Outside and far from the tube, the liquid reaches a ground level in contact with the atmosphere. Liquids in communicating vessels have the same pressures at the same heights, so a point , inside the tube, at the same liquid level as outside, would have the same pressure . Yet the pressure at this point follows a vertical pressure variation as
where  is the gravitational acceleration and  the density of the liquid. This equation means that the pressure at point  is the pressure at the interface plus the pressure due to the weight of the liquid column of height . In this way, we can calculate the pressure at the convex interface