There is a correlation between humid climates and tonal languages [1], [2].<p>And there is some evidence that there have been shifts in tonal complexity within languages over time [3].<p>[1] <a href="https:&#x2F;&#x2F;www.mpg.de&#x2F;8879447&#x2F;tonal-languages-institutes" rel="nofollow">https:&#x2F;&#x2F;www.mpg.de&#x2F;8879447&#x2F;tonal-languages-institutes</a><p>[2] <a href="http:&#x2F;&#x2F;www.pnas.org&#x2F;content&#x2F;112&#x2F;5&#x2F;1322.abstract" rel="nofollow">http:&#x2F;&#x2F;www.pnas.org&#x2F;content&#x2F;112&#x2F;5&#x2F;1322.abstract</a><p>[3] <a href="http:&#x2F;&#x2F;languagelog.ldc.upenn.edu&#x2F;nll&#x2F;?p=4906" rel="nofollow">http:&#x2F;&#x2F;languagelog.ldc.upenn.edu&#x2F;nll&#x2F;?p=4906</a>