Isotherm lines that are placed close together on a weather map represent a weather front. A weather front is a boundary between different densities of air in a given region.
There are many different variations of weather fronts. A cold front is a boundary where a mass of cold air is pushing beneath and displacing a mass of warmer air. This process may lead to thunderstorms, rain, and hail.
Warm fronts are located at the head of a mass of warm air as it moves across a region. Warm fronts are normally uniform in temperature and are accompanied by a large embankment of featureless, low atmospheric clouds called strati. Fog is common along a warm front, with thunderstorms appearing occasionally if the warm air mass begins to become unstable.
An occluded front is a situation where a cold front and a warm front mix. For the most part, because cold fronts move almost twice as fast as warm fronts, an occluded front is a boundary where a cold front has caught up to a warm front. As the two fronts mix, they curve toward either pole. In addition to this curvature, the air in the warm front tends to become increasingly more and more dry.