definite decrease in height over the grass
Speculation: Perhaps the reason for this is that grass has a lesser surface friction (smooth) than does concrete (rough).
_____________
A thought that just keeps bubbling up to the surface.
- Meteorological charts show pressure and each line represent a difference of 4 hectopascals (hPa)
- Tire pressure is measured in PSI.
- A tire pressure of 36 PSI = 2,500 hPa
- We tend to think of pressure in the context of the much higher tire pressure.
- A meteorological pressure difference of a few hectopascal can create a wind.
- This wind can remove the 'whats the thing' out from under a helicopter when it is hovering in ground effect over a fixed point.
- I would suggest that meteorically induced air pressure is blowing away the rotor induced air pressure.
Perhaps aerodynamists do not talk in terms of pressure because 99% of their computations relate to aerodynamic activities that have nothing to do with the ground.
However; pressure is pressure.
Dave