Hardware.Info posted High static pressure case fans test: the best 120mm and 140mm fans for your radiator or CPU cooler A quote from the article:

Many manufacturers of case fans offer separate fans with high static pressure. These fans are capable of easily pushing air through small openings, which makes them ideal for CPU cooler or water cooling radiators. But what are the best options?

There are roughly two ways to measure the performance of case fans. Firstly, you can measure the amount of airflow produced by the fan, in other words the amount of air moved by the fan. The amount of static pressure represents the force that pushes the air. A case fan that is capable of pushing large amounts of air, without actually pushing the air through a radiator, is less useful for such a purpose. A consequence of high static pressure is that the produced air stream tends to be more targeted, which is why there is a difference between 'airflow' fans with a wide air stream through your case and 'static pressure' fans that can be used for radiators of CPU coolers.

 High static pressure case fans test: the best 120mm and 140mm fans for your radiator or CPU cooler @ Hardware.Info