Like its daytime counterpart, the mesoscale circulation of air associated with the down-valley wind is superimposed on the mountain-valley circulation driven by the nocturnal cooling of the mountain slopes. The convergence of drainage winds into the valley below paves the way for a secondary mesoscale circulation of air that results in nighttime down-valley winds. To get a visual for how a cold-air avalanche can occur, check out this animation showing an idealized cold-air avalanche from the Cooperative Program for Operational Meteorology, Education, and Training (COMET) University Corporation of Atmospheric Research. That's really fast in comparison to typical cases! Remarkably, cold-air avalanches can occur more than once during a single night. In special cases, cold-air avalanches, which occur when cold air builds up on plateaus and then spills downward into the valleys below, can attain speeds of several tens of meters per second. As you might expect, the down-valley wind tends to be stronger in deeper valleys. I note, however, that the speed of nighttime katabatic winds can sometimes exceed 8 meters per second (16 knots), depending on the local terrain (steepness, etc.) and prevailing weather conditions. For the record, the typical speed of drainage winds ranges from one to five meters per second (approximately 2-10 knots). Because drainage winds move relatively slowly, the rate of adiabatic warming is usually less than the effects of net radiational cooling. Of course, cold, dense air draining down mountain slopes does warm some by way of adiabatic compression during descent, but not enough to offset net radiational cooling. Clearly, the two circulations go hand in hand, with their formation optimized when synoptic-scale pressure gradients are weak and the early daytime sky is generally clear, which favors differential heating. In other words, the circulation associated with the up-valley wind is superimposed on the mountain-valley circulation. Note the up-valley wind at the surface and the counter flow of air aloft that help to complete a circulation of air whose axis parallels the valley axis. I emphasize here that there are really two distinct circulations of air during the daytime (check out the schematic below). By 'up-valley,' I'm implying that the valley floor isn't perfectly flat (which is true in most cases). The pattern of upslope winds associated with the mountain-valley circulation then creates an area of low-level divergence that, in turn, promotes an up-valley wind. Indeed, there's a secondary circulation that can develop, that results in something called an up-valley wind. With our daytime mountain-valley mesoscale circulation established, our story isn't quite finished, however.

As a general rule, the daytime upslope flow associated with the mountain-valley circulation is typically several hundred meters deep, and upslope speeds typically average about 3-6 meters per second (approximately 6-12 knots). I also point out that the mountain-valley circulation along initially shaded west-facing slopes begins later in the day (shaded north-facing slopes are pretty much exempt from this discussion). The compensating subsidence over the valley now explains the clearing we previously observed in the river valley south of the Black Sea. The end result is that a thermally direct mesoscale circulation develops (bottom schematic shown below). Over time, solar heating establishes a dry adiabatic lapse rate in a layer roughly one kilometer deep surrounding the mountain (the boundary layer) and air in the boundary layer becomes positively buoyant and starts to rise. Meanwhile, higher up in the air column, developing higher pressure sets the stage for air to move away from the mountain. Note, near the bottom of the air column, how the 900-mb pressure surface dips toward the mountain slope, indicating lower pressure (also notice that the 900-mb pressure surface remains relatively flat over the valley). Now focus your attention on the sample air column shown in the top schematic (below) and keep the generic animation you just reviewed in mind. In other words, the air in contact with the mountain is warmer than air not in contact with the mountain at the same altitude. In effect, this differential heating creates a local hot spot on the mountain slope. Incoming solar energy strikes east- and south-facing slopes sooner and more directly than the relatively flat, adjoining valley.

The set-up for a daytime mountain-valley circulation hinges on differential heating. In mountainous regions this terrain induced circulation is called a mountain-valley circulation. When the prevailing synoptic-scale pressure gradient is weak, the differential heating and cooling of the earth's surface often promotes circulations that run counter to the large-scale wind flow.

Mountain and valley breeze