In oceanography, pycnocline is the layer of the water column where a rapid change in vertical density gradient occurs. This increase in density with increasing depth is generally more intense in low latitude regions, as at high latitudes the density has little variation with depth. The term pycnocline is analogous to the terms halocline (intense vertical salinity gradient) and thermocline (intense vertical temperature gradient). Generally the three features occur at the same depths, typically between 300 and 1000 m below the ocean surface. Pycnocline is the result of the combination of thermocline and halocline, as temperature and salinity influence the density of seawater. The relationship between these three factors (temperature, salinity and density) conditions the distribution of water bodies in the ocean.
When air resistance is equal to the sack's weight, it will no longer accelerate (so its acceleration will be zero) and will be falling at terminal velocity.
