 San Diego experiences marine layer clouds, most often between May and August, which cause cool temperatures, cloudy weather and fog in the morning.Marine layer conditions linger until the heat of the sun becomes strong enough to evaporate the clouds.The local sayings "May gray" and "June gloom" refer to the way in which San Diego has the most trouble shaking off the early morning fog during those months, and cool, cloudy conditions often last into the afternoon or even all day.