BLOG: Why the fog? - | WBTV Charlotte

BLOG: Why the fog?

(Credit: Cotton Ketchie) (Credit: Cotton Ketchie)

We have had many foggy nights and mornings over the past week or more around here and I thought it was time to discuss why. 

Water is the only element (or compound if we’re being technical) on earth that exists in all three phases within the normal temperature ranges we experience. We know water as a liquid, a solid (ice) and also as an invisible gas known as water vapor (or humidity).

The air that surrounds us always has water vapor in it, even on those very low humidity days that make you think it’s Arizona. Because water vapor is a gas, it behaves like any other gas.  Water vapor of course contributes its (partial) pressure to the other atmospheric gases (Oxygen, Nitrogen, etc.). 

We call that vapor pressure. The amount of water vapor in the air determines its Saturation Vapor Pressure.  More water, higher SVP.  Less water, lower SVP. 

Thus, as the temperature of the air falls, typically at night, the vapor pressure also falls, and if it falls far enough, it can hit the Saturation Vapor Pressure.  Once the air is saturated, the gaseous water molecules then begin to escape the gaseous phase and condense as tiny liquid droplets. 

That might be seen in the form of dew, fog or even steam coming out of your mouth. 

The dew point you hear us refer to represents the saturation vapor pressure and we express it in degrees Fahrenheit. 

So what are the conditions that cause air to fall to the dew point?  Well moist air helps. 

If the dew point is high, that means the air temperature doesn’t have very far to fall to hit it. 

That’s one easy way to form fog. But other conditions are important. Clear, cloudless nights allow the ground-level heat to radiate away causing rapid cooling of the surface. That’s a good start toward hitting the dew point. It’s also important we have little or no wind. 

People think of wind as blowing in a straight line all the time. But it doesn’t. It swirls, twists, turns and does flips in the atmosphere like a Ferris Wheel.  We call those Eddies, and that can mix down drier air from above and evaporate the fog layer. 

But even on a clear, calm night, there must be enough cooling to continue forcing the fog to form. 

Once condensation begins, that process gives off heat and can quickly arrest the cooling process. So without a continuous source of cooling to force the process along, the fog can stop forming before it ever really gets started.

In the past week or so, you’ve probably noticed all the warm air that returned to the southeastern US. That warm air has to come from somewhere and typically it’s the Gulf of Mexico. 

As the warm, moist air moves into our region, it is sliding over the relative chilly earth that we now have in place here this time of the year. Thus, the chilly ground is helping to cool this air down to the juicy dew point. 

So on nights when the weather is clear and calm, and you then add in the cooling effect from below, you have the perfect setup for foggy nights!

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