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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 As it expands, it cools through the adiabatic process and no heat is added or withdrawn from the system in which it operates.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
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Figure 4-3. High and low pressure systems.
currents, but instead creates a stable air mass with poor surface visibility. The poor surface visibility is due to the fact that moisture, smoke, dust, and other particles cannot rise out of the air mass and are instead trapped near the surface. A stable air mass can produce low stratus clouds and fog.Pressure Systems
The differences that occur with heating and cooling the atmosphere in the lower levels also cause density variations. These variations cause small horizontal pressure differences that are only about one ten-thousandth of the magnitude of the normal change of pressure with altitude, but they significantly impact atmospheric circulation and most weather phenomena. [Figure 4-3]
A low or cyclone is a pressure system in which the barometric pressure decreases towards the center and the wind flow around the system is counterclockwise in the Northern Hemisphere. Unfavorable flying conditions in the form of low clouds, restricted visibility by precipitation and fog, strong and gusty winds, and turbulence are common in low pressure systems. Thermal low pressure systems caused by intense surface heating and resulting low air density over barren continental areas are relatively dry with few clouds and practically no precipitation. Thermal lows are stationary and predominate over continental areas in the summer. General airflow in a low pressure system, since the atmosphere is attempting to achieve equilibrium, is in (towards the center of the low pressure system), and up. This tendency can affect the overall dynamic of the low pressure system.
A high is a pressure system in which the barometric pressure increases toward the center and the wind flow around the system is clockwise in the Northern Hemisphere. Flying conditions are generally more favorable in highs than in lows because of fewer clouds, light or calm winds, and less concentrated turbulent areas. But, in some situations, visibility may be reduced due to early morning fog, smog, or haze at flight levels. High pressure systems predominate over cold surfaces where the air is dense. General airflow in a high pressure system, in reverse of the low pressure dynamic, is out (away from the center of the pressure system) and down. Again, these airflow tendencies can affect the dynamic of the high pressure system, much like the low.
　
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