the air mass over the ocean is warm & the air mass formed close to the ocean is humid in moisture ->  the air mass over the ocean is warm and humid  unstable air mass can cause a thunderstorm & warm and humid air masses are usually unstable ->  warm and humid air masses can cause thunderstorms  the air mass over the ocean is warm and humid & warm and humid air masses can cause thunderstorms ->  the warm and humid air mass over the ocean can cause thunderstorms  the warm and humid air mass over the ocean can cause thunderstorms & thunderstorms are a source of rainfall -> the warm and humid air mass over the ocean can cause rainfall