ATPL · Meteorology · Meteorology Atpl Ground Training Series 2014 lesson — Page 167, Lesson 146

I want to walk you through two important local wind phenomena that every pilot needs to understand: anabatic winds and föhn winds. Let's start with anabatic winds. On a warm sunny day, the slope of a hill becomes heated by insolation — that's the sun's radiation heating the ground directly. This effect is especially strong on a south-facing slope in the northern hemisphere, because that side gets the most direct sunlight. The air in contact with that heated ground is warmed by conduction — heat transfers from the warm ground into the air touching it. That warmed air becomes less dense and rises up the hill. As that warm air lifts away, free cold air from the surrounding area flows in to replace it. This creates a continuous cycle: air warms, rises up the slope, and cold air moves in to take its place. The result is a light wind blowing up the hillside. We call this an anabatic wind. It's a light wind — typically around 5 knots — that blows up a hill or mountain during the daytime. You can see this formation illustrated in Figure 10.30, which shows the air warmed by conduction rising, cold free air replacing the lifted air, and clouds forming above the hilltop. Now let's move to a completely different phenomenon: the föhn wind. This is a warm, dry wind that blows on the downwind side of a mountain range. It's a local wind in the Alps, but similar winds exist elsewhere. For example, on the east side of the Rocky Mountains in Canada, it's called the Chinook. In South America, to the east of the Andes, it's called the Zonda. These are all the same basic mechanism under different regional names. Here's how the föhn effect works. When moist air is forced to rise up a mountain in stable conditions, it cools adiabatically — that means it cools because it expands as pressure decreases with altitude, with no heat exchange with the surroundings. Initially it cools at the Dry Adiabatic Lapse Rate, or DALR, until it becomes saturated. Once saturated, it continues cooling, but now at the Saturated Adiabatic Lapse Rate, or SALR, which is slower because latent heat is released as water vapor condenses. During this ascent, precipitation occurs, which removes water from the air. As a result, the dew point — the temperature at which the air becomes saturated — decreases. Now here's the key. When the air descends on the leeward side — that's the downwind side — the cloud base is higher than it was on the windward side. So the air warms at the DALR over a greater height than it cooled at the SALR on the windward side. Because the DALR is a faster rate of temperature change than the SALR, the air ends up warmer at the base of the mountain on the downwind side than it was on the upwind side. The result is a warm, dry wind blowing down the lee side of the mountain. Temperature increases in excess of 10°C can occur. On the windward side, you can expect low cloud and precipitation. On the leeward side, you'll see clear but turbulent conditions. The presence of a föhn wind can also indicate the presence of mountain waves — those are standing wave patterns in the airflow downwind of the mountain, which can create significant turbulence and even rotor clouds. Föhn winds aren't just an Alpine phenomenon. They can occur over the east and west coasts of Scotland when moist winds come over the highlands off the Atlantic Ocean or the North Sea. So this is directly relevant to flying in the UK and Ireland. Take a look at Figure 10.31, which illustrates the föhn effect with temperatures at different altitudes. You can see on the windward side, the cloud base is lower, precipitation is falling, and the temperature profile shows cooling at the SALR above the cloud base. On the leeward side, the cloud base is higher, and the temperatures at corresponding altitudes are warmer — notice at ground level on the leeward side it's +20.8°C, while on the windward side at the same elevation it's only +1°C. That's the föhn effect in action.