Similarly, if forced to descend, it becomes warmer than the  surrounding air; like a cork in water, it rises to its original  position upon removal of the outside force. Absolute instability exists when the actual lapse rate in  a layer of air is greater than the dry adiabatic lapse rate;  that air is absolutely unstable regardless of the amount of  moisture it contains. A parcel of air lifted even slightly will  immediately be warmer than its surroundings, and, as with  a hot air balloon, will be forced to rise. Conditional instability exists when the temperature lapse rate  of the air involved lies between the moist and dry adiabatic  rates of cooling. Before the displaced air actually becomes  unstable, it must be lifted to a point where it is warmer than  the surrounding air. When this point has been reached, the  relatively warmer air continues to rise freely until, at some  higher altitude, its temperature has cooled to the temperature  of the surrounding air. In the instability process, numerous  variables tend to modify the air. One of the most important  of these variables is the process called entrainment. In this  process, air adjacent to the cumulus or mature thunderstorm  is drawn into the cloud primarily by strong updrafts within  the cloud. The entrained air modifies the temperature of the  air within the cloud as the two become mixed. results, especially near industrial areas. Such conditions  frequently persist for days, notably in the Great Basin region  of the western United States. Turbulence is the irregular motion of the atmosphere as  indicated by gusts and lulls in the wind. Since turbulence is  associated with many different weather situations, knowledge  of its causes and its behavior will help a balloon pilot avoid  or minimize its effects. Turbulence can be divided into four categories according to  the specific causes: Two or more of the above causative factors often work  together. In addition, turbulence is produced by manmade  phenomena. Neutrally stable air is air with the same temperature, and there  is no parcel to rise or descend. For example, the surface area  in contact with that air is of the same temperature. The term convective instability refers to a condition in  which the air becomes unstable after lifting. From a physical  standpoint, it closely resembles a conditionally unstable  air mass, but has the mechanical lifting of thermal activity  impacting on the overall characteristics of the air. A thermal is simply the updraft in a small-scale convective  current. Convective currents (vertical or horizontal air  movements) develop in air, which is heated by contact with  a warm surface. This heating from below occurs when either  cold air is advected (moved horizontally) over a warmer  surface or the ground is strongly heated by solar radiation. The degree of stability of the atmosphere helps to determine  the type of clouds formed, if any. For example, if very  stable air is forced to ascend a mountain slope, clouds will  be layer-like with little vertical development and little or no  turbulence. Unstable air, if forced to ascend the slope, would  cause considerable vertical development and turbulence in  the cumulus-type clouds. The strength of convective currents depends in part on the  extent to which the Earth’s surface has been heated, which  depends upon the nature of the surface. Barren surfaces, such  as sandy or rocky wasteland and plowed fields, are heated  more rapidly than surfaces covered in vegetation. Thus,  barren surfaces generally cause stronger convection currents.  In comparison, water surfaces are heated more slowly.  When air is very dry, convective currents may be present  although convective-type clouds (cumulus) are absent. The  general upper limits of the convective currents are often  marked by the tops of cumulus clouds, which form in them  when the air is moist, or by haze lines. However, turbulence  If air is subsiding (sinking), the heat of compression  frequently causes an inversion of temperature which increases  the stability of the subsiding air. When this occurs, as in  winter high pressure systems, a surface inversion formed  by radiational cooling is sometimes already present. The  subsidence-produced inversion, in this case, will intensify  the surface inversion, placing a strong “lid” above smoke and  haze. Poor visibility in the lower levels of the atmosphere  4-21 Effects of Stable and Unstable Air Thermal Weather Hazards Turbulence  •  Thermal—caused by localized convective currents due  to surface heating or unstable lapse rates and cold air  moving over warmer ground or water •  Mechanical—resulting from wind flowing over  irregular terrain or obstructions •  Frontal—resulting from the local lifting of warm air  by cold air masses, or the abrupt wind shift (shear)  associated with most cold fronts •  Wind shear—marked gradient in wind speed and/or  direction due to general vibrations in the temperature  and pressure fields aloft