Advection Fog/Radiation Fog Comparisons
The following tables are taken from the DLAC 1 Lesson 1 teletraining session:
Characteristics
Characteristics |
Advection Fog |
Radiation Fog |
| Duration | Can last for several days | Generally short duration (< 24 hrs), often dissipating by afternoon |
| Intensity | Can range from thin to dense, but dense conditions may cover large area. Changes in intensity tend to be more gradual than with radiation events. | Varies with denser fog likely over open areas or near water bodies |
| Coverage | May be advected over large areas and across great distances | Typically remains in one place, patchy and localized |
| Depth | Varies with the boundary layer but tends to be deeper than radiation fogs since it is often driven by synoptic scale factors | Varies with the depth of the radiation inversion. Can be as deep as advection fogs, but tends to be shallower as it is formed more by local factors |
| Time of day | Can form and advect into a region almost any time of day. Some tendency to develop in late afternoon or evening hours over coastal areas. | Tends to form late at night or in early morning hours. Can also form following precipitation that clears near or after sunset. |
Dominant Processes in Advection or Radiation Fog Events
Advection Fog |
Radiation Fog |
|
Fog that develops when warm air moves over a colder underlying surface
|
Surface-based cloud caused by nocturnal cooling at and/or near the ground surface |
Formed primarily by boundary layer dynamic and adiabatic processes including advection of moisture, temperature
|
Forms and completes its life cycle in situ (Can be advected under the right conditions) |
| Can occur with light or moderate low-level winds that can be less than 10 kts, but can also occur with winds stronger than 10 kts | Boundary layer dynamic and adiabatic processes are negligible. Winds generally 5 kts or less. |
Low-Level Factors
Advection Fog |
Radiation Fog |
|
Differential heating between the underlying surface and the air mass being advected
|
Moist low-level conditions below a capping inversion |
| Air parcel trajectories that originate over a moisture source sufficient to establish a moist boundary layer condition | Rapid cooling of the lower boundary layer below the inversion |
| Depth of the surface-based moist layer increases as a result of mechanical turbulence and convective mixing of buoyant moist air | Presence of a low-level anticyclone creating favorable conditions by
|
Large-scale anticyclonic winds and subsidence
|
Moderate vertical shear often exists near the capping inversion |
