A temperature inversion is the culprit for the recent weather conditions
By NAREN KRISHNA JEGAN — science@theaggie.org
The Central Valley is known for its Mediterranean climate of cold, wet winters and dry, hot summers. Recently, the temperature of the Central Valley hit its most significant low in over 30 years.
Professor Paul Ullrich leads the UC Davis Climate and Global Change Group, along with earth and energy system resilience efforts at the Lawrence Livermore National Laboratory. He explained the climatic conditions that garnered this effect.
“The Central Valley was colder than usual in early December because of this mid-level, anomalously warm air that effectively trapped air in the Central Valley,” Ullrich said. “This phenomenon is known as a temperature inversion. Temperatures within the valley were then driven colder by persistent Tule Fog that filled the valley and reflected incoming sunlight. The inversion was broken up in late December by the big storm system that came through, allowing temperatures to moderate.”
Alexander Hort, a first-year atmospheric sciences student and representative of the Atmospheric Aggies American Meteorological Society (AMS) Student Chapter, provided some more context regarding the heat blanket effect.
“An inversion layer forms when, on a cloudless night, the surface cools faster than higher up in the atmosphere, but it persists into the mid-day when clouds or fog condense nearer to the surface and there isn’t enough wind to encourage mixing — preventing the sun from heating the surface and breaking the inversion layer,” Hort said.
Normally, less dense warm air rises higher into the atmosphere, leaving cold air to linger near the surface. The sun plays a crucial role in the movement of air between the lower and upper atmosphere; the cold air becomes heated and causes convection currents within the atmosphere. The persistent cooling and strong sun coverage prevents the cold air from being heated, forming the effect that we saw this past December.
In addition to the cool climate, many residents also noticed the emergence of the Tule Fog. Ullrich explained the emergence of this phenomenon.
“The Tule Fog originates in the valley, as it is a low, flat valley floor that can trap moisture and cold air throughout the day,” Ullrich said. “The fog largely forms at night, when radiation cooling lowers surface temperatures and moisture in the air condenses into ‘radiation fog.’ When the fog is dense enough that it can resist evaporation, even under the mid-day sun, it prevents sunlight from warming the surface and breaking the inversion layer that forms at night.”
Ullrich further explained the significance of this fog.
“This is the reason why the Tule Fog can last for days to even weeks at a time when these favorable conditions persist, and is also the reason why — when the Tule Fog is present — the valley remains significantly cooler during the day,” Ullrich said.
Clouds contain water molecules that are stuck to tiny airborne particles called cloud condensation nuclei (CCN), which either reflect radiation through the albedo effect or scatter radiation onto the surface.
“The clouds are trapped close to the surface because that’s where all the cold air is,” Ullrich said. “If you go up higher, you’re in a warm air layer where there’s less propensity for the water vapor to condense into liquid water.”
This consistent band of cold, dense air and wind fosters the ideal conditions for a positive feedback cycle that maintains the cooling.
“The air that is in [the Central Valley] is being funneled in through the San Francisco Bay […] and it’s basically sitting in the Central Valley,” Ullrich said. “It can’t go anywhere because those relatively warm temperatures in the mid-tropospheric level are blocking that cold air from going up and over […] The winds kept blowing so that it was funneling cold marine air from the Pacific into the Central Valley, [thus] continuing to maintain those relatively cold temperatures within the Central Valley. And so, that’s why for those two weeks in December, it just felt incredibly cold.”
Written by: Naren Krishna Jegan — science@theaggie.org
