Atmospheric pressure
Atmospheric pressure.. maybe
Downforce
Downforce
The atmospheric pressure at that place and at that time.
Yes, pressure increases with depth as a result of the weight of water above a given depth. Weight is caused/determined by gravity.
WEIGHT
The measurement will be in terms of force or weight just as the measurement of the force holding a person to the earth is given by their weight.
You cannot. You do not have the necessary information.
The amount of force is called stress.
The Amount Of Force
What do you mean "... based on"? The athmosperic pressure is "weight of air" and it can be measured. Strikly speaking: "force per unit area exerted against a surface by the weight of air above that surface at any given point" (wikipedia) Ap could be given in force/area units for example lb/sq ft, kg/sq meter etc.
If you know the force of gravity then mass = weight/gravitational force. If you don't then you cannot. Knowing the volume is useless.