A '''front''' is a boundary or transition zone between two air masses of different density, and thus (usually) of different [[temperature]]. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.<ref>http://www.weather.gov/glossary/index.php?letter=f</ref> | A '''front''' is a boundary or transition zone between two air masses of different density, and thus (usually) of different [[temperature]]. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.<ref>http://www.weather.gov/glossary/index.php?letter=f</ref> |