How is underwater pressure affected by ice? Let's say I have a bucket of freshwater exposed to an environment where ice can form (less than 0C, normal atmospheric pressure, etc.). The bucket is on the ground so ice will only form on the top and sides. How is the pressure underwater affected by this ice growth?
 A: To a very good approximation, the pressure near the bottom of the bucket will not be impacted by the freezing of the water at the surface so long as the ice doesn't form a sheet which adheres to the sides of the bucket restricting its movement up and down as things expand/contract. The pressure at the bottom of the bucket will be the sum of two pressure terms which are a function of the thickness of each layer, the density of each layer, and the localized acceleration due to gravity. One term will correspond to the layer of ice and one corresponds to the liquid water. As the water at the top of the bucket freezes its density will decrease but the height of those freezing water molecules once frozen will increase with a 1:1 proportionality to the change in density meaning the contribution to the overall pressure from those frozen molecules remains unchanged as those two changes offset. The overall pressure contribution from the water molecules which didn't freeze of course remains unchanged as well.
