The International Standard IEC 60529 specifies the full testing procedure for degrees of protection provided by enclosures. Products are tested in accordance with the IEC 60529 testing procedures and where manufacturers claim compliance to a specific IP rating they are only confirming that the product has passed the relevant test. Suitability for any specific application is not claimed or inferred and it is strongly recommend that users carry out their own tests to establish suitability of a product to specific site environmental conditions.
As an example part of the the IEC 60529 IP65 test procedure requires that a stream of water is sprayed from a 6.3mm diameter nozzle at 12.5 litre/min from all practical directions from 2.5m-3m for 3 minutes. Passing an IP65 test does not imply that a product is suitable for outdoor use in any weather.
In addition to particle and water ingress, other factors to consider when evaluating suitability of a product for a specific application should be daily and annual temperature range, humidity, UV exposure, color of fixture (black fixture in direct sunlight will easily exceed specifications).
It is recommended that users check with manufacturers directly regarding suitability of a product for each specific installation.