An air mass is a large body of air that has relatively uniform temperature and humidity characteristics throughout. It can cover hundreds or thousands of square miles and extends vertically through the atmosphere, sometimes up to the top of the troposphere (about 10–18 km above the Earth's surface)

. Air masses form when the atmosphere remains over a large, relatively uniform surface area (land or sea) for a sufficient time, acquiring the temperature and moisture properties of that surface. The characteristics of an air mass are primarily determined by its source region, which can be polar, tropical, arctic, equatorial, continental (land), or maritime (sea)

. Air masses are classified by their moisture and temperature properties using a system such as the Bergeron classification:

• The first letter indicates moisture: "c" for continental (dry) or "m" for maritime (moist).
• The second letter indicates temperature source region: "T" for tropical, "P" for polar, "A" for arctic/antarctic, "E" for equatorial, etc.
• A third letter may indicate stability or modification relative to the surface below (e.g., "k" for colder than surface, "w" for warmer)

Examples include:

• Continental Polar (cP): cold and dry air from land in polar regions.
• Maritime Tropical (mT): warm and moist air from tropical oceans

When air masses move, they can modify their characteristics depending on the surfaces they pass over, and when two air masses with different properties meet, they form weather fronts that influence weather patterns

. In summary, an air mass is a large, relatively uniform body of air defined by its temperature and humidity, originating from a specific geographic source region, and influencing weather over large areas