Question

hydrostatic pressure in water varies significantly over small change in depth whereas the atmospheric pressure does not change unless the difference in altitude is very large explain why

Answer 1

Hydrostatic pressure is the pressure that is generated by the weight of liquid above a measurement point, when the liquid is at rest. The height of a liquid column, of uniform density, is directly proportional to the hydrostatic pressure.

The hydrostatic properties of a liquid are not constant and the main factors influencing it are the density of the liquid and the local gravity. Both of these quantities need to be known in order to determine the hydrostatic pressure of a particular liquid.

Answer 2

Since hydrostatic pressure is the pressure solely generated by a column of liquid, it maybe the case that the pressure measured includes the additional component of atmospheric pressure acting on the liquid surface.

In the cases of measuring absolute pressure it is necessary to also measure and subtract the atmospheric pressure to determine the true hydrostatic pressure.

How local gravity affects hydrostatic pressure

The local gravity depends on latitudinal position and height above sea level.

For convenience the most popular pressure units for measuring hydrostatic pressure are metres of water and feet of water at 4 deg C (39.2 degF) with a standard gravity of 9.80665 m/s2. The density of pure water at 4 deg C is very close to 1000 kg/m3 and therefore this has been adopted as the standard density of water. Another reason for the significance of choosing 4 deg C is that it is very close to the temperature that water reaches its maximum density.