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stationary front. Occasionally, a slow-moving winter cold front with light wind may generate fog. This fog forms in the polar air behind the surface front and is known as postfrontal fog.
Steam fog forms when cold stable air flows over a nonfrozen water surface that is several degrees warmer than the air. The intense evaporation of moisture into the cold air saturates the air and produces fog. Conditions favorable for steam fog are common over lakes and rivers in the fall and over the ocean in the winter when an offshore wind is blowing.Atmospheric Stability and Instability
A stable atmosphere resists upward or downward movement, and small vertical disturbances dampen out and disappear. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical or convective current allowing small vertical air movements to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather.
Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. This adiabatic process (heating or cooling) 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 water vapor is lighter than air, moisture decreases air density, causing it to rise. Conversely, as moisture decreases, air becomes denser and tends to sink. Since moist air cools at a slower rate, it is generally less stable than dry air since the moist air must rise higher before its temperature cools to that of the surrounding air. The dry adiabatic lapse rate (unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 feet. The combination of moisture and temperature determine the stability of the air and the resulting weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally clear weather. The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the instability of the surrounding air.
The normal flow of air tends to be horizontal. If this flow is disturbed, a stable atmosphere will resist any upward or downward displacement. It will tend to return quickly to normal horizontal flow. An unstable atmosphere, on the other hand, will allow these upward and downward disturbances to grow, resulting in rough (turbulent) air. An example is the towering thunderstorm which grows as a result of large and intensive vertical movement or air. It climaxes in lightning, thunder, and heavy precipitation, sometimes including hail.
Atmospheric resistance to vertical motion, called stability, depends upon the vertical distribution of the air’s weight at a particular time. The weight varies with air temperature and moisture content. In comparing two parcels of air, warmer air is lighter than colder air, and moist air is lighter than dry air. If air is relatively warmer or moister that its surroundings, it is forced to rise and would be unstable. If the air is colder or dryer than its surroundings, it will sink until it reaches its equilibrium, and would be stable. The atmosphere can be at equilibrium only when light air is above heavier air.
Temperature has a significant effect on the stability or instability of the air mass. Air heated near the Earth’s surface on a hot summer day will rise. The speed and vertical extent of its travel depends on the temperature distribution of the atmosphere. Vertical air currents, resulting from the rise of air, can vary from the severe downdraft and compensating downdraft associated with thunderstorms to the closely spaced upward and downward bumps that are felt on warm days when flying at low levels. Since the temperature of air is an indication of its density, a comparison of temperatures from one level to another can approximate the degree of the atmosphere’s stability, or how much it will tend to resist vertical motion.
Types of Stability
The five types of atmospheric stability are:
• Absolute stability
• Absolute instability
• Conditional instability
• Neutral stability
• Convective stability
Absolute stability occurs when the actual lapse rate in a layer of air is less than the moist adiabatic lapse rate; that air is absolutely stable regardless of the amount of moisture it contains. A parcel of absolutely stable air which is lifted becomes cooler than the surrounding air and sinks back to its original position as soon as the lifting force is removed.
　
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