If PSULLYKEYS method doesn't work, you can download and install Cfo_weather, available here:
http://www.mnwright.btinternet.co.uk/Cfsops/Modules/cfo_weather/Cfo_Weather_75.htm
Cfo_weather allows you to change all the weather conditions from your dropdown in-flight menu during missions, quick combat, and free flight. Cfo_weather also offers some other interesting options, including a handy autopilot.
To set your visibility at 15 nm with Cfo_weather:
1. In CFS2, max out your "max visibility slider" - all the way to the right. This is important to make the Cfo visibility option work.
2. Start a mission or other flight and click on Cfo from your in-flight dropdown menu.
3. Select "time and weather", "advanced weather", and "visibility".
4. Set "tops" at 50,000, "base" at 0, and set the visibility slider at 15 nm.
5. Exit back to your flight or mission.
If you can't see the wierd stuff off in the distance, it won't bother you. This has to be done after you start your mission or flight. As mentioned, you can also change other weather conditions during missions and flights this way.
Setting haze this way also makes other aircraft in the distance show up better. It really shouldn't, but it does. It's a graphics thing.