Science calls it a phenomena. But the word “phenomena” makes it sound as though the Nocturnal Boundary Layer (NBL) is something inexplicable. In reality, we know that it is a relatively thin layer of cool air separate from the rest of the Boundary Layer which appears at night. Its creation is spurred by net radiative loss from Earth’s surface. In the unique case of the solar eclipse of 2017, the NBL appeared from the lack of sun in under a half-hour. We also know there is an effect from cooling of the air after sunset, but the Earth constantly emits longwave radiation (infrared) into the universe.
“Many interacting processes can occur within the statically stable nocturnal boundary layer: patchy sporadic turbulence, internal gravity waves, drainage flows, inertial oscillations, and nocturnal jets.”
– American Meteorological Society
Schematic drawn by me, adapted from Garratt 1992.
The Nocturnal Boundary Layer is also called the Stable Boundary Layer. At night, parcels of air won’t rise – they stay low to the ground and move horizontally rather than vertically. As a result, emissions will get trapped at a lower level and pollution can travel longer distances before being dissipated at night.
During the daytime, buoyancy and turbulence allows pockets of air to move around more. Hence why the daytime layer is referred to as the convective or mixed boundary layer. At nighttime, turbulence is rare. It occurs unexpectedly and is largely due to friction from surface elements. Prevailing wind velocity also has an impact.
How a Solar Eclipse Created an NBL
Surveying equipment dedicated to the Land-Atmosphere Feedback Experiment (LAFE) at 3 different field sites in the Southern Great Plains of Oklahoma recorded the solar eclipse on August 21st, 2017. Scientists were able to measure exact changes in meteorological conditions and their time scales.
First off, the convective boundary layer turned into an NBL. This was characterized by a drop in air temperature of 2-3 degrees C and a 1-2 meter per second decrease in wind speed 10 meters above the ground just 10-15 minutes after the sun was eclipsed (Turner et. al, 2018). Finally, a nocturnal jet formed, before dissipating shortly after the sun reappeared. Two things were completely unaffected by the change: 1. water vapor mixing ratio and 2. temperature at 200 meters above the surface.
What is the Nocturnal Jet?
The Nocturnal Jet, or low-level jet, is a high velocity (~13-26 m/s) current that appears at night. It is spurred by cool air at high elevation, but may depend on nightly atmospheric conditions and landscape (i.e., nocturnal jets are common in the Great Plains).
Think of how night-time air just feels different. For instance, small valleys have colder nighttime temperatures than flatter landscapes because cool, dense air sinks down into the space and the lack of sunlight allows it to settle early on in the night. Valleys are an example of a geographic feature that has small-scale impact on temperature, weather, wind, day light (more shading), and humidity. This is just one example of how land has an influence on the atmosphere- particularly at night.