Any city with a happiness of less than 74 has a chance of rioting each turn. The probability of a riot occurring is equal to 10% times 74 - the current Happiness rating. For example, a city with a happiness of 72 has a 20% chance of rioting at the beginning of each turn. Rioting cities create no food, gold, science, or production.
Great Library entry[]
Throughout history discontented city dwellers have shown a tendency to explode into acts of mob violence when living conditions have suffered from poor rulership. Riots are generally short-lived but can be extremely costly. After the population has expended its energy looting and burning the city, the people will return to their homes and jobs and order will be restored.