Question: is 20mm of rain a lot?
Answer: Moderate rain: Greater than 0.5 mm per hour, but less than 4.0 mm per hour. Heavy rain: Greater than 4 mm per hour, but less than 8 mm per hour. Very heavy rain: Greater than 8 mm per hour. ... Heavy shower: Greater than 10 mm per hour, but less than 50 mm per hour.