A downdraft is a column of air sinking through or near a thunderstorm. It is driven by the weight of precipitation, cooling from evaporation and melting, and downward momentum carried from mid-levels.
What drives the sinking air
A parcel becomes negatively buoyant when it is cooler and denser than its surroundings. Three mechanisms produce that condition inside a storm. Precipitation loading adds the weight of rain, graupel, and hail to the parcel. Evaporation of raindrops falling into drier subcloud air removes heat. Melting of ice near and below the freezing level does the same.
In sheared environments, a fourth mechanism matters. A descending parcel can carry the horizontal momentum of the mid-level winds down with it. When the parcel reaches the surface, the transported momentum reinforces the outflow speed and biases the strongest winds in the direction of the mid-level flow.
How forecasters anticipate downdraft strength
Downdraft CAPE, or DCAPE, estimates the negative buoyancy available to a parcel descending from mid-levels. Large DCAPE with a deep, dry subcloud layer favors strong, cold downdrafts. Steep low-level lapse rates and mid-level dry intrusions increase the potential for surface wind damage.
On radar, descending reflectivity cores, mid-level convergence signatures, and pronounced rear-inflow notches on line segments all suggest that a downdraft is accelerating toward the ground. Forecasters watch these signatures together, not in isolation.
Where the picture misleads
Downdraft potential is not evenly distributed across the storm. A thunderstorm can have a benign forward-flank downdraft and a violent rear-flank downdraft at the same time. Reading a single downdraft parameter without asking which part of the storm it applies to hides that spatial contrast.
A downdraft that never reaches the surface still shapes the storm. Mid-level cooling changes the buoyancy field that the updraft is competing with, and the resulting adjustments can either strengthen or shorten the storm's life.
Wet-microburst and dry-microburst environments produce downbursts through different balances of these mechanisms. A dry microburst leans on evaporation into a deep, dry subcloud layer, while a wet microburst leans on precipitation loading beneath a moist boundary layer. Both can be dangerous, so an absence of visible rain is not, by itself, a sign that the ground below is safe.