In weather, fronts are defined as the boundaries between different air masses. Depending on the direction of movement and the characteristics of the air involved, different types of fronts form. In this visualization from McDougal Littell/TERC, see movement of warm and cold fronts as well as the characteristic clouds that are generated by each.
Air masses can differ from each other in temperature, moisture, and pressure. These characteristics are generally the result of conditions in the area over which the air mass originates. Global winds such as the jet streams flow between air masses and direct their movement across Earth. Although air masses maintain their individual identities, their movement causes them to interact with each other, and it is at the boundaries between them that weather fronts form. The movement of a front over an area brings characteristic changes in temperature, pressure, winds, cloud cover, and precipitation.
A cold front forms when a mass of relatively cold air advances toward a mass of warmer air. Because cold air is denser than warm air, it wedges underneath the mass of warm air, forcing the warm air to rise and cool. The cold front moves quickly and has a steep edge that creates the rapid uplift needed to form cumulus clouds. A strong cold front may produce cumulonimbus clouds and thunderstorms, tornadoes, or snow squalls. Cold fronts also bring abrupt temperature changes. As a cold front advances, temperatures can drop more than 8°C (15°F) within a couple of hours.
When a mass of warmer air advances to replace colder air, the boundary is called a warm front. This type of front moves more slowly than a cold front. Warm fronts also have gentle slopes and are associated with less severe weather. As the warm air advances, it gently slides over and replaces the cold air. Typically, the leading edge of the warm front first produces high cirrus clouds. Later, the gradual lifting of the rest of the front produces layered clouds such as cirrostratus, altostratus, and nimbostratus clouds. Because of the gentle slope and slow movement, warm fronts can produce steady precipitation that lasts for days.
On a weather map, fronts are shown as colored lines with markings. Cold fronts are drawn as blue lines with triangles pointing in the direction of movement. Warm fronts are drawn as red lines with scallops that face the direction of movement. Additionally, two other types of fronts may be shown: stationary and occluded. Stationary fronts — where there is little or no movement of air masses — are illustrated with alternating warm- and cold-front symbols. Occluded fronts — where three different air masses meet and keep warm air trapped away from the surface — are shown as purple lines with alternating triangles and scallops.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.