A boundary is a narrow zone across which air properties change quickly, whether that is temperature, moisture, density, or wind, and where converging flow can lift air enough to initiate or organize storms. Fronts, drylines, outflow boundaries, sea breezes, and terrain-forced wind shifts are all boundaries in this sense.
Why boundaries lift air
When two air masses meet, the denser one wedges under the less dense one and forces the lighter air upward. Even where the density contrast is small, converging winds on either side of the line pile air together and squeeze it out of the boundary layer in the only direction it can go, which is up.
The lift a boundary provides is usually only a few tens of centimeters per second, but it is sustained and it is exactly at the level where the boundary layer's most moist, warmest air lives. That combination is often enough to break a weak cap that broader synoptic lift cannot.
How forecasters read them
Forecasters look for boundaries in surface plots, satellite fine lines, and radar reflectivity or velocity. A thin arc of clear-air return on radar, a shift in wind barbs across a mesonet, and a temperature or dewpoint discontinuity together mark a boundary worth watching.
Where two boundaries intersect, the local convergence is stronger and the low-level flow can turn sharply. These intersections tend to be the first places storms initiate in a marginal setup, and they can enhance low-level rotation in storms that cross them.
Important limits
Boundaries move, decay, and re-form on their own schedule. An outflow boundary from morning storms may still be lifting air by afternoon or may be gone by then, and a dryline can push east all day and retreat west after sunset. Position at initiation time matters more than position at forecast issue time.
A boundary that focuses storms also constrains them. Storms riding the cool side of an outflow boundary feed on modified air and often behave differently than storms on the warm side. Knowing which side a cell is on changes how it should be interpreted.