Terdiurnal Surface-Pressure Oscillations over the Continental United States

The small terdiurnal pressure oscillation S3(p) is determined over the conterminous United States by analyzing long time series of hourly barometer data from 180 stations. Spectral analysis of these time series reveals that the terdiurnal band is dominated by three or four spectral peaks, separated in frequency by 1 cpy. The central peak at 3 cpd is invariably smaller than the two immediate side peaks, indicative of extraordinarily strong seasonal variations in the tide. The largest terdiurnal tide occurs over the south-central United States in winter where amplitudes exceed 300 μbar. Summertime amplitudes are roughly one-half as large. Summer and winter tides are almost completely out of phase, with rapid 180° shifts occurring in the equinox seasons when amplitudes are very small.