On the Formation and Development of Radiation Fog: An Observational Study

Observations from the Local and Non-local Fog EXperiment field campaign are used to investigate the formation and initial development of radiation fog. Data from six locations have been selected: five sites in Shropshire, and one in Bedfordshire, UK. These data show that during night-time clear-sky conditions, thermally-stable radiation fog only forms when the vertical velocity variance near the ground (at 2m) decreases to 0.002-0.005m(2)s(-2) or less. The variation in this threshold appears to be a function of the vertical relative humidity gradient near the ground. When the vertical velocity variance exceeds these values, saturation may still occur near the ground, but fog formation is inhibited due to the more efficient transport of moisture to the ground in the form of dew. Furthermore, once stable radiation fog forms, any increase in the vertical velocity variance to values greater than 0.002-0.005m(2)s(-2) may cause the fog to dissipate. The subsequent development of shallow stable fog into deeper, weakly-unstable fog is also examined. Possible mechanisms for this are discussed, including in situ processes and non-local larger scale processes. Clear evidence of the latter is presented, but the relative importance of in situ and non-local processes for fog deepening requires further clarification.