Several parts combine into one character.

雷 is a compound ideograph: 雨 (rain) + 田 (now "field," but originally a pictograph of lightning forking out in cross patterns ⊞). The oracle bone form was more dramatic — what is now 田 was a clear lightning-bolt shape splitting in four directions. The composite picture is a complete weather scene: rain above, lightning below = thunder. The rain radical 雨 anchors the entire vocabulary of meteorological precipitation: 雷 (thunder), 雪 (snow), 雲 (cloud), 霜 (frost), 露 (dew), 霧 (fog), 霰 (hail). All things that fall from the sky live under the rain radical.

Korean reading "roe" (or "noe" word-initially per phonotactic rules). 落雷 (nangnoe, lightning strike), 地雷 (jiroe, "earth thunder" = land mine — military term used widely in Korean security and historical contexts due to mines along the DMZ), 雷管 (noegwan, detonator), and the literarily charged 靑天霹靂 (cheongcheon-byeongnyeok, "thunderbolt from the blue sky" — the universal Sino-Korean idiom for "shocking unexpected event"). 雷聲大發 (noeseong-daebal, "thunder erupting greatly") describes a violent outburst of anger.

Mandarin léi, 2nd tone. 雷 (léi, thunder), 打雷 (dǎléi, "to hit thunder" = to thunder — note the verb 打 used for atmospheric phenomena), 地雷 (dìléi, land mine), 雷电 (léidiàn, thunder and lightning). And the famous personal name 雷锋 (Léi Fēng), a Communist soldier whose model behavior was held up for emulation during 学雷锋运动 ("Learn from Lei Feng" campaigns) of the 1960s. Modern internet slang: 雷人 (léirén, "thunder-person" = shocking / outrageous / makes you feel struck by thunder).

Japanese on-reading ライ (rai) — 雷雨 (raiu, thunderstorm), 落雷 (rakurai, lightning strike), 地雷 (jirai, land mine), 雷神 (Raijin, "Thunder God" — a major figure in Japanese Shinto-Buddhist mythology, depicted as a fierce ogre-like deity beating drums to make thunder; paired with 風神 Fūjin, the Wind God). Kun-reading かみなり (kaminari) — 雷 (kaminari, thunder), and the famously vivid idiom 雷が落ちる (kaminari ga ochiru, "thunder falls") = "to be severely scolded / get a tongue-lashing from a superior." Japanese mythology personifies thunder more elaborately than Korean or Chinese, with the imagery embedded in temples, festivals, and tattoo art.

Memory aid: rain (雨) above, with lightning forking below (originally cross-pattern, simplified to 田) — the full picture of thunder.