result of the continuous alternation of the influences of warm  and cold air masses. Warm air masses predominate in the  summer, and cold air masses predominate in the winter.  However, both cold and warm air, alternately, may prevail  almost anywhere in the temperature zone at any season. The  basic characteristics of any air mass are temperature and  humidity. These properties are relatively uniform throughout  the air mass, and it is by measurement of these properties that  the various types of air masses are determined.  method of heat exchange only at the Earth’s surface, where  the lowest few centimeters of the atmosphere are actually in  contact with the ground or water. Convection is the transfer of heat energy in a fluid. This type  of heating is most commonly seen in the kitchen when a liquid  boils. This type of heat transfer occurs in the atmosphere  when the ground is heated by the sun. The warm ground heats  the air above it by radiation and conduction, causing the warm  air to rise. Meanwhile, the dense cooler air aloft moves in to  take the warm ground air’s place to be heated. Air masses acquire the characteristics of the surrounding  area, or source region. The characteristics of an air mass  consist of the basic properties of moisture and temperature,  which include: Heat can be transferred by convection in either a vertical or  a horizontal direction. In meteorology, “advection” is the  term used for the horizontal transport of heat by wind. It is  important to differentiate between the vertical and horizontal  paths of convection. In the atmosphere, the amount of heat  transferred horizontally over the surface of the Earth by  advection is about 1,000 times greater than that transferred  by convection. The adiabatic process is the change of the temperature  of air without transferring heat. In an adiabatic process,  compression results in warming, and expansion results in  cooling. The adiabatic process takes place in all upward and  downward moving air. When air rises into an area of lower  pressure, it expands to a larger volume. As the molecules  of air expand, the temperature of the air lowers. As a result,  when a parcel of air rises, pressure decreases, volume  increases, and temperature decreases. When air descends,  the opposite is true. A source region is typically an area in which the air remains  relatively stagnant for a period of days or longer. During  this time of stagnation, the air mass takes on the temperature  and moisture characteristics of the source region. Areas of  stagnation can be found in polar regions, tropical oceans,  and dry deserts. Air masses are classified by region of  origination: The terrain surface underlying the air mass is the primary  factor in determining air mass characteristics. Since air is composed of a mixture of gases subject to heating  when compressed and cooling when expanded, air will rise,  seeking a level where the pressure of the body of air is equal  to the pressure of the air that surrounds it. Whatever the  cause of the lifting, the air rises, and the pressure decreases,  allowing the “parcel of air” to expand. This continues until  it reaches an altitude similar in pressure and density to its  own. As it expands, it cools through the adiabatic process  and no heat is added or withdrawn from the system in which  it operates. As air rises, it is cooled because it is expanding  by moving to an altitude where pressure and density is less.  This is adiabatic cooling. When the process is reversed and  air is forced downward, it is compressed, causing it to heat  by a process called adiabatic heating. A continental polar air mass forms over a polar region and  brings cool, dry air with it. Maritime tropical air masses  form over warm tropical waters like the Caribbean Sea and  bring warm, moist air. As the air mass moves from its source  region and passes over land or water, the terrain it passes  over modifies its qualities, and thus modifies the nature of  the air mass.  An air mass passing over a warmer surface will be warmed  from below, and convective currents form, causing the air  to rise. This creates an unstable air mass with good surface  visibility. Moist, unstable air causes cumulus clouds,  localized showers, and turbulence to form. Conversely, an air  mass passing over a colder surface does not form convective  An air mass is a large body of air (usually 1,700 kilometers  or more across) whose physical properties (temperature and  humidity) are horizontally uniform. The weather is a direct  4-5 •  Polar or tropical •  Maritime or continental Air Masses The Adiabatic Process Characteristics Convection •  Stability, •  Cloud types, •  Sky coverage, •  Visibility, •  Precipitation, •  Icing, and •  Turbulence.