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 

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. 

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 

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. 

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 As it expands, it cools through the adiabatic process and no heat is added or withdrawn from the system in which it operates.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. 

## Air Masses 

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 

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. 

## Characteristics 

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: 

• Stability, 

• Cloud types, 

• Sky coverage, 

• Visibility, 

• Precipitation, 

• Icing, and 

• Turbulence. 

The terrain surface underlying the air mass is the primary factor in determining air mass characteristics. 

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: 

• Polar or tropical 

• Maritime or continental 

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 

