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Estimating the Likelihood of Fog on the River

In the absence of better means, you can use the 48 hour forecasts to get a rough idea of the potential for fog on the river at the time you want to row. Check the graph or the table to see if the following conditions are predicted to exist when you want to be on the water. Conditions 1 and 2 must be present for fog to form. The other conditions listed below influence probability of fog in close cases.

1. The predicted relative humidity must be at or close to 100%. For fog to form, this level of humidity must exist at the surface. The forecast shows the predicted relative humidity for the general area – not the surface or the river area specifically. The relative humidity close to the river surface is likely to be higher than that shown in the forecast because the water normally imparts humidity to the air. This effect is even more pronounced in the lower portion of the ARC river stretch due to the warm water being discharged from the power plant. So you can generally assume for your fog estimate that the relative humidity on the river will be higher, on average, than the value indicated in the forecast.
   
   
2. The dewpoint shown in the forecast is predicted to be within 5 degrees (plus or minus) of the predicted air temperature.
   

Fog is most common when the air temperature is around 40 degrees. Wind and cloud cover tend to inhibit fog development so check the projected wind speed and sky cover percentages as well – the more of either, the less the likelihood of fog.

Fog is most likely to form on clear nights in the Fall and most likely to be thickest right around sunrise. Fog usually starts to evaporate after sunrise as the sun warms the fog from the top down. The thicker the fog the longer it takes to dissipate. Some rowers report that fog near the river surface sometimes becomes even denser shortly after sunrise and before it finally burns off. There are a variety of reasons why this might be the case. On-site, eye-balled “to row or not to row” decisions should account for this possibility.

See also: Fog Information at the Airport Sites for notes on fog in the past 24 hours.