For the past week or so, Nevada County residents at different elevations may have been experiencing very different weather. If you’ve driven up or down the hill recently, you may have been surprised to see how bright and sunny (and warm!) it is at higher elevations and cold and foggy at lower elevations. This is because there has been a persistent inversion layer with its boundary right at the elevation of about Penn Valley/Lake Wildwood.
Inversion Layers and Tule Fog
Inversion layers are exactly what they sound like—inversions in the normal temperature gradient of air from the earth’s surface up into the atmosphere. Under average conditions, air temperature is cooler as we go higher into the atmosphere. This is due to what’s known as the lapse rate, which is about 9.8°C per kilometer of vertical rise, meaning we should expect temperatures to decrease by that amount for each kilometer we go into the atmosphere. Note that this is variable depending on how moist air is, and varies from place to place (and as you go higher into the stratosphere as noted below), but for our purposes here, we’ll just point out that the general trend is: higher = cooler temperatures
During an inversion, the opposite can become true, with cold air capped by a layer of warm air, becoming “trapped” near the surface, unable to escape (remember that typically warm air rises, and in doing so, becomes cooler).
Plots of temperature relative to atmospheric height under “normal” conditions (top), and inversions (bottom). Note the declining temperature with height in the troposphere on the top, with increasing temperature with height during an inversion until the cold air layer is hit in the image on the bottom (this actually shows two inversions!).
So what causes inversions, and how are they correlated with super foggy days in the valley or this slow start to the winter?
There can be multiple kinds of inversions, but they’re typically related to warming from the earth versus from above, or to high pressure air forming a cap. The earth actually radiates quite a bit of heat from below our feet! In the summer months, incoming solar radiation from the sun, which is out for many hours and more directly overhead in our northern latitudes than in winter, can provide the majority of the radiation and warming that a region experiences. In the winter months, however, we can actually receive more heat at surface level from natural radiation from earth than from incoming sunlight. This air radiates up (and continues upward through the process of convection), and typically escapes out into the atmosphere, cooling in the process (until much higher in the stratosphere, but we’ll leave that alone for now…). During the cold winter months, as the surface air then cools, it becomes trapped under the warm air above it, forming a sandwich of cold-warm-cold—an inversion layer. Add settling cool air from the nearby mountains that warms as it “drains” into the valley, and you get even more of a “capping” effect.
Tule Fog is the name for the dense fog that forms in the valley (in fact so dense it is the leading weather-related cause of vehicle accidents in the state!) during fall and winter months. This fog is typically associated with a wet period (like the rains received in early November), followed by clear, cold nights. That moisture contributes to radiative fog; in the winter, rapid cooling of the surface at night condenses the remaining moisture (just like the outside of your glass of cold water touching hot air on a summer day). Add in an inversion layer that doesn’t allow the fog to naturally disperse and BANG—Tule Fog!
But I thought you said the surface is warmer in the winter due to natural radiation from the Earth? Yes! But…still cool enough to condense the fog and get trapped under the warm air…AND…how about that high pressure ridge out over the ocean and this slow start to winter?
Another atmospheric phenomenon that can be associate with inversions is that of high pressure ridges, where areas of high pressure create mass air subsidence, with dense air masses pushing down into the region, settling in the valley, rapidly warming, but still receiving more air above it, eventually creating the inversion cap.
For the past few weeks, just such a high pressure system has been parked out over the Northern Pacific. This high pressure system not only continues to shove masses of air into the valley, intensifying inversion and fog, it also has the effect of “blocking” storms from hitting California.
Areas of high pressure can almost be visualized as “walls” of air. In the winter, storms that provide California with most of its moisture can either be atmospheric rivers (long streams of warm moisture that form out over the Pacific and take aim at the state directly from the west) or “inside sliders” from the North (cold storms, typically drier than atmospheric rivers). Conditions like El Nino or La Nina (we are currently in a slight La Nina), the Pacific Decadal Oscillation, and other such phenomena can influence these storms, but in general, none of them can make it California if there is a high pressure wall in their way! Instead, storm systems have to take the scenic route, being pushed around the high pressure ridge into the central United States.
These persistent high pressure ridges can be so significant that they are often associated with long term droughts. For example, the 2012-2017 “megadrought” that was tied to mass tree mortality in the Sierra Nevada was thought to be primarily driven by a blob of high pressure that was so constant it was eventually dubbed the “Ridiculously Resilient Ridge”.
It remains to be seen how the winter will play out, and current forecasts do seem to point to a pattern shift in coming weeks (fingers crossed for more moisture!), but for now, enjoy the natural phenomenon that is the inversion layer (while driving as safely as you can if you’re in the fog!), the unusually warm dry days, and start prepping for what we can hope will be a snowy winter, and if you’re like me and like to check data every chance you get on current versus average winter conditions, bookmark the Central Sierra Snow Lab website to keep tabs in real time!
Do you have a science topic you’ve been wanting to learn more about? send an email to: sam@sierrastreamsinstitute.org with the subject “Ask A Scientist”
About Sierra Streams Institute
Sierra Streams Institute is a regional watershed science organization based in Nevada City, California, dedicated to increasing watershed stewardship capacity throughout the Sierra Nevada region and beyond. We work with local, state, and federal agencies as well as universities and community groups to find solutions to the problems that afflict Deer Creek, Bear River and other watersheds throughout the region that share the challenges resulting from a century and a half of gold mining, development, and agriculture. SSI’s emphasis on rigorous science and consistent data collection provides the basis for restoration decisions that are made on behalf of watersheds, and makes us an especially valuable partner of local, state and federal government agencies who lack the funding and capacity to gather their own data.