 Photo Credit: White Squirrel Weather DISCUSSION: With the autumnal equinox behind us, the days are starting to get shorter and it looks like flannel and pumpkin-spice drinks are all the rage again. And as that fall mood sets in, so too does the Halloween spirit. Several horror-movies and TV-shows come to mind this time of year, including the classic Nightmare on Elm Street or the latest season of American Horror Story on FX. While watching one of those shows, the scientist in me started noticing that a lot of horror-scenes take place in foggy night-time settings. This begged the question “Why is it that fog seems to happen so frequently late at night"? And that’s exactly what we’ll be discussing in today’s article. There are many different types of fog, but the focus in this article will be on radiation fog, which usually is the type of fog that’s portrayed in horror movies. After sunset, the surface of the earth immediately loses its biggest source of incident radiation. And with no more solar radiation for the surface to absorb, all of the heat that has been collected over the entire course of the day simply begins escaping freely into the atmosphere in a process that’s referred to as radiational cooling. This process can be slowed down on cloudy nights when radiation is absorbed by low-hanging clouds and emitted back down to the surface but on nights with dense radiation fog, the night sky is crystal-clear more often than not.  Photo Credit: The Weather Network As the radiation continues to scatter outward in all directions, the temperature of the surface will rapidly decrease while the air immediately above it cools more slowly, causing what’s referred to as a temperature inversion. In other words, the temperature actually increases with height for a small column of air that stretches from the surface to a few feet above the ground. But this situation alone won’t be enough for the eerie October fog to develop.  The next key ingredient is moisture, which can come from many sources, including lakes and rivers, and be carried to the surface by light turbulent mixing. Surface-level moisture will also increase since colder air gets saturated a lot faster than warmer air. If enough moisture is transported to the surface and the temperature manages to drop to what’s known as the dewpoint temperature, then the air will saturate to the point where dew droplets will develop! Once the air just above the ground begins to saturate, more droplets form, only this time the droplets that form are condensing in the air. This is the start of the radiation fog!  Photo Credit: Meteorologist Jeff Haby As these fog droplets float around and slowly fall to the surface, they absorb and emit the radiation still coming off the ground, slowing the rate of cooling at the ground-level and pushing the area of greatest radiational cooling to the top of the expanding fog layer. The air mass inside this fog layer soon becomes isothermal (same temperature) throughout, making it eerily stable. Anyone caught under it will observe a sharp decrease in visibility and a calm, quiet ambiance that is sure to raise some hairs in the middle of the night. It will then maintain itself for as long as the radiative cooling at the fog-top introduces newly-formed droplets to the rest of the layer. If the fog-top reaches a point where the air is no longer moist or stable enough to continue this process, then it will reach its maximum depth. Walking around this quiet, misty environment in the middle of the night would immediately feel like something out of a horror movie! By daybreak, solar radiation is reintroduced and immediately begins to warm the entire fog-layer, evaporating the fog droplets. Vertical wind shear introduces drier, windier air down to the surface and dries out the fog top. As the ground continues to heat up, the lower levels of the fog thin out. Any remaining fog droplets eventually settle on the ground as well. And of course, the ghosts, ghouls, and witches all go back into hiding. Sources: The Weather Network: The Science Behind the ‘Classic Fall Storm’ https://www.theweathernetwork.com/news/articles/the-science-behind-the-classic-fall-storm/73106/ Fog Forecasting: Meteorologist Jeff Haby https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/forecasting_fog/FORECASTING_FOG.htm White Squirrel Weather, Western Kentucky University: On Air http://www.wkuweather.com/ To learn more about various weather education topics, click here! ©2018 Meteorologist Gerardo Diaz Jr.
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