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. 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.  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. 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. 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.  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. 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.  The five types of atmospheric stability are: Types of Stability 4-20 •  Convective stability •  Neutral stability •  Conditional instability •  Absolute instability •  Absolute stability Atmospheric Stability and Instability