In most cases, employers can force employees to work outdoors in the rain. The only time an employer would be prohibited is if the weather can cause risk of serious injury or death.
Under federal labor law, an employer is only required to provide an employee with a safe working environment. An employee cannot be subject to working in an environment that could cause serious physical injury or death. Therefore, an employer could force employees to work in the rain, unless the inclement weather could result in severe unsafe conditions. For example, an employer can't require employees to work outdoors in a hurricane.
If an employee is forced to work in conditions that are so severe he is fearful for his safety, he can file a complaint with the Occupational Safety and Health Administration. The Occupational Safety and Health Act of 1970 allows employees to file a complaint and have their workplace inspected by a government official. Employers are prohibited from terminating an employee for filing a complaint with OSHA, even if the complaint is found to have no merit.
Certain federal and state government workers are exempt from OSHA regulations concerning a safe work environment. Similarly, self-employed individuals and employees working on farms for immediate family members are exempt.