How high must the ceiling be to depart from a controlled airport under IFR?

To depart from a controlled airport under IFR (Instrument Flight Rules), the ceiling must be at least 1,000 feet above ground level. This requirement is based on FAA regulations which state that when operating under IFR, pilots must ensure that they have the requisite visibility and cloud cover to safely conduct their departure. A ceiling of 1,000 feet provides enough vertical space to ensure a safe climb-out from the airport environment, allowing pilots to safely navigate through the initial phases of flight while keeping the aircraft clear of terrain and obstacles.

Higher ceiling requirements are not typically necessary for departing under IFR, making the stated 1,000 feet the appropriate regulation to adhere to. Therefore, this choice aligns with the current IFR departure requirements set by aviation authorities.