It has a revolving screen/mat that is soaked with water (from an included pan); air from forward motion goes through the mat and evaporates the water, which takes energy out of the air, thus cooling it. Works great in desert heat and 5% humidity. Works less well as the humidity rises, hence the nickname "swamp cooler." Most such coolers require the user to pull a lever or otherwise move something to revolve the screen/mat and keep it soaked with water.

Obviously, the water needs to be replenished frequently – most such things are good for only an hour or two before the reservoir runs dry. OTOH, they are inexpensive and use no power from the car except for a slight reduction in fuel economy due to air resistance. I doubt that they are even made anymore due to the prevalence of a/c in most cars. The one in the photo looks like it dates from the 1950s. I may have owned one a few decades ago.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970