We woke up to thick dense fog in Jacksonville Monday morning and the weekend rain made it slow to clear through the day.
Using a technique called the Crossover Temperature rule I could determine fog would be very likely.
The way to know fog is coming is to watch to see if the dew point and temperature are in lockstep together through the warmest part of the day.
The dew point is useful for gauging this moisture.
In practice, if the dew point remains constant or rises, the humidity in the air is sufficient enough to promote fog development or initiate it earlier.
You can use this site to track the hourly weather conditions. Using the crossover rule, 5 p.m. was the warmest part of the day reaching 69° and the dew point was 63. We should expect fog once the temps cool down to 63.
Sure enough, notice how the fog first developed in Jacksonville at 8 p.m. when the air temperature dropped past 63 ° to 62°.
Fog forms when the temperature and dew point of the air are the same value or less than 5°F. The fancy word is called dew point depression.
If the dew point drops as the temperature rises, the drier air can inhibit or prevent fog development.
Most of the time fog is not an issue in Jacksonville because when the day heats up, the amount of moisture in the air typically drops.
So when does this rule not work?
- When warm or cold air is moving into the area like during the passing of fronts.
- If there is rain or if the airmass is expected to change due to dry air intrusion.
Sometimes we only get the dew without fog because the vertical humidity profile drops quickly and is too dry for foggy situations.
Hope this helps!