Left-side fire radical — pictograph of flames rising. Indexes
燥 in the fire family with
烧 to burn,
烤 to roast,
烫 scalding,
烟 smoke. Fire dries things out, so
燥 means 'dry, parched, arid' (
干燥 dry weather,
烦燥 hot-and-bothered restless). The radical is the whole semantic spine here.