Convergence is a net inflow of air into an area or along a line, and by mass balance it forces air upward when it happens near the surface. It is one of the primary ways the atmosphere translates horizontal wind changes into vertical motion.
The physics of convergence
Air is nearly incompressible on the scales that matter for severe weather. When more air flows into a column than flows out at the same level, the extra mass cannot pile up in place. It has to go somewhere.
Low-level convergence pushes that excess air upward. Upper-level convergence pushes it downward as subsidence. Convergence lines form wherever winds change direction or speed across a short distance. A cold front, dryline, sea breeze, or outflow boundary all show up in the wind field as a sudden shift, and along that shift the flow has nowhere to go but up.
How forecasters use it
Surface analyses highlight convergence axes as boundaries where new convection is most likely to initiate. Chasers position along drylines, cold fronts, and outflow boundaries for the same reason. Radar velocity products can identify convergence along the leading edge of a gust front or beneath the base of a maturing updraft.
Boundary intersections are especially productive. Where two convergence lines cross, the combined ascent is often enough to punch through a residual cap that neither boundary alone could break, and the locally backed low-level winds at the intersection can also help storm rotation once cells form.
Where it misleads
Convergence by itself does not make a storm. The rising air still needs sufficient moisture, sufficient buoyancy, and either a weak cap or enough sustained lift to break through. A vigorous convergence line in a strongly capped, dry environment produces a wind shift and little else.
Convergence is also scale-dependent. Broad synoptic-scale convergence at a few centimeters per second matters for the daily setup but does not initiate individual storms. It is the sharp, mesoscale convergence along a boundary that does most of the trigger work.