PROBLEMS AND METHODS OF RAINFALL INVESTIGATION 
and is then unevenly affected by the rain-cool- 
ing. This might also partly explain why many 
writers have found irregular hurricane paths. 
In fact, a path need not be so irregular, it may 
only seem so because of the multiple eyes. 
Radar has revealed to us the spiral structure 
of the hurricane precipitation. The hurricane- 
cloud spirals might at first be composed by sepa- 
rate convective cells, arranged in rows that are 
gradually dragged spirally into the Low, at last 
joining up into real bands. Figure 10 contains a 
continuation of the series of precipitation maps 
17 
of Figure 9, showing that the precipitation pat- 
tern based on rain-gage data also may have a 
spiral structure. Certainly, it is founded on a 
rather loose network, so that in some cases one 
might be able to combine the places that have 
got precipitation in a different way and thereby 
fail to attain a clear spiral rain pattern. How- 
ever, in the map of September 19, 02h EST, 
which also has isobars and winds, the direction 
of the spiral arms lies somewhere in between the 
direction of the wind at the Earth’s surface and 
the gradient wind. It seems quite reasonable, 
°F Gr 
/| 
4 
HO/ 
aed a 4 
1000 5°) 
ia = 
aE 
tl Ls 
itd 
SRA 
i 
| Vy 
a 
a | 
57°N —_| : 
6° T5erveron /H8 
Coast line 
1000 ° 
—_—mM 
Jsobar of 995 mb 
20, | 
POT TE IIT | 
Wind ENE 14 "%s 
500 m above MSL 
(oe oxen Jsotherm 0c at sea-level 
Fic. 11—Distribution of precipitation, wind, pressure, and temperature in SW Scandinavia during 
the period March 24-27, 1927 (72 hr), showing marked coastal maxima of precipitation mainly due to 
orographically conditioned convergence 
