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Lunar Phase Affects Rainfall

03.02.2016 13:57 Age: 1 year

Click to enlarge. Satellite data over the tropics, between 10 degrees S and 10 degrees N, shows a slight dip in rainfall when the moon is directly overhead or underfoot. The top panel shows the air pressure, the middle shows the rate of change in air pressure, and the bottom shows the rainfall difference from the average. The change is 0.78 micrometers, or less than one ten thousandth of an inch, per hour. Courtesy: University of Washington


by Hannah Hickey, University of Washington


Classical tidal theory predicts that the lunar gravitational semidiurnal tide (L2) should induce perturbations in relative humidity (RH). Adiabatic expansion in divergent flow in advance of the L2 pressure minimum cools the air and reduces its saturation vapor pressure, thereby increasing the rate of condensation in saturated air parcels and causing the relative humidity (RH) of unsaturated parcels to rise. Here we detect a statistically significant L2 signature in precipitation rate (P) in a 15 year, eight times daily, global precipitation data set acquired in the Tropical Rainfall Measuring Mission. Consistent with tidal theory and with the notion that L2 modulates P mainly by perturbing RH, P varies out of phase with pressure, and it increases at a rate of about 10% per 1% increase in RH. These measurements thus provide a measure of the sensitivity of P to planetary-scale changes in RH. Analysis of solar semidiurnal (S2) tidal statistics yields similar results.


Rainfall variations induced by the lunar gravitational atmospheric tide and their implications for the relationship between tropical rainfall and humidity by Tsubasa Kohyama John (Michael) Wallace published in Geophysical Research Letters, DOI: 10.1002/2015GL067342

Read the abstract and get the paper here.


University of Washington news release here.