What’s The Difference? It Can Be Important.

Fog Over Roof South Hills | Photo by Tim Chuey

In today’s world there are people pointing out differences sometimes for good purposes and sometimes not so good. Color differences, political differences, religious differences, geographical differences and the list goes on. That’s also the case with the weather, particularly this time of the year when some differences can lead to confusion and even accidents.

Dense Fog
Dense Fog Seen From Peace Health Riverbend Hospital | Image Tim Chuey

We have had and will continue to have advisories for fog, dense fog, freezing fog, and frost throughout the winter months.The subsidence caused by high pressure closes off the mixing of the air at the surface which means anything that is at or near the surface stays there. Some of those things include smoke, which I discussed in my last column article, fog, freezing fog, and frost. The differences can be subtle, but they can also be dangerous.

Let’s start with fog. I remember in school the first definition I was taught. We were told that fog is a cloud that is close to or touches the ground. The National Weather Service officially defines fog this way: “Fog is water droplets suspended in the air at the earth’s surface. Fog is often hazardous when the visibility is reduced to 1/4 mile or less.”

Radiation Fog
Radiation Fog | Image by slideplayer.com

Fog is just fog, right? No. Actually there are eight major kinds of fog. Let’s take a look at them to see how they differ. The following descriptions are from weather.com.

1) Radiation fog: This forms when all solar energy exits the earth and allows the temperature to meet up with the dew point. The best condition to have radiation fog is when it had rained the previous night. This helps to moisten up the soil and create higher dew points. This makes it easier for the air to become saturated and form fog. However, the winds must be light, less than 15 miles per hour, to prevent moist and dry mixing.

2) Precipitation fog: This is fog that forms when rain is falling through cold air. This is common with warm fronts but it can occur with cold fronts as well only if it is not moving too fast. Cold air, dry at the surface, while rain is falling through it evaporates and causes the dew point to rise. This saturation forms fog.

Advection Fog
Advection Fog | Image by weather.gov

3) Advection fog: This type of fog forms from surface contact with horizontal winds. This fog can occur with windy conditions. Warm air, moist air blows in from the south and if there is snow or cool moisture on the ground will come in contact with the warm, moist winds. This contact between the air and ground will cause the air blowing in to become cool. Then dew point rises and creates high humidity and forms fog.

4) Steam fog: This type of fog is commonly seen in the Great Lakes but can be seen on any lake. This forms in the fall season. As summer ends, water temperatures don’t cool right away but air temperature does. As a mass of dry cool air moves over a warmer lake the the warm lake conducts warm, moist air into the the air mass above. This transport between the lake and air evens out. This corresponds to the second law of thermodynamics and this law states  “any two bodies that come into contact, the system will become equilibrium state.” Steam fog does not become very deep but enough to block some of the sunlight.

5) Upslope fog: This fog forms adiabatically. Adiabatically is the process that causes sinking air to warm and rising air to cool. The cooling of the air from rising causes it to meet up with the dew point temperature. Fog forms on top of the mountains.

6) Valley fog: This fog forms in the valley when the soil is moist from previous rainfall. As the skies clear solar energy exits earth and allows the temperature to cool near or at the dew point. This forms deep fogs so dense it’s sometimes called tule fog.

Fog With Freezing Fog On The Grass| Photo by Tim Chuey

7) Freezing fog: Freezing fog occurs when the temperature falls to 32 degrees F (0 degrees C) or below. This fog produces drizzle, and these tiny droplets freeze when they come into contact with an object. But at the same time sublimation is going on.

8) Ice fog: This type fog is only seen in the polar and arctic regions. Temperatures at 14 degrees F (-10 degrees C) are too cold for the air to contain super-cooled water droplets it forms small tiny ice crystals.

Now we’ll discuss frost. According to nationalgeographic.com “Frost is water vapor, or water in gas form, that becomes solid.” “Frost forms when an outside surface cools past the dew point. The dew point is the point where the air gets so cold, the water vapor in the atmosphere turns into liquid. This liquid freezes. If it gets cold enough, little bits of ice, or frost, form. The ice is arranged in the form of ice crystals.”

There are four main types of frost. Here’s how The National Geographic Society describes them.

Radiation Frosty (Hoarfrost) | Photo by pixabay.com

1) Radiation frost ( also known as hoarfrost) is frost in the form of tiny ice crystals that usually shows up on the ground or exposed objects outside. Hoarfrost also forms in refrigerators and freezers.

Advection Frost
Advection Frost | Photo by nationalgeographic.org

2) Advection frost is a collection of small ice spikes. Advection frost forms when a cold wind blows over the branches of trees, poles, and other surfaces.

3) Window frost forms when a glass window is exposed to cold air outside and moist air inside. Window frost is familiar to winter residents of cold climates. Indoor heat and cold outdoor temperatures form this type of frost. Window frost was much more common before people began using double-paned windows.

4) Rime frost is frost that forms quickly, usually in very cold, wet climates. Rime also forms in windy weather. Rime sometimes looks like solid ice. Ships traveling through cold places like the Arctic Ocean often end up with rime covering at least part of the exposed part of the ship.

We have been experiencing a combination of fogs and frost. Our fog combination is radiation fog and freezing fog with some mainly radiation frost. The one good thing about having dense fog when freezing fog and frost are present (if there can be anything good about the situation) is that the fog can lower visibility enough that drivers must slow down to see where they are going. If they are moving at a slower speed when they encounter slippery spots on the roadway there is a better chance they will be able to keep the car under control and not experience an accident

Let me know what you would like me to talk about or explain. You can comment below or email me at: [email protected].

Tim Chuey is a Member of the American Meteorological Society and the National Weather Association and has been Awarded Seals of Approval for television weathercasting from both organizations.

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