Night Inversions

night inversion

smoke and inversions

Air cooled at night, primarily by contact with cold, radiating surfaces, gradually deepens as the night progresses and forms a surface inversion. This is a surface layer in which the temperature increases with height. Such an inversion may involve a temperature change of as much as 25° F. in 250 vertical feet. The cold air is dense and readily flows down slopes and gathers in pockets and valleys. Surface inversions forming at night are commonly referred to as night inversions. Night inversions are so important in fire behavior that we should consider them in some detail.

Night inversions are common during clear, calm, settled weather. They are usually easy to identify. Inversions trap impurities, smoke, and factory and traffic fumes, resulting in poor visibility. Smoke from chimneys rises until its temperature matches that of the surrounding air. Then it flattens out and spreads horizontally. If fog forms in the cold air, it is generally shallow ground fog, its depth generally less than that of the inversion. Ground fog in patches in surface depressions along highways is formed in small-scale inversions.

Cloudiness and water vapor in the atmosphere limit the formation and strength of night inversions by reducing the rate of outgoing radiation from the earth.

On windy nights, compared with calm nights, turbulence and mixing distribute the cooling through a deeper layer, and the temperature decrease is less. Winds may reduce and sometimes prevent the formation of a night inversion. The drop in temperature near the ground at night is thereby often abruptly stopped or even reversed when the wind picks up.

Night inversions are shallow but more intense when the overall temperature structure of the atmosphere is stable. Under unstable conditions, convection distributes available heat. Inversions are therefore less likely, and those occurring will not be as intense. However, if a night inversion is able to form, mixing is reduced in the lower layers.

Topography plays a decided role in both the formation and intensity of night inversions. Cold air layers are quite shallow on slopes and in open canyons or ravines where the cold, dense air can drain away as it is formed. This descent of cold air results in the formation of deep, cold layers and inversions in valleys. Inversion layers are both more common and intense in lower mountain valleys or in basins with poor air drainage, than in flat areas.

In mountainous areas, the height of the top of night inversions, although it varies from night to night, is usually below the main ridges. The height of the warmest air temperature at the inversion top can be found by measuring temperatures along the slope. From this level, the temperatures decrease as one goes farther up or down the slope. At this level are both the highest minimum temperatures and the least daily temperature variation of any level along the slope. Here also are the lowest nighttime relative humidity and the lowest nighttime fuel moisture. Because of these characteristics of the average level of the inversion top, it is known as the thermal belt. Within the thermal belt, wildfires can remain quite active during the night. Below the thermal belt, fires are in cool, humid, and stable air, often with downslope winds. Above the thermal belt, temperatures decrease with height. The effect of the lower temperatures, however, may be offset by stronger winds and less stable air as fires penetrate the region above the thermal belt.

Night inversions in mountainous country increase in depth during the night. They form early in the evening at the canyon bottom or valley floor and at first are quite shallow. Then the cold layer gradually deepens, the top reaching farther up the slope with the continued cooling from the surface and the flow of cold air from adjoining slopes. A maximum depth is reached during the middle of the night, and the depth may then remain constant or decrease slightly just before sunrise. If the air is sufficiently cold and moist, fog may form.

After sunrise, surface heating begins to warm the cold air, and the inversion top may actually rise slightly from this expansion. As heating destroys the inversion along the slopes, upslope winds begin. The transport of air from the valley bottom up the slopes may actually cause the inversion top to lower over the middle of the valley. Finally, with continued heating and mixing, the inversion layer is completely dissipated. The behavior of a fire burning beneath an inversion may change abruptly when the inversion is destroyed.

See also: Marine inversions.