Field observations were conducted at Ibaraki Airport, Japan, to determine the vertical development speed of shallow radiation fog. The development speed in shallow (less than several meters) fog was 3-16 cm min⁻¹, and was slower at higher wind speeds. The speed decreased when the air above the fog layer became drier, possibly due to the mixing of the fog layer with the dry air above it. The speed data presented here can be used in nowcasting fog development for aviation decision making.