A front is a transition zone between two air masses with different densities, most often produced by different combinations of temperature and moisture. The classic fronts are cold, warm, stationary, and occluded, and each is drawn on surface maps with its own symbol so a reader can tell at a glance which way the boundary is moving and which air mass is winning.
How fronts form
Air masses acquire their character over source regions where they sit long enough to take on the temperature and moisture of the surface below. When flow patterns push two of these air masses together, the denser one wedges under the lighter one and the sharp gradient between them becomes a front.
The intensification of that gradient by converging flow is called frontogenesis, and it is what turns a broad, weak air mass boundary into a narrow, well-defined front with clearly organized cloud and precipitation.
How forecasters use fronts
Fronts anchor the day's synoptic story. Their position at forecast time tells a forecaster where the air mass boundaries are, where lift is likely to be organized, and where the wind shift will change how convection behaves.
In severe weather setups, fronts do most of their work at their intersections with other boundaries. A cold front meeting a dryline near a surface low, or a warm front meeting an outflow boundary, is often the first place storms fire and often where they organize best.
Important limits
A drawn front is an interpretive product, not a measurement. Different analysts can place the same boundary slightly differently on the same data, and models can carry a front tens of kilometers away from where the mesonet and satellite say it really is.
Fronts also lose or gain identity through the day. A cold front can wash out as the temperature contrast erodes over land, or a stationary front can revive when new thermal advection kicks in. Watching how the frontal zone actually behaves in observations is more reliable than trusting the map from six hours earlier.