According to the National Weather Service, mobile devices are safe to use indoors during a lightning storm, as they do not use wires that could carry the electrical charge of a lightning bolt toward your body. However, phones and other metal-containing devices should be used with caution when outside, as they may worsen the effects of a direct lightning strike. In addition, interference from storms can decrease the effectiveness of wireless equipment.
Indoors
Staying indoors does not eliminate the risk of being struck by lightning during a storm. Lightning charge can travel into your house through your power wiring or plumbing, and could strike you if you are using a wired device or a corded phone. However, battery-powered devices and cell phones are fine when used indoors, as their lack of cords means that they will not be affected even if a bolt does travel along your power wiring.
Outdoors
You should always look to come indoors or seek appropriate shelter during a lightning storm, as all outdoor areas can be vulnerable to lightning strikes. Whilst finding shelter, do not use a mobile phone or other wireless device, especially if it has a metal chassis. Although such devices do not increase the chances of you being struck, they may make the effects of a strike worse. Metals conduct electricity very well, and having metal close to your skin may result in more severe injuries if you are struck.
Rain
You should avoid using wireless devices outside in a storm even if you cannot see any lightning, as they may be damaged by rain. All electrical devices can be harmed if water gets into their circuitry, with touchscreen devices such as smartphones also vulnerable to surface water. If your device has been water-damaged, turn it off and remove its battery if possible, as well as any peripherals. Place it in an airtight container and cover it with dry rice. Leave it to dry out for 24 to 48 hours, and retest.
Interference
Irrespective of any damage or safety concerns, wireless devices may suffer reduced performance during a storm. This is because devices such as cell phones and Wi-Fi equipment use radio frequency to broadcast, which can be interfered with by the radio noise produced by a lightning strike. This phenomenon occurs when wireless radio waves collide with radio noise of a similar frequency, causing both to become disrupted. Interference often results in slow and unstable wireless transmission between devices.