Determination of total atmospheric water vapour content from solar observations at millimetre wavelengths

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Abstract Observations of the absorption of solar radiation in a broad band centred at 1.3 mm wavelength have been made at Zaria, Nigeria. By supplementing the observations with radiosonde data taken at Niamey, Niger Republic, absolute estimates of water vapour content with a mean fractional error of 0.15 may be obtained. Relative estimates of water vapour content with a mean fractional error of 0.03 are however possible. The present observations show that during Harmattan conditions, not only the absolute water vapour content but also its scale height increases during the day. A similar increase is found from Harmattan to wet season conditions. Calibration procedures are discussed which should produce a greater absolute accuracy in the water vapour content measurements.