calamity, disaster, catastrophe
灾 centers on a destructive event that brings great harm or misfortune: a calamity, disaster, or catastrophe.
灾 is a simplified form of 災. The traditional form 災 combines 巛 (river) and 火 (fire), suggesting a flood or fire as a disaster. The modern 灾 replaces the upper part with 宀 (roof), visually suggesting a fire under a roof as a calamity.
A fire 火 breaks out under a roof 宀, turning a home into a disaster zone. This image captures the sudden calamity of a house fire.
For サイ, imagine a sigh of despair when a disaster strikes: sigh -> サイ, as you watch the fire under the roof.