Analysis of a 12-sol period of wind speed, wind direction, temperature, pressure, and optical depth at the Viking lander 2 site presents the first in situ evidence of high- and low-pressure systems, complete with fronts, on the surface of Mars. The discovery of these systems in the landed data occurred while analyzing a period during which the north polar hood was advected over the site at midday, dramatically decreasing the surface illumination and surface-to-atmosphere heat flux. This obscuration immediately preceded a global dust storm in the southern hemisphere and low latitudes of the northern hemisphere. The direct effects of the dust storm, other than pressure variations, reached 48¿N, the lander 2 latitude, later and more gradually than they reached the 22¿N latitude of lander 1. The system responsible for the polar hood passage is a major disturbance, and it appears that radiational damping is inadequate to stop strong frontal formation. The front analyzed, although it was the strongest observed in this sol, is characteristic of a repetitive series of systems that pass roughly every 3.3 sols. These systems are similar to those predicted by theoretical analyses and by general circulation modeling of the Martian atmosphere and those observed in laboratory experiments.