A ridge is an elongated region of relatively high pressure or high geopotential height, drawn on a map as a northward bulge in the height contours and typically associated with warmer, less dense air in the column below. Warmer air is less dense, so any given pressure surface sits at a higher altitude within a ridge.
How a ridge behaves
Air in a ridge flows anticyclonically, curving clockwise around the crest in the Northern Hemisphere. Broad, gentle subsidence dominates through much of the ridge, which warms and dries the middle troposphere and reinforces the cap that suppresses convection at the surface.
The ridge itself amplifies when warm advection enters its lower half or when cold air is advected over its top, both of which increase the height gradient. Because it is generally a slow-moving feature, a strong ridge often anchors the pattern for days.
How forecasters read it
The most productive severe weather corridors often sit on the western edge of a summer ridge over the central United States. Northwest flow aloft rides over a moist surface layer, and shortwaves rotating over the top of the ridge provide focused lift into an unstable warm sector.
Directly under the ridge axis, however, forecasters expect a strong cap, weak flow, and few storms. When the ridge builds westward or eastward, it takes the storm corridor with it. Tracking ridge axis position from day to day is often the first cut at where the action will be.
Where the pattern misleads
A ridge that sits unmoved for a long stretch can produce prolonged heat and drought under its axis, and those effects are their own hazard. Fire weather concerns often align with a persistent ridge because subsidence and low humidity stack together.
A collapsing or retreating ridge can also produce a rapid transition to active weather as the cap breaks and the jet returns overhead. A ridge should be read as a moving boundary of a corridor, not as a permanent shield.