A huge area of air in the lower section of the troposphere that has similar characteristics throughout is called an air mass. Air masses are named based on their characteristics. These variables are the temperature and moisture content. Air masses coming from colder areas are labeled as polar (P), whereas tropical masses (T) come from warm regions. Extremely cold regions supply arctic (A) air masses. If the source region is over the ocean, it is moist and labeled as maritime (m). Air originating from over land is called continental (c). The combination of the five labels and conditions describes air masses. For example, an mT air mass could originate in the Gulf of Mexico, whereas a cP air mass could come from the middle of Canada. The other possible air masses are cA (continental arctic), cT (continental tropical), and mP (maritime polar). As the masses move along into other areas, their characteristics can moderate. The weather experienced while in a particular air mass doesn't change much. The clashing of air masses creates other weather conditions that can lead to changes in weather.