Temperature Inversions and Smoke

When the ambient temperature increases with height, an inversion is said to be present. It usually marks a layer of strong stability. When a heated air parcel from the surface encounters an inversion, it will stop rising because the ambient air is warming faster than the expanding parcel is cooling. The parcel being cooler than its surroundings will sink. Although the heat from some fires is enough to break through a weak inversion, inversions often are referred to as lids because of their effectiveness in stopping rising air and trapping pollutants beneath it. Smoke trapped under an inversion can substantially increase concentrations of particles and gases, aggravating respiratory problems and reducing visibility at airports and along roadways.

There are three ways that surface-based inversions typically form: (1) valley inversions are very common in basins and valleys during clear nights when radiation heat losses cause air near the ground to rapidly cool: the cold surface air flows from the surrounding slopes and collects in hollows and pockets, allowing warmer air to remain aloft; (2) advective inversions are caused by cold air moving into a region from a nearby lake or ocean, usually during the afternoon when onshore lake and sea breezes tend to form; and (3) subsidence inversions can occur at any time of day or night as cold air from high altitudes subsides or sinks under a region of relatively stagnant high pressure. Valley inversions cause tremendous problems when managing long-duration fires that continue into the night. Advective inversions can surprise smoke managers who are unfamiliar with local lake- and sea breeze effects, creating poor dispersal conditions in an afternoon when typically good dispersion is expected. Subsidence inversions are difficult to predict even for a well-trained meteorologist. Figure 7.7 shows smoke caught under a valley inversion that is being transported by downvalley winds in the early morning.

Surface inversions also occur in the gaps (passes and gorges) of mountain ranges. Approaching storms usually have an associated center of low pressure that causes a pressure gradient across the range. If cold air is on the opposite side of the range, the gradient in pressure causes the cold air to be drawn through the gap, creating an inversion in the gap. Gap inversions are most common in winter but also are frequent during spring and autumn.

In addition to surface-based inversions, temperature inversions also occur in layers of the atmosphere that are above the ground surface, which sometimes are called thermal belts. Upper-level inversions usually are associated with incoming warm fronts that bring moisture and warmth to high altitudes well ahead of a storm. The inversion lowers to the ground as the front approaches. Upper-level inversions also may be associated with subsidence or surface-based inversions that have been lifted, usually by daytime heating.

See: Inversions in the Fire Weather section for more information.