FAR 33.78, CS E.790 Rain and hail ingestion
The ingestion of large hailstones (0.8 to 0.9 specific gravity) at the maximum true air speed, for
altitudes up to 4 500 metres, associated with a representative aircraft operating in rough air, with
the Engine at Maximum Continuous power/ thrust, must not cause unacceptable mechanical
damage or unacceptable power or thrust loss after the ingestion, or require the Engine to be shut
down.
Pilots are not supposed to perform a take-off in a hailstorm.
In addition to complying with (CS/FAR specific) and except as provided in (CS/FAR specific), it must be
shown that each Engine is capable of acceptable operation throughout its specified operating
envelope when subjected to sudden encounters with the certification standard concentrations of
rain and hail as defined in Appendix (CS/FAR specific).
Appendix (CS/FAR specific) gives a diagram which requires 20 g/m³ Water up to an altitude for 20.000ft, which is the value relevant for take-off.
Pilots are not supposed to perform a take-off in extreme rain exceeding 20 g/m³ (which
is extreme, but happens). In most countries you only encounter such rain in heavy thunderstorms, pilots are not supposed to take off in that situation, especially as you typically just have to wait a few minutes before it has passed.
Aircraft are certified to flown reasonably by the pilot. Every pilot is trained to avoid severe weather.
Sometimes the job of a pilot is to just say no.