Subsidence is broad, gentle descent of air in the atmosphere. As the sinking air is compressed, it warms adiabatically and dries out, and it commonly builds a stable layer or capping inversion above the boundary layer.
The physics
An air parcel that descends encounters higher pressure. If it is unsaturated, it warms at the dry adiabatic lapse rate of about 9.8 degrees Celsius per kilometer. Because its water vapor content does not change while its temperature rises, its relative humidity falls sharply. Subsidence therefore warms and dries every parcel it acts on.
In synoptic-scale weather, subsidence is driven by upper-level convergence and by mass balance around ridges and high-pressure systems. NWS training materials note that in a deep high, the strongest sinking motion is typically in the middle troposphere, often between about 850 and 600 mb, and that this layer is where the subsidence inversion tends to appear.
How subsidence shows up in forecasting
The most familiar operational fingerprint of subsidence is the cap. A warm, dry layer aloft, sitting above a cooler, moister boundary layer, prevents parcels from rising into deep convection until surface heating or forcing can overcome it. Elevated mixed layers advected off elevated terrain, especially the Mexican Plateau, are a related product of subsidence and daytime mixing at the source region.
On a Skew-T, subsidence is visible as a layer where temperature increases (or barely decreases) with height and dewpoints drop off sharply. On visible satellite, it appears as clear or nearly clear skies inside a ridge, with only shallow cumulus over land and stratocumulus decks over cool oceans.
Where subsidence misleads
Not all sinking motion is created equal. A shallow, transient subsidence signature during the morning may be gone by afternoon, while a deep, persistent subsidence layer under a strong ridge can suppress storms across a multi-state area for days. Reading the depth and persistence of the subsidence, not just its presence, is what makes the forecast useful.
It is also easy to overstate how well a cap will hold. Capping inversions built by subsidence look impressive on paper but can rupture along a mesoscale boundary, over a dryline bulge, or under the nose of a strong low-level jet. When they do, the instability stored beneath them often erupts into intense convection, which is why capped days sometimes turn into some of the more explosive severe weather events.