The dome-shaped formation of stagnant and polluted air above a city is known as a dust dome. As any summer city-dweller eager to get to the country for a cool weekend knows, the air in an urban environment is often signiﬁcantly warmer than air in the surrounding rural area, creating a phenomenon called the urban heat island. Industrial machinery and furnaces, manufacturing complexes, cars, and even air conditioners heat up the city’s air; building materials such as concrete, asphalt, and brick retain and radiate that heat well into the night. The large number of windows and other reﬂective surfaces serve to trap heat, and the lack of areas of open water sustains it. Soon the city is cooking; an inversion layer forms, which, because it’s capped at a relatively low level in the atmosphere, causes a dome of air pollutants to form over the city. If there is no wind, the dust dome remains intact, its pollutants sometimes growing a thousand times more concentrated over the urban area than in a nearby rural area. If the winds begin to blow strongly enough, however, the dust dome will elongate downwind, forming a dust plume. The city’s pollutants are then spread to its neighbors in the country.