As the vapor rate increases even more in the pipe during stratified flow, the top most layer begins to form waves. As the vapor rate increases, the amplitude of the waves increases. Wavy flow can occur when flow moves uphill, in horizontal pipe or in downhill flow. Most often, it is found in piping that is horizontal but it may occur in uphill flow. When the flow is moving downhill, wavy flow can still occur but the amplitude of the waves is not as great.