Atmospheric tides

(geophysics) Periodic global motions of the earth's atmosphere, produced by gravitational action of the sun and moon; amplitudes are minute except in the upper atmosphere.

Those oscillations in any or all atmospheric fields whose periods are integral fractions of either lunar or solar days. Oscillations with a period of a day are called diurnal, with a period of a half day semidiurnal, and with a period of a third of a day terdiurnal. The sum of all tidal variations is referred to as the daily variation. As a practical matter, the subject of atmospheric tides is generally restricted to oscillations on a global spatial scale (thus excluding sea breezes). The bulk of attention is devoted to migrating tides, which are those tidal oscillations that depend only on local time.

Atmospheric tides tend to be rather small in the troposphere, although the tidal oscillations in rainfall are surprisingly large. Their importance stems from two primary factors: (1) Tidal oscillations tend to increase in amplitude with height and become major components of the total meteorology above about 50 km (30 mi). (2) The subject has played a prominent role in the intellectual history of meteorology, and it still provides a remarkable example of scientific methodology in an observational science. Tides are unique among meteorological systems in that they have perfectly known periods and relatively well known sources of forcing.

The determination of an oscillation by means of data requires at least two measurements per period. Since most meteorological upper air soundings are taken only twice each day, such data can be used only to marginally determine diurnal oscillations. Occasionally, stations obtain soundings four times per day, which in turn permits determinations of semidiurnal oscillations. Rain gages assign rainfall to specific hours, and averages over many years allow the determination of the daily variation of rainfall. Surface pressure is monitored at a great many stations with effectively (from the point of view of tidal analyses) continuous time resolution. Therefore, surface pressure has traditionally been the field most thoroughly analyzed for tides.

The lunar semidiurnal tide in surface pressure is similar in distribution to the migrating part of the solar semidiurnal tide but only about one-twentieth its strength; maximum lunar semidiurnal surface pressure typically occurs about 1 lunar hour and 13 lunar hours after lunar transit. Clearly, the solar semidiurnal tide dominates the surface pressure. The solar diurnal component is not only smaller but also far more irregular.

Rainfall is commonly observed to have a daily variation. The diurnal component, though often quite large, has a very irregular phase; on the other hand, the solar semidiurnal component is surprisingly uniform, amounting to about 10–20% of the mean daily rainfall in the tropics with maximum rainfall at about 4 A.M. and 4 P.M. local time. Maximum semidiurnal rainfall appears to occur somewhat later in middle latitudes. See also Precipitation (meteorology).

Data become more sparse when attempts are made to analyze tides above the surface. Analyses of radiosonde wind measurements have shown that solar semidiurnal oscillations in horizontal wind are primarily in the form of migrating tides. Diurnal oscillations, on the other hand, are significantly affected by regional, nonmigrating components up to at least 20 km (12 mi). Above this height, the diurnal oscillations also tend to be dominated by migrating components. There is a tendency for the diurnal oscillations to have more phase variation with height, especially at low latitudes. As a rough rule of thumb, oscillations in temperature tend to have magnitudes in kelvins comparable to the amplitudes in wind in meters per second. There is also no longer a clear dominance of the semidiurnal oscillations over the diurnal oscillations once the upper-level fields are considered. The amplitude increase with height renders the detection of tidal oscillations at greater altitudes somewhat easier since the tides are becoming a larger feature of the total fields. See also Oscillation.

While the classical theory of atmospheric tides is adequate for many purposes, recent years have seen a substantial development of theory well beyond the classical theory to include the effects of mean winds, viscosity, and thermal conductivity. See also Atmosphere; Earth tides.