Most of this weeks background information is available in Chapter 5. Below is a summary of some additional information.

There are four causes of turbulence

1. Convection
2. Surface Roughness
3. Gravity Waves
4. Wind Shear

Convection: In convection, hot air rises freely becuase it is warmer than the environment. This is sometimes referred to as thermal turbulence. There are two basic types of atmospheric convection: forced and free. In forced convection the air is mechanically pushed, such as along fronts or mountains. In free convection, air moves vertically without any external help. Forced convection involves neutral or stable air. Free convection requires unstable air. Stable air reduces convection and thus has less thermal turbulence.

Surface Roughness: When a plane lands on a windy day we experience increased turbulence during the descent. These eddies are caused by surface friction or the roughness of the surface. The degree of turbulence depends on the wind speed, stability, and the roughness of the surface. This type of turbulence is also referred to as mechanical turbulence.

Atmospheric Gravity Waves: Periodic motions can result when stable air is moved upward or downward. Since the vertical motion is in the vertical, gravity plays a major role in restoring the displaced air to is original location. Theiwave-like oscillations move away from the original point of disturbance and are known as atmospheric gravity waves. They often occur in clear skies and so are difficult to see.

Wind Shear: Wind shear is a gradient in the wind speed and/or wind direction over some distance. Horizontal wind shear is a change in wind over a horizontal distance. Vertical wind shear is a change in wind over a vertical distance. Vertical wind shear occurs near the surface due to friction and in the vicinity of certain stable layers. Horizontal wind shear near the ground is caused by the wind interacting with the surface. Away from the surface, horizontal wind shear is caused by fronts, jet streams and strong convection.

There are four classifications of turbulence:

1. Light- Unsecured objects will stay at rest. A half-filled cup of coffee won't spill.
2. Moderate- Unsecured objects move about; passengers will wake-up if they are asleep, and a half-filled cup of coffee will spill.
3. Severe - Passengers who aren't buckled in fumble around and try to buckle-up as they are bounced about avoiding your coffee cup which is bouncing around.
4. Extreme - Control of aircraft might be impossible, let alone a control over a cup which once contained coffee.

Remember, initially a parcel has the same temperature as air around it. When we raise the parcel the air pressure is lower; molecules push on "sides" to expand parcel - this is work; less energy (kinetic energy) cooler temperature. Rising parcels of air always cool due to expansion. Sinking or subsiding air always warms by compression.