First, let’s look at the kinds of roof vents.
Gable/louvers vents. Louvers are covered openings that allow air to escape the attic. They are located on the gable (non load-bearing) ends of the house and allow limited airflow. This system relies on wind direction to create airflow through the attic. When the wind blows perpendicular to the ridge, it circulates air around the louver, which acts as it own intake and exhaust vent. If the wind blows parallel to the ridge, the airflow pattern acts as a draft that moves in one louver and out the other.
Power fans. Fans draw air through the attic by creating an air flow. They are effective, but they don’t allow natural forces to ventilate the attic. Plus, it’s somewhat expensive to buy, install and operating power fans, a cost that is greater than the fans’ benefit. Fans do not create airflow over the roof sheathing. Even fans that automatically turn on and off at selected temperatures are not worth the investment because the summer heat in the attic will cause the fan to run almost continually.