Relative Humidity at Armagh Observatory, 1838-2008

C.J. Butler and A.M. García-Suárez

Figure 5. The wavelet power spectra for the mean seasonal and annual RH series: at 09:00 GMT, 1838-2008. The large panels show the power plotted against period (vertical axis) and epoch (horizontal axis). The right-hand panels show the power, integrated over all epochs. The dotted line shows the 95% confidence level (see Torrence and Compo, 1998 for details).

Abstract: We present a calibrated atmospheric humidity series for Armagh Observatory, Northern Ireland. Beginning in 1838 and continuing to the present day, this series is believed to be the longest presented so far. Daily wet-bulb and dry-bulb temperature readings are corrected for the time of reading and for known instrumental errors to determine mean monthly, seasonal and annual relative humidity (RH) at 09:00 GMT from 1838 to 2008 and at 21:00 GMT from 1844 to 1964.

The mean seasonal and annual time series of RH at this site show only marginal evidence for a trend in RH since the 1880s, however there is significant decadal and multi-decadal variability, some of which appears to be cyclic. Wavelet analysis indicates the presence of two quasi-periodic components in RH, one ranging from 23.4 to 25.5 years and the other from 36 to 51 years. We discuss potential physical origins of these cycles such as changes in the atmospheric and thermohaline circulation and the solar magnetic cycle. There is a small but significant anti-correlation between RH and northerly winds.

Two conspicuous peaks in RH during the 19th century coincide with major infestations of potato blight in Ireland.

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Last Revised: 2010 November 18th