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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.
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.
Effects of Stable and Unstable Air
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.
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 results, especially near industrial areas. Such conditions frequently persist for days, notably in the Great Basin region of the western United States.Weather HazardsTurbulence
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:
• 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
Two or more of the above causative factors often work together. In addition, turbulence is produced by manmade phenomena.Thermal
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 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
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Balloon Flying Handbook(52)
