Causes of valley fog on the northwest coast of the United States

The northwest coast of the United States belongs to temperate maritime climate, with humid climate, high water vapor content in the air, poor air mobility in the valley, difficult water vapor diffusion, and large daily range in summer, which is easy to form foggy days.

When the relative humidity reaches 100%, the water vapor in the air will condense into fine water droplets suspended in the air, which will reduce the visibility on the ground. This weather phenomenon is called fog. Fog is more common in spring from February to April. When the visibility in the atmosphere is lower than 1 km due to the condensation of suspended water vapor, meteorology calls this weather phenomenon fog. Conditions for fog formation: cooling, humidifying and increasing water vapor content.