Final answer:
Temperature inversions occur when surface temperatures are cooler than overlying air, which is option 3). This phenomenon leads to warmer air being held above cooler air, often trapping air pollutants and affecting weather patterns.
Step-by-step explanation:
In meteorology, a temperature inversion occurs when the usual temperature decrease with altitude is altered so that a layer of cool air is trapped beneath a layer of warmer air. This situation is option 3): when surface temperatures are cooler than overlying air. During a temperature inversion, warmer air is held above cooler air, which can lead to the accumulation of air pollutants near the ground, negatively affecting air quality. Moreover, temperature inversions can suppress convection and potentially lead to violent weather events if this cap is broken.
Normally, the air temperature decreases with height. However, under certain conditions such as clear nights, long-wave radiation from the Earth's surface can escape into space, cooling the surface and the nearby air. When this cool air does not mix with the warmer air above it, an inversion forms. Inversions are significant because they can lead to poor air quality and influence local weather patterns.