• Air masses are meteorological phenomena responsible for affecting weather conditions. Air masses are identified by formal classifications, including Arctic, Polar and Tropical. Sub-classifications indicate the surface below the air mass, so that a classification of "mT" indicates a maritime tropical air mass, where "m" refers to the surface type beneath the air mass, and "T" indicates that the air mass is located in a tropical latitude.
  
  

High Dew Point

• Dew points are indicators of the amount of moisture in the air. The dew point is expressed in terms of the temperature that air would have to cool to in order to reach its saturation point. A dew point of 70 degrees Fahrenheit would feel very sticky and humid to most people. Maritime tropical air masses often reach dew points of 80 to 90 degrees Fahrenheit in summer. The instrument used to measure the dew point is known as a "hygrometer."
  
  

High Temperatures

• Maritime tropical air masses are characterized by high temperatures. The source regions of these air masses are the the ocean areas falling between the Tropic of Cancer in the northern hemisphere and the Tropic of Capricorn in the southern hemisphere. Air temperatures (in Fahrenheit) range from the 80s to 90s in summer and even in winter remain in the 70s and 80s.
  
  

High Humidity

• Humidity describes the capacity of air to hold water in the form of water vapor. As temperatures rise, the capacity of air to hold water increases. Air at 60 degrees Fahrenheit may contain 70 percent of the water that it could support at that temperature, so it would have a relative humidity (amount of water vapor in the air, relative to the temperature) of 70 percent at that temperature. If the temperature were to increase to 80 degrees Fahrenheit, the relative humidity would fall to just 55 percent, as air at higher temperatures can support more water, but the actual amount of water vapor in the air has stayed the same. Thus,100 percent humidity at 80 degrees means that there is more actual water vapor in the air than a reading of 100 percent humidity at 60 degrees, because at the higher air temperature, the air mass can hold more water vapor.
  
  Since temperatures of maritime tropical air masses are high, a high humidity level means that there is more water vapor in the air than could likely be contained by the same amount of air at lower temperatures, and this is what makes these air masses uncomfortably sticky. Maritime tropical air masses can have a relative humidity of 80 percent or higher.