Cold air has greater density. The greater the density, the better the performance of your engine. As you gain speed, the pressure of the air box increases improving airflow by decreasing the restriction of air traveling through the filter. Besides better performance, we have also even seen increases in MPG's and decrease in EGT's when used on diesel applications.
If you're worried about water ingestion when it rains, we conducted extensive independent testing to the SAE J2554, Engine Intake Air Water Separation Test Procedure to make sure that your engine is protected. You can find these results on the product page for the scoop. Just look up your vehicle and select the scoop.
Below are links to some of the test results done for our scoops.