A limited test of methods of pressure reduction to mean sea level

A limited test has been made of five methods of correcting pressures at high level stations to sea level. The mean sea level pressures were calculated using each of these methods and were compared with pressures found by inter-polation on a mean sea level chart, which was drawn on the basis of the estimated mean sea level pressures at stations below 500 ft (calculated by the method at pre sent being used in Australia) supplemented by estimates of pressure gradients assuming geostrophic flow at 3000 ft. The methods were compared on the basis of the means and root mean squares of the differences between the calculated and inter-polated pressures over the period studied. In this test the method which most frequently gave the best results was that based on a constant monthly mean surface temperature and vapour pressure. Mean sea level charts for the synoptic hours 0600 EST on 6.7.64 and 15.1.64 and 1500 EST on 3.1.64 and l4.7.64 were drawn for pressures reduced by three methods – the two most successful methods in the preceeding test and the method at present being used in Australia. This limited test again showed the superiority of the se two methods but did not indicate any marked difference between them.