A mean storm can definitely cause dropped calls, but not because the weather is interfering with the microwave frequencies your mobile phone uses (1–3 GHz). Rather, it's that cell towers transmit to each other and to base antennas at higher radio frequencies that occasionally straddle the resonant frequency of water vapor, causing wireless signals to be blocked and absorbed by passing storms. These "backhaul" frequencies can be so sensitive to humidity that researchers in Israel are deploying tower-mounted sensors that can determine which rainstorms are capable of causing flash floods. With GPS devices, it's a different story. They contact satellites using lower-frequency radio waves that aren't affected by rain, clouds, glass, or clothing but can be blocked by dense foliage and buildings that obscure the sky. So trust your GPS during a storm, but keep the unit itself dry: Even a thin coating of water on the antenna's exterior housing can scramble a satellite signal.