Forecasting Radiation Fog

Goals and Objectives

Module Goal

The goal of this training module is to help you increase your understanding of how radiation fog forms, grows, and dissipates. Such understanding, in turn, can help you more efficiently and accurately evaluate the ability of a given atmospheric environment to generate and/or maintain radiation fog.

Performance Objectives

At the end of the module you should be able to do the following things:

With Regard to the Preconditioning Environment:

  • Identify key conditions and ingredients necessary for the development of radiation fog
  • Discriminate between large-scale low-level environments that are favorable or unfavorable for development of radiation fog
  • Describe the sequence of key surface and boundary-layer processes that prepare the low-level environment for development of radiation fog
  • Demonstrate an understanding of how surface cooling dries the micro-boundary layer and prevents low-level condensation from being deposited onto the surface
  • Rank various surface and surface-cover types in terms of the relative speed with which low-level air in contact with them will reach saturation

With Regard to Formation:

  • Identify levels at which radiative cooling is most active at various stages of the fog initiation and growth process
  • Demonstrate an understanding of the effects that various condensation nuclei types and concentrations have on fog formation
  • Sequence the key processes and events that occur during formation of a layer of radiation fog
  • Demonstrate an understanding of how the fog-top inversion is created by the fog itself
  • Demonstrate an understanding of influences that heat flux from the surface has on a fog layer during its initiation and growth

With Regard to the Maintenance Phase:

  • Describe key processes that balance one another to allow a fog layer to maintain a relatively constant depth
  • Identify conditions in and above a fog-top layer that support continued condensate production
  • Identify conditions in and above a fog-top layer that restrict further deepening
  • Demonstrate an understanding of the effects that various condensation nuclei types and concentrations have on fog maintenance
  • Demonstrate an understanding of the effects that introduction of an overlying cloud layer has on a mature fog layer at the surface
  • Demonstrate an understanding of influences that heat flux from the surface has on a mature fog layer
  • Identify the typical level of a fog-top inversion
  • Demonstrate an understanding of how the fog-top inversion is maintained by various processes at and above the top of the fog layer

With Regard to the Dissipation Phase:

  • Identify key processes that contribute to the dissipation of a fog layer
  • Apply a droplet settling rate calculation to predict the time required for a given depth of fog layer to settle to the ground in the absence of any new condensate production
  • Demonstrate an understanding of how radiative heating contributes to dissipation of a fog layer
  • Demonstrate an understanding of how turbulent mixing contributes to dissipation of a fog layer
  • Demonstrate an understanding of how changes in low-level winds can contribute to dissipation of a fog layer
  • Demonstrate an understanding of how introduction of an overlying cloud layer can contribute to dissipation of a fog layer

With Regard to Detecting Fog:

  • Identify surface observations that show atmospheric conditions conducive to radiation fog
  • Identify soundings that show atmospheric conditions conducive to radiation fog
  • Identify fog in satellite images
  • Describe the limitations of infrared satellite images for detecting radiation fog

With Regard to Forecasting Fog:

  • Describe the diurnal cycle of radiation fog occurrence
  • Demonstrate an understanding of the strong seasonal dependence of radiation fog occurrence in at least two localities
  • Describe which forecast products best show the atmospheric conditions conducive to radiation fog
  • Describe the limitations of numerical forecast models in predicting radiation fog

Prerequisite Knowledge

To most effectively learn from this module, you should have knowledge of basic physical processes including conduction, radiation, evaporation, condensation, deposition, sublimation, and absorption. You should also be able to read and interpret sounding diagrams in skew T-log p data format. You should also be familiar with the structure and physics of large-scale low-level anticyclones.