Divergence is a net outflow of air from an area, and depending on where in the column it occurs it either pulls air upward from below or forces air downward from above. It is the mirror image of convergence, and together the two describe how horizontal wind patterns generate vertical motion.
How divergence produces vertical motion
At any level in the atmosphere, air that flows out of a column faster than it flows in creates a mass deficit. That deficit has to be filled. Upper-level divergence removes air aloft, and the atmosphere responds by drawing air up from below. Low-level divergence spreads air out at the surface, and air must sink from above to replace it.
The classic severe weather setup pairs low-level convergence with upper-level divergence over the same region. The low-level inflow supplies mass and moisture. The upper-level outflow evacuates it. Together they produce sustained ascent through a deep layer of the atmosphere.
How forecasters read upper-level divergence
Jet streaks are the most familiar source of upper-level divergence. As air enters a jet maximum, it accelerates, and as it exits, it decelerates. That speed change forces ageostrophic circulations that produce divergence in the right-entrance and left-exit quadrants. Storms that fire under a favorable jet-streak quadrant often organize more efficiently than those without upper support.
Shortwave troughs and difluent flow patterns aloft can also produce divergence. Water vapor imagery is useful here, because rising motion and drying signatures at upper levels help mark where mass is being removed from the column.
Where it misleads
The four-quadrant jet-streak model is an idealization. Real jets curve, tilt, and interact with other features, so the divergence pattern rarely lines up as cleanly as a textbook diagram suggests. Verify with observed data before leaning on it.
Upper-level divergence also does not, by itself, force a storm. It provides broad synoptic-scale ascent that helps destabilize the column and moisten the mid-levels. A boundary or another mesoscale trigger is almost always still needed to actually initiate convection.