses the celestial equator from the Southern to the Northern Hemisphere, that is approximately on March 21st, for the northern latitudes.

Vertical development

Cloud that grows to significant heights, presenting a considerable vertical distance between the level of the cloud base and that of the cloud top.

Vertical mixing


Greatest distance in a given direction at which it is possible to see and recognize a dark object with the unaided eye, against the horizon during the day (6 miles / 10 km on a clear day) or at which an object could be seen and recognized during the night if a moderately intense light source was raised to the normal daylight level. In general, the clarity with which an object can be seen.

Volcanic cloud

Convoluted rolling mass of water vapor, ash and dust, highly charged with electricity that hangs over a volcano during an eruption.


Any flow that possesses vorticity.


Rotational circulation of air about an axis, with an arbitrary orientation. Vector measure of local rotation in a fluid flow, defined mathematically as the curl of the velocity vector, relating vorticity vector and wind vector. Vertical component of the vorticity vector defined above. This is the most used sense.