14 CFR § 121.637 - Takeoffs from unlisted and alternate airports: Domestic and flag operations.
(a) No pilot may takeoff an airplane from an airport that is not listed in the operations specifications unless -
(1) The airport and related facilities are adequate for the operation of the airplane;
(2) He can comply with the applicable airplane operating limitations;
(3) The airplane has been dispatched according to dispatching rules applicable to operation from an approved airport; and
(4) The weather conditions at that airport are equal to or better than the following:
(i) Airports in the United States. The weather minimums for takeoff prescribed in part 97 of this chapter; or where minimums are not prescribed for the airport, 800-2, 900-1 1/2, or 1,000-1.
(ii) Airports outside the United States. The weather minimums for takeoff prescribed or approved by the government of the country in which the airport is located; or where minimums are not prescribed or approved for the airport, 800-2, 900-1 1/2, or 1,000-1.
(b) No pilot may take off from an alternate airport unless the weather conditions are at least equal to the minimums prescribed in the certificate holder's operations specifications for alternate airports.