Both "effective in" and "effective at" can be correct, depending on the context. Use "effective in" when referring to a specific area or situation where something is effective. Use "effective at" when referring to a specific task or action at which something is effective.

The reverberation time for a room depends on the ratio of the volume of the room to the total effective absorbing area of the room. The effective absorbing area will depend on the materials which make up the room and which are in the room. The effective absorbing area will depend on frequency. As an example, if a wall absorbs 30% of the sound (at a given frequency) for each reflection, the effective area for the wall will be 0.3 times the actual area of the wall. There are tables of typical effective areas, often reported in "sabins," for other objects, such as people, chairs, etc.

The maximum effective range of a point target on the M249 is 800 meters. The maximum effective range of an area target is 1000 meters. The maximum range is about 3600 meters.

Wet climates with mountains.

Yes, but only in an enclosed area.