Vertical wind shear is the derivative of the wind field with height. It is often expressed as the magnitude of the vector difference between the horizontal winds at two specified altitudes or bulk shear. We will do so here as well although strictly speaking it is incorrect. Wind shear has an important influence on deep convection because...

it plays a role in the generation of new convective cells along the outflow of an older cell and hence influences the propagation of a storm system

it affects vertical speeds in storm updrafts and downdrafts, by causing pressure perturbations. This is especially important in supercell storms (see below).