When you open your eyes in the early morning and look out your window, you might see a dim and gray image of the outside world. No wind, no birdsong yet; just a thick fog blanketing your view.

Checking the weather forecast, there is no chance of rain. You go outside, touch a leaf, or watch a spider web spinning between two oak trees, and then you see it. Drip. Drip. Drip. It is not dew. Dew usually occurs on clear nights due to rapid cooling of the earth surface. This is Fog Drip, and it is a cloud that is watering your forest without a single drop of rain falling from the sky.

A map graphic with arrows pointing to fog veins.

A satellite image of Valley Fog; characterized by fog veins (Photo credit: NOAA).

As fog develops, tiny water droplets floating in the air begin to collect on tree branches, needles, or any other surface. Water really loves to be connected with more water, and this is referred to as cohesion. The water molecule consists of two Hydrogen atoms and one Oxygen atom; these atoms are joined with bonds between each element. Unlike ice or vapor, the bonds that hold liquid water together are not rigid, yet not completely nonexistent. They break and reform very easily. As mentioned before, they actually seek out contact with other water molecules. The result is that, as the water droplets grow larger, and due to gravity, they begin dripping from all those surfaces. So, they roll down the bark of trees, gently touching and soaking into the ground below each tree trunk. For a tree, fog might be the difference between a dry week and a drink.

Fog highlights the view of the Pennsylvania Grand Canyon in Colton Point State Park Aug. 18, 2021, in Tioga County (Photo credit: Brian Whipkey, Erie Times-News).

Not all fog is created equal, and not all trees are equally good at drinking it. Think of two trees that grow side by side on the ridge of a mist-covered mountain. On one side is a younger tree, a Tulip Poplar, whose leaves are wide. Oppositely is an older Eastern Hemlock that has thousands of needles in its branches. As the fog comes in, which tree is more efficient in catching the water? There is a clear winner in this scenario; the tiny needles offer a massive surface area. Every drop of fog hits the needles and combines with other drops until falling onto the forest floor. Broad leaves also work, but their wide surface allows more droplets to evaporate before they can join or make it to the ground.

Next foggy morning, take a walk with a purpose. Pick one hemlock, one oak, or one maple. Stand under your tree for 30 seconds. Look at the ground at the base of the trunk, and see the water finding its way to the forest floor. When the forecast says “dry” but the fog says otherwise, take that walk in the woods; watch for the drip. Your trees are drinking from the clouds (no rain required).

This month’s What’s Poppin’ was written by Lauren Tushar, Watershed Intern