noun
thunder on a clear day; thunder from a blue sky
Refers to thunder occurring when the sky is otherwise clear, often considered an unusual or ominous phenomenon. Typically written as 日雷.
今日は晴れているのに日雷が鳴った。
Today, even though it's sunny, there was thunder from a clear sky.
noun
lightning that starts a fire; fire-causing lightning
Lightning strike that ignites a fire. Often written as 火雷 or 火神鳴り. Related to 水雷 (みずがみなり, lightning accompanied by heavy rain).
See also: 水雷 (みずがみなり)
火雷が落ちて山火事になった。
Fire-causing lightning struck and started a forest fire.
Irregular okurigana usage; rare variant of 火神鳴り.
水雷 refers to lightning accompanied by heavy rain, while 火雷 (sense 2) is lightning that causes fire.
Compound of 日 (sun, day) or 火 (fire) + 雷 (thunder). The reading ひがみなり is a combination of ひ (hi, sun/fire) and かみなり (kaminari, thunder), with rendaku voicing. The exact historical derivation is uncertain.