cold
冽 centers on piercing coldness, often of water or air. It describes a sharp, clear chill rather than a mild coolness.
冽 combines 冫 (ice) with 列, which likely contributes the sound and reinforces the idea of a sharp, cutting cold. The exact historical development is uncertain.
The ice radical 冫 beside 列 (line up) suggests a line of ice crystals: a sharp, piercing cold.
For レツ, imagine a line of ice so cold it makes you shiver and say 'let's get out of here!' — 'let's' cues レツ while the ice radical reinforces the cold.