Researchers claim that sunshine occurs most often in cities that have more palm trees. As a result, the mayor has the city plant 500+ trees to increase the number of sunny days. Is this a correlation...

Researchers claim that sunshine occurs most often in cities that have more palm trees. As a result, the mayor has the city plant 500+ trees to increase the number of sunny days. Is this a correlation or is there a hidden factor (and what would it be)?