Buildings and vast stretches of pavement in dense cities trap and generate heat, forming urban heat islands. Similarly, urban development can boost rainfall. Around the world, these so-called urban wet islands have seen precipitation almost double on average over the past 20 years, according to a study published in the Proceedings of the National Academy of Sciences of the United States of America.  “What we knew up to now has been very focused on particular cities,” said Jorge González-Cruz, an urban climatologist at the University at Albany in New York who wasn’t involved with the work. Places such as Beijing and Houston have served as case studies showing that cities can influence temperature, rainfall, and storms. But the new study shows that the phenomenon occurs at a global scale. The analysis revealed certain factors that influence the wet island effect.