The American Meteorological Society defines a down-valley wind as:
Although sometimes referred to as a mountain breeze, down-valley winds develop at night and are a component of the mountain-valley wind system that forms in response to daytime warming and nighttime cooling.
The series of images to the left shows this daily cycle. Once the sun comes up (upper-left panel), warming of the ridge-tops and peaks promotes upslope flow that is generally parallel to the slope and perpendicular to the valley. As daytime heating continues (upper-right panel), an up-valley wind parallel to the orientation of the valley forms in addition to the upslope flow.
Just after sunset (lower-left panel), radiational cooling begins just above the surface of the slope. If this radiational cooling continues unabated, the air may become colder and therefore denser than air lower in the valley. This negative buoyancy promotes the development of the downslope, or katabatic, wind. As the image shows, the downslope wind is generally parallel to the incline. This katabatic flow is relatively thin (10 to 100 meters) and typically reaches a speed of six to eight knots. Katabatic flow can be enhanced by the presence of snow cover.
Several hours after sunset (lower-right panel) if net radiational cooling has persisted, the down-valley breeze will form and may persist until sunrise. Down-valley breezes are typically more robust than downslope winds and may reach speeds of 16 to 20 knots. It is easy to see why this flow of cold air down the valley floor is sometimes referred to as the drainage wind.
The graphic below shows the interaction between the up-valley and upslope winds; and the down-valley and downslope winds. In the nighttime panel (right), note the upward motion associated with the downslope's interaction with the down-valley wind.
If it persists, a down-valley wind may continue to increase in height until it completely fills the valley.
Occasionally an outflow jet, reaching speeds of approximately 20 knots, may form where the down-valley wind spills onto the plain below.
The development and persistence of the down-valley wind (and the mountain-valley wind system) is dependent upon a weak synoptic pressure gradient. Strong synoptic-scale winds and the attendant mixing of momentum from aloft towards the surface, tend to disrupt the formation of the thermally forced flow that is the hallmark of the mountain-valley system. The strength of downslope and down-valley winds also tend to be strongest on clear, calm nights when radiational cooling is allowed to proceed with vigor.
A satellite image of the Grand Valley from Google Earth (below) has been annotated to show the likely flow of downslope wind (in blue) and down-valley wind (in red) in the Gunnison River valley. The southeasterly flow of air associated with the down-valley wind is consistent with the observed pattern at Grand Junction during the month of November.
Predicting the speed and direction of the synoptic-scale wind is a challenging proposition. Fortunately, output from numerical weather models and other computer guidance provide forecasters with a sporting chance. In stark contrast, predicting mesoscale wind patterns in such a complex environment as the Grand Valley is nearly impossible for a forecaster, particular one who is unfamiliar with the topography.
However, familiar or not, the WX Challenge requires each participant to submit a forecast for maximum sustained wind. Under conditions where the synoptic pattern is likely to control the wind, the forecaster can rely upon a multitude of resources, including observations of analogous locations upstream. In a valley location, however, the forecaster must anticipate the conditions that favor the development of down-valley flow that could potentially exceed the speed of the synoptic-scale winds and adjust his forecast accordingly. Failure to do so could result in an unwelcome accumulation of error points.
© 2005-2006 Mark A. Thornton