Devereux. — Rainfall and Meteorology of Waihi. 411 



and forms to leeward of the Te Aroha Range. It is often a remarkable 

 spectacle in an otherwise cloudless sky. 



Reference is also made to the occurrence of " foehn "-like winds from 

 the north-west in early spring. It is not certain whether these winds 

 blow in front of an advancing disturbance or whether they are due to the 

 influence of high country. The ranges to the north-west are of only mod- 

 erate height. Certain it is that these Avinds bring a marked increase in 

 temperature, and have the parching characteristics of the foehn. 



CONCLUvSION. 



It would appear that the cause of the remarkable rainfall of Waihi may 

 be found in — (1) its nearness to the ocean ; (2) its situation as regards the 

 track of cyclonic storms ; (3) its topographical features ; (4) Nos. 2 and 3 

 operating in combination. 



Hann has shown how mountainous districts often produce " islands " 

 of heavier and more frequent rainfall, and that the precipitation does not 

 begin at the foot of the ranges but at some distance therefrom : local 

 observations confirm this. 



It may be well to note that Athenree, situated five miles distant to the 

 south-east, almost invariably recorded smaller rainfalls during the passage 

 of a storm. This feature can, no doubt, be accounted for when it is stated 

 that the slope of the land fi'om the sea at this latter station is small, and 

 it presents no bold features. 



Frequent reference has been made to the passage of wave-depressions* 

 in conjunction with cyclones as a factor for heavy precipitation. Helmholtzf 

 has found that atmospheric waves or undulations may exist on a most 

 gigantic scale, in which the wave-length is several kilometres, and so great 

 is the amplitude that when they occur at elevations of one or even more 

 kilometres above the earth's surface their action is felt at the ground. 

 This explanation would appear to account for the waviforni barographic 

 trace hereinbefore referred to, and which is generally coincident with in- 

 termittent wind-gusts and heavier bursts of rain. Since the upheaval of 

 air in the truly mountainous waves must amount to hundreds of metres, 

 causing a mixture of air-layers, it is easy to conceive that precipitation 

 would occur ; and this impulse would accelerate and extend higher up- 

 wards the movement already begun. 



It may be that the passage of a wave-depression concurrently with a 

 cyclonic area would set up wave-interferences attended with " breaking " 

 phenomena. Helmholtz explains that the counterparts in the air of 

 breaking spraying water-waves would cause a mixture of air-layers. HannJ 

 has shown how saturated layers of air, being forced to rise, expand, and by 

 expansion become cooled below the dew-point. If this process, already 

 begun, is accelerated, further cooling would result, and it is easy to conceive 

 of the enormous quantities of rain which may fall under these conditions. 



EXPLANATION OF PLATE XXXIX. 



The upper part of the plate is the barogram for the week ending 5th April, 190't. 

 In the lower part of the plate the Waihi record of 3rd July, 1909, shows similar 

 waviform trace and sudden fall to that of the St. Louis tornado of 27th May, 1896. 



* Captain Edwin. Trans. N.Z. List., vol. xxxvii, p. 557. 

 t Waldo, " Modern Meteorology," p. 335. 

 j Hann, " Handbook of Climatology." 



