Welding can be safe when done indoors especially when everything about your machine is hooked up properly and without any damage to its cables. But when welding is done outdoors and it’s raining, I should say you are putting yourself at high risk and that is mainly from getting electrocuted. So obviously even the answer to the question can you weld in the rain seems possible, the hazards always remain high.
According to OSHA (Occupational Safety and Health Administration) which is a US government agency that oversees the safety and health of workers in workplaces, there is no law that prohibits people from welding in the rain. However, it would be the employers’ problem if something bad happens to their welders and no employer would want to endanger his employees’ lives and face consequences under the law.
On the other hand, not all welders are doing their work under employment which means anyone can weld anywhere they want even under the rain. The only thing they need to do is to take a lot of safety precautions, secure their machines, and wear rain gear which as much as possible should be waterproof and fireproof.