Why Does Fog Form?

Over the past several days, we have seen patchy dense fog develop during the overnight and morning hours across Northern Illinois and Southern Wisconsin. Fog is one of those weather phenomena that is oftentimes forgotten. In addition, it is typically not something people take seriously, however it can be one of the more dangerous weather phenomena if the conditions are prime.

So what is fog exactly? Fog is essentially a cloud. It is a visible mass of cloud water droplets and ice crystals suspended near the surface of the Earth. In order for fog to form, you need a temperature-dew point difference of 4°F or less. Without this, it is nearly impossible for fog to form. So how does it all start? First, you need water vapor present near the surface. It condenses into tiny liquid water droplets in the air near the surface. Ways to get this water vapor into the air include transpiration from plants, the evaporation of moisture from bodies of water or the ground, and precipitation falling from above, among other processes. This is why whenever it rains, it typically gets foggy as well. Also, whenever there is a cold air mass trapped below a warm air mass -- what we call a temperature inversion in meteorology (likely the cause for our fog formation over the past few days).

Then, this water vapor condenses onto condensation nuclei near the surface, such as dust and salt, thus forming the cloud we see as fog. Most of the time when we get fog, the relative humidity outside is at or near 100%, which occurs by either adding moisture to the air (see above paragraph) or lowering the temperature closer to the dew point reading. A relative humidity value of 100% essentially means the air is saturated and cannot hold any additional moisture. This is similar to clouds forming in the sky. When you reach the level in the atmosphere where the air becomes saturated, clouds form. However, there are cases when fog can develop without the relative humidity value being at 100%, and just because the relative humidity is at 100% does not mean fog is guaranteed to form.

So for us, we have had nights recently with temperature inversions ( temperatures near the surface being cold amid a stable air mass trapped below a warmer air mass above the surface). This has provided the region with a set up that is conducive to fog development.