15. on a surface weather map, a stationary front is drawn in purple. 16. the top part of a thunderstorm usually has a negative charge. 17. on average, cities tend to be warmer and more polluted than rural areas. 18. when the base of an inversion lowers, pollutants are able to be dispersed throughout a greater volume of air. 19. the terms typhoon and cyclone refer to the initial weaker stages in development of a hurricane. 20. the region of strong winds of a jet stream is called a jet maximum or jet streak.