Air masses are large bodies of air that create distinctive weather conditions across the globe.
An air mass is defined as a body or 'mass' of air with uniform weather conditions, such as similar temperature and humidity. Air masses may cover several million square kilometres and extend high up into the atmosphere.
They are primarily defined by the area in which they originate, this is called the 'source region'. The characteristics of an air mass can then be modified as they travel across the globe. Where two air masses of different temperatures meet, a boundary forms which is termed a 'front'.