Caleb Everett, Damián E. Blasi and Seán G. Roberts recently published the results of their research into common characteristics of languages based on native climate. They conclude that as languages evolve, their dominant sounds reflect the climates where they develop. In particular, humidity plays a major role. Tonal languages are more likely to develop in wet climates. Science Daily explains that:
. . . languages with complex tones --those that use three or more tones for sound contrast -- are much more likely to occur in humid regions of the world, while languages with simple tone occur more frequently in desiccated regions, whether frigid areas or dry deserts. […]
One explanation, supported by extensive experimental data discussed in the study, is that inhaling dry air causes laryngeal dehydration and decreases vocal fold elasticity. It's probably more difficult to achieve complex tones in arid climates--particularly very cold ones--when contrasted to warmer and more humid climates. The result is that deviations of sounds, including increased jitter and shimmer, are associated with very cold or desiccated climates, the study says.
-via Seriously, Science?