Cedric Scherer explains some of the psychology behind understanding of color in visuals:
Without any intention, the two variations of my visualization triggered different emotional reactions. While the red chart likely leads you to think “Wow, Berlin summers are quite hot,” the blue version may push you to think of summers as rainy and rather cold.
In general, we should have in mind that different details might spark different emotions and expectations in our viewers. Some of these details will make it easier for them to understand the chart in the manner the designer intended.
Having experienced (parts of) a Berlin summer, let me confirm that they are not hot when compared to the midwestern or southeastern US.