Unoccupied Cars Sliding Backwards in Paradise Lot

Excellent video Amar!
Big brother is watching at all times.
I once had an incident at Crystal where the car in front slid back and did some damage to my vehicle. I took several pictures of the contact and their insurance company tried to deny fault saying it was an act of God since no one was operating the vehicle. I called BS on that!

I love it !   I did think one time that something had changed, but discarded the thought as 'pre-senior'   

Perhaps you have found evidence of the subtle yet irresistible draw of the Tatoosh.

I feel it sometime when I am laying in bed.

Well done! I expect to see the PRL within the month. . That effect is remarkably large.

Amar -

Quick, delete this thread...the NPS risk management folks will never open the gate to access the parking lot if they catch wind of this. 

How am I going to write a PRL if I don't really know what's causing the motion?

But after pondering it some more today, I think that pressure melting and regelation are almost certainly involved. Ice melts when pressure is applied to it by some object (such as underneath an ice skate blade), and refreezes when the pressure is released. The melting produces a thin film of water which lubricates the interface between the pressing object and the ice.

However, the effect is very tiny, with only a 0.0072 °C reduction in melting point per atmosphere of applied pressure. The pressure on the surface under the tires of a car is roughly equal to the air pressure in the tires, about 30 psi or 2 atm. This means that pressure melting can only occur in this situation at temperatures within about 0.014 °C below 0 °C (0.025 °F below 32 °F).

This would explain why the cars only slide on certain days when the ice temperature is hovering right near the melting point. On colder days, there can be no pressure melting and thus no lubricating film of water can form. On slightly warmer days, the ice will quickly soften to slush before the cars have a chance to slide much. The cars need a firm surface of ice, lubricated by meltwater, in order to slide an appreciable distance. Otherwise, this would be a much more commonly observed phenomenon.

The observed rates of cars sliding are very small, about 1-10 mm per minute in these webcam images. Obviously, the effect should be much larger and more noticeable in steeper sloped parking lots, but also less surprising than when it happens on a nearly-flat lot like Paradise.