Terrain features give rise to their own special meteorological conditions. On warm, sunny days, the heated air in a valley is laterally constricted, compared with that over an equivalent area of lowland, and so tends to expand vertically. The volume ratio of lowland/valley air is typically about 2 or 3:1 and this difference in heating sets up a density and pressure differential, which causes air to flow from the lowland up the axis of the valley. This valley wind (Figure 6.10) is generally light and requires a weak regional pressure gradient in order to develop. This flow along the main valley develops more or less simultaneously with anabatic (upslope) winds, which result from greater heating of the valley sides compared with the valley floor. These slope winds rise above the ridge tops and feed an upper return current along the line of the valley to compensate for the valley wind. This feature may be obscured, however, by the regional airflow. Speeds reach a maximum at around 14:00 hours.
At night, there is a reverse process as denser cold air at higher elevations drains into depressions and valleys; this is known as a katabatic wind. If the air drains down-slope into an open valley, a 'mountain wind' develops more or less simultaneously along the axis of the valley. This flows towards the plain, where it replaces warmer,
Figure 6.10 Valley winds in an ideal V-shaped valley. (A) Section across the valley. The valley wind and anti-valley wind are directed at right angles to the plane of the paper. The arrows show the slope and ridge wind in the plane of the paper, the latter diverging (div.) into the anti-valley wind system. (B) Section running along the centre of the valley and out on to the adjacent plain, illustrating the valley wind (below) and the anti-valley wind (above).
Source: After Buettner and Thyer (1965).
less dense air. The maximum velocity occurs just before sunrise at the time of maximum diurnal cooling. As with the valley wind, an upper return current, in this case up-valley, also overlays the mountain wind.
Katabatic drainage is usually cited as the cause of frost pockets in hilly and mountainous areas. It is argued that greater radiational cooling on the slopes, especially if they are snow-covered, leads to a gravity flow of cold, dense air into the valley bottoms. Observations in California and elsewhere, however, suggest that the valley air remains colder than the slope air from the onset of nocturnal cooling, so that the air moving down-slope slides over the denser air in the valley bottom. Moderate drainage winds will also act to raise the valley temperatures through turbulent mixing. Cold air pockets in valley bottoms and hollows probably result from the cessation of turbulent heat transfer to the surface in sheltered locations rather than by cold air drainage, which is often not present.
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