What exactly is fog?
Fog is just water droplets that are suspended in the air at the surface of the Earth. It is essentially a 'surface cloud.' However, though similar, fog is not mist and vice versa. Fog and mist are only distinguishable through density. Mist can reduce visibility to NO LESS than a mile, where as fog can reduce visibility to LESS THAN a mile.
Fog forms when the difference between the dew point and temperatures is less than 4 degrees F (or 2.5 degrees C) and when water vapor condenses into liquid water droplets (very tiny) in the air. Normally fog occurs when the relative humidity is at or near 100% (relative humidity of 100% means the air can no longer hold anymore moisture). Getting to 100% can be achieved by adding more moisture to the air or dropping the air temperature. If more moisture is added to the air when the relative humidity is already 100%, then the air will become 'supersaturated.' Whew! That was A LOT! My nightmare of my undergraduate Thermodynamics class just came back to haunt me!
Now getting back...
The following are ways water vapor can be added to the air:
- precipitation or virga falling from above the surface
- daytime heating and evaporating water from surface bodies of water i.e. the oceans, lakes, any wet surface
- wind convergence into areas with upward motion
- cool drier air moving over warmer water
- air lifting over mountains (temperature cools when rising in elevation = condensation)
- plant transpiration
Water vapor begins to condense on dust, ice, and salt; condensation nuclei (cloud seeds), to form clouds. Fog is said to be a stratus cloud; it is a stable cloud deck that forms when a cool, stable air mass is trapped under a warm air mass. If you can remember back to the Summer Olympics that were held in China a few years ago, scientists were discussing 'cloud seeding' as an option to break the heat and drought-like weather and to encourage rainfall over the Olympic grounds.
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| Example of Cloud Seeding |
Cloud seeding is a form of weather modification in effort of change the amount to precipitation falls from clouds usually in the form of rain, however other uses are to used at airports to suppress hail and fog from forming. The main chemicals used in this process are silver iodine and dry ice (frozen CO2). Now, other methods are being researched due to the risk of human and environmental health from (over) exposure to these chemicals.
Back to the fog.
There are many kinds of fog...yes, many kinds, ranging from Valley fog to frozen fog (that's fun!). The main kinds of fog that is prevalent in New Jersey are the following:
Radiation Fog (or Ground Fog): formed by the cooling of the land after the sun sets by thermal radiation in calm, clear weather and sky conditions. A.K.A. radiational cooling. The cooling ground produces condensation in the nearby air by heat conduction. This is most common in the Fall and Winter seasons.
Advection Fog: forms when (warm) moist air passes over a cool surface by advection (wind) and is cooled. This is most common when a warm front passes over a cooled surface, such as a surface with significant snowpack.
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| My poor attempt of creating a Paint doc. illustrating advection fog |
I leave you with this: an absolutely beautiful timelapse of fog in San Fransisco Rolling Fog
Near term forcasted NJ weather: Sun, clouds and scattered shower and isolated thunderstorms ending early this afternoon. Clearing skies this evening as a weak high pressure move in for Wednesday. More clouds and rain roll in Wednesday evening through Friday, as a pretty intense low move in from off the coast . Could this be (another) significant Nor'easter? Stay tuned tomorrow for the updated forecast!
~V.S.
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