Help Needed: What causes this effect?

Lift

A bit of aerodynamics, a touch of thermodynamics and pinch of meteorology here.

The aeros’ first:

In order to produce lift the wing must produce a pressure differential between the upper and lower surfaces. The section of the wing accelerates the airflow over the upper surface and thus creates a lower pressure than the lower surface.

Now the thermo’s:

If you lower the pressure of a gas you lower its temperature. This converse of this is rather easier to demonstrate, think of how warm a bicycle pump becomes as you use it. The temperature rises as you repeatedly compress the air in the body to inflate the tyre.

The met’ bit:

The atmosphere contains water vapour in varying amounts at all times. The amount of water vapour present will govern the temperature at which it becomes visible moisture, i.e. fog. This temperature is the “Dew Point” and it the point at which the air becomes “saturated”. On the day the photo was taken the ambient temperature would have been fairly close to the Dew Point and the small temperature drop over the upper surface of the wing was sufficient to saturate the air and produce a local “fog” The pressure drop also plays a part as the lower the pressure the lower the Dew Point for a given air mass.

The Flap vortices are produced by the mixing of the upper and lower surface flows at the tip of the flap. The pressure differential gives rise to a rotating vortex. This again produces a presure drop and leads to a local saturation, the visible vortex.

The above is very simplified explanation of the mechanisms involved. A further frequently seen example of this is the mist that forms in the intake if a jet engine as it spools up to takeoff thrust on a damp day.

exmpa