Most of the current on-board and cable communication systems are based on differential signaling. If any asymmetry happens in the differential lines then unwanted RF radiation will occur. On the other hand, placement of the communication equipment that contains differential signaling can be located anywhere in the world with different environmental conditions. To this effect, researchers have explored the effect of the humidity and temperature on PCB boards but mostly in single mode communication systems. In fact, environmental issues are very important to the signal integrity (SI) community since they have a profound effect in the performance of communications systems. In this paper, we explore relative humidity and temperature effects on PCB boards with differential signaling. Of particular interest is mode conversion, since this increases unwanted RF radiation.