hazy below due to METAR visibility?

Flying FSX, I often run into sudden changes in the visibility, not due to clouds but to a haze covering everything. Since I am using realtime weather information (FSrealWXlite), I assume this happens because a weather update contains a nearby METAR with low visibility. As suddenly as it appears it can disappear leaving unlimited visibility. Can anyone confirm this? I've searched for a way to make the change gradual (more realistic) but haven't found a handle for that. In addition, the haze appears to be of uniform density; there is no light haze or heavy haze, it can't accommodate variation in visibility, so there must be a threshold below which it kicks in.