When wind blows against a mountain, it's impeded — the mountain blocks it — Page 167, Lesson 144

I want to walk you through two more local wind effects that are important for your understanding of low-level flying conditions: valley or ravine winds, and katabatic winds. Let's start with valley winds. When wind blows against a mountain, it's impeded — the mountain blocks it. But if that barrier is broken by a gap or a valley, the wind will blow along the valley at an increased speed. Why? Because the same volume of air is being forced through a narrower opening, so it accelerates. That's the restriction effect. I want you to look at Figure 10.27, which illustrates this. Now here's an interesting point: if there's only a relatively small change in the general wind direction, it's possible for the valley wind to reverse completely. Figure 10.28 shows this reversal. The combination of high wind speed and rough terrain is likely to give rise to considerable turbulence at low level. That means landing at airfields in such areas may be difficult — something you need to anticipate as a pilot. There are named examples of valley winds you should know: the Mistral in the Rhone Valley, the Genovese in the Po Valley, the Kosava along the Danube, and the Vardarac in the Thessalonika area. Valley winds also occur in fjords. Now let's move to the Venturi Effect. The increase in speed as wind flows through a valley causes the Venturi Effect, with a consequent reduction in pressure. That pressure drop has a practical consequence for you as a pilot: the true altitude will be less than the indicated altitude. So your altimeter will over-read in these conditions. The same effect can be experienced above a mountain range as the wind blows over the range, particularly in stable conditions. Now let's talk about katabatic winds. A katabatic wind is caused by a flow of cold air down a hill or mountain side at night. Here's how it works: if the side of the mountain is cooled by radiation — that is, it loses heat to space on a clear night — the air in contact with that slope is also cooled. That cooled air becomes denser and heavier than the surrounding air, so it flows down the mountain side under gravity. The katabatic effect is most marked under three conditions: first, if the mountain side is snow-covered; second, if the sky is clear to assist radiation cooling; and third, if the pressure gradient is slack — meaning the large-scale pressure differences are weak, so the local drainage flow dominates. Speeds average about 10 knots. The flow of cold air into the valley helps frost and fog to form. Another effect is that with the sinking of cold air down the slope, the air at higher levels will be warmer, and an inversion results — a temperature inversion where warmer air sits above colder air near the surface. The katabatic effect can also occur by day when relatively warm air comes into contact with snow-covered slopes. Figure 10.29 shows katabatic wind formation. An example of a katabatic wind you should know is the Bora in the Northern Adriatic, which we'll cover in more detail in Chapter 21. So to summarise: valley winds are accelerated flows through gaps, with possible reversal and turbulence; the Venturi Effect from that acceleration causes altimeter errors; and katabatic winds are cold air drainage down slopes at night, averaging 10 knots, promoting frost, fog, and inversions.