What indicates the presence of stable air?

Stable air is characterized by limited vertical movement, which results in a shallow lapse rate and a close temperature-dewpoint spread. When the lapse rate is shallow, it means that temperature decreases with altitude at a slower rate than normal, suggesting that the air is not prone to significant upward motion. This stability helps to prevent the development of strong convective activity and turbulence.

Additionally, a close temperature-dewpoint spread indicates that the air is near saturation and can contribute to a stable environment because there is less likelihood of moisture rising to form clouds and precipitation. In stable air, typical weather conditions include calm weather or low clouds without significant vertical development.

In contrast, the other choices describe conditions that are associated with instability or do not directly indicate the presence of stable air. A gentle slope in terrain does not intrinsically signify air stability, as it may exist regardless of the atmospheric conditions. A strong vertical wind pattern typically leads to turbulence and instability, while a rapid change in temperature with altitude suggests a steep lapse rate indicative of unstable air, where buoyant forces can promote vertical movement.