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.