Internal waves in llyn Tegid

Summary The paper deals with the formation of internal waves of periods of about 12 hours, 24 hours, and in one case 48 hours on Lake Tegid (Bala) in North Wales. It was possible to isolate these waves by means of a numerical filter, applied to the outputs of 30 thermistors place below each other on a chain suspended from a buoy. The wave characteristics are very closely related to those of the wind and a high correlation can be found in most cases with the internal waves lagging behind the corresponding filtered components in the wind stress by about a third of a period.The movement of the waves round the lake was investigated by using two buoys, one of which was moved round the lake perimeter and then after using the filter, obtaining the phase difference between the two for the various components. The results indicated a clockwise movement which is contrary to what one would expect of simple Kelvin waves. The effect can be caused, however, by the interference of several waves of approximately the same frequency but of different mode and wavelength.The filter was also used to examine the vertical profile of the water movement. The profiles indicated, as expected, that there is a mixture of many modes present but some of these can be identified by comparison with the theoretical mode profiles.