Turbulence — Page 104, Lesson 95

I want to walk you through the topic of turbulence, starting with the different types and their causes. We'll begin with thermal turbulence. Insolation — that's the solar radiation reaching the Earth's surface — gives rise to convection currents. The intensity of these currents depends entirely on how much the surface is heated. Surfaces like rock and concrete heat up rapidly, and they produce strong vertical currents. Grass and wooded areas, on the other hand, heat only slowly, so they create weak convection currents. What this means for you as a pilot is that flight within the friction layer — the lowest part of the atmosphere where the wind is affected by the Earth's surface — on a sunny day will be affected by variable-speed vertical currents. Those vertical currents produce windshear, and that windshear gives you turbulence. Thermal turbulence is greatest around 1500 hours local time on clear, sunny days — that's when the sun has had the most time to heat the ground. There is no thermal turbulence over the sea, because the sea surface temperature doesn't vary enough to produce those strong convection currents. Now let's move to mechanical turbulence. This is caused by physical obstructions to the normal flow of air. The obstructions include hills, mountains, coasts, trees, and buildings. When the wind hits these obstacles, it gets disrupted — the smooth flow breaks into eddies and chaotic motion, which is what we call mechanical turbulence. Next, I want to cover mountain waves, which may also be referred to as standing waves or lee waves. These are a specific type of wave motion that forms downwind of a mountain range. For mountain waves to form, three conditions must exist simultaneously. First, the wind direction must be perpendicular to the mountain range, within plus or minus 30 degrees, and there must be no significant change in wind direction as altitude increases. Second, the wind speed at the summits — at the top of the mountains — must be at least 15 knots, and the wind speed must increase as altitude increases. Third, there must be a marked layer of stability around the altitude of the summits. That stable layer could be an isothermal layer — where temperature doesn't change with height — or an inversion, where temperature actually increases with height. Above and below that stable layer, the air must be less stable. These conditions together allow the air to flow over the mountains and then oscillate in waves on the downwind side, which can produce moderate to severe turbulence. The low-level effects of mountain waves and other phenomena are discussed in the Aeronautical Information Circular, or AIC, at the end of this chapter — that's a document you'll refer to for operational guidance. So to summarise: thermal turbulence comes from surface heating and peaks around 1500 on sunny days, with no thermal turbulence over the sea. Mechanical turbulence comes from physical obstructions like hills and buildings. And mountain waves — also called standing waves or lee waves — require wind perpendicular to the range, at least 15 knots at summit level increasing with height, and a stable layer at summit altitude with less stable air above and below.