The predominantly horizontal transport of an atmospheric property by the 
wind, e.g. 
moisture[?] or 
heat advection.  Mathematically it is the operator (in 
z and 
p vertical coordinates)
- <math>\mathbf{v} \cdot \nabla = u \frac{\partial}{\partial x} + v \frac{\partial}{\partial y} + w \frac{\partial}{\partial z} = u \frac{\partial}{\partial x} + v \frac{\partial}{\partial y} + \omega \frac{\partial}{\partial p}</math>.
 
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