Vent louvers (also called slats) are usually made at a 45 degree angle for rain to run down and stay outside the attic. The design is simple yet proven to be very effective over the centuries. The face or front of the louver acts as a stiffener to help brace against high winds. The louver then rises as it goes back for any rain collectd to run down. Then the rear is turned up so blowing rain will also be blocked.
The spacing of these slats is also concidered serious. Too large of a gap will make the slats unsuccessful in capturing and draining water. Yet, too small of an air gap will block too much air flow and choke the attic. The louver back turn-up should reach to just above the bottom of the next louver. Our 1 inch deep louvered vent has a 1 inch slat spacing. The 2 inch deep has 1 1/2 inches of spacing. 4 inch deep vents (actual 3.5 inchs) have 2 inch spacing. And the 6 inch deep (5.5 inch depth actual) has 2 1/2 inches of louver spacing. The deeper the gable vent, the larger the louver allowing more water resistant rise. Now, these louvers are designed for falling water and wind blown rain... Standing below the vent with a waterhose spraying straight up between the slats will not stop water intrusion. Hurricanes and tornadoes also can produce updrafted rain and rarely can cause water to enter the attic. Louvered vents are designed for the normal and everyday weather and will add a lot of protection during extreme weather but nothing is tornado proof nor hurricane proof.To be water proof would also mean being air proof as well. If air can enter, then with the right circumstances water will enter also.