Katabatic and Anabatic Winds
Anabatic winds flow upslope when sunshine warms a slope and the adjacent air, while katabatic winds flow downslope when air over elevated terrain cools, becomes denser and drains under gravity. They often reverse through the daily cycle, but snow cover, clouds, terrain shape and larger weather systems can change or…
Timeline
- 2026-10-12: Current National Weather Service definitions, training material and forecaster references were rechecked.
- 2026-10-11: Primary and authoritative sources were reviewed for this guide.
Anabatic and katabatic winds are local flows along sloping terrain. Anabatic means upslope: sunshine heats a mountain face, the air touching it becomes warmer and less dense than air at the same elevation away from the slope, and it rises along the terrain. Katabatic means downslope: air over an elevated surface cools, becomes denser and drains downhill under gravity. The words identify direction and the thermal mechanism, not a fixed wind speed. [1][2][4]
The classic anabatic cycle starts after sunrise on a clear day. Sun-facing rock, soil and vegetation warm faster than the free air, producing an upslope pressure difference and a valley-breeze circulation. Air drawn toward higher terrain may help build cumulus clouds or showers when enough moisture and instability are present. Slope orientation matters, so an east-facing slope can begin its upslope flow before a shaded west-facing slope nearby. [1][2][3]
After sunset, a slope radiates heat and cools the air directly above it. That air becomes relatively dense and slides into hollows and valleys as a katabatic or mountain breeze. Repeated drainage can create a cold pool on the valley floor, allowing frost or fog even while higher slopes remain warmer. Snow and ice enhance surface cooling and can support downslope flow during daylight, so katabatic does not strictly mean a wind that occurs only at night. [1][2][3]
Slope winds and valley winds are related but the terms describe different parts of a terrain circulation. An anabatic flow climbs the sidewall, while a daytime valley wind commonly moves up the valley axis; at night, katabatic drainage descends slopes and may feed a down-valley mountain breeze. Terrain channels and merges these currents, which is why observations on a ridge, slope and valley floor can show different directions at the same time. [2][3][4]
Not every wind blowing downhill is a simple katabatic wind. Foehn or Chinook winds can descend, warm and dry through compression after crossing a mountain barrier, and severe downslope windstorms can involve mountain waves and strong pressure gradients. Some regional wind names combine several processes. Calling all of them katabatic hides the difference between cold gravity drainage and a dynamically forced, potentially warm downslope wind. [1][4]
The larger weather pattern can weaken, reverse or mask the daily cycle. Strong gradient wind may mix away a shallow drainage layer, while stable air can confine it close to the surface. Clouds reduce daytime heating and nighttime cooling. Gaps, gullies and narrowing valleys accelerate or redirect the flow, and rough terrain produces turbulence. A textbook sunrise-to-sunset reversal is therefore a useful starting model rather than a timetable for every mountain. [2][3][4]
These winds matter because they rearrange temperature, moisture, smoke and pollutants. Katabatic drainage can carry cold air and smoke into populated valleys and contribute to inversions; upslope flow can transport moisture toward cloud-forming levels. For aviation, changing wind across ridges and runways can produce shear or turbulence. For wildland fire, slope flow influences local spread, but crews must use official fire-weather information because terrain and the regional wind can change behavior quickly. [2][3][4]
To interpret a mountain forecast, note the slope's aspect, sun and shade, snow cover, cloud cover, stability, time of day and the regional pressure gradient. A light-wind clear day favors a recognizable anabatic-to-katabatic sequence; strong synoptic flow makes it less reliable. Nearby station readings may represent another elevation or exposure, so local observations and official forecasts are better evidence than applying the label from wind direction alone. [1][2][3][4]
Sources
- National Weather Service Columbia — Weather Terms
- National Weather Service — Prevailing Winds and Mountain or Valley Breezes
- National Weather Service Albany — StormBuster: Mountain and Valley Breezes
- National Weather Service — Forecasters Reference Book