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Subsidence Inversions

Recognition and Application in Lighter-than-air Flight

by Harold F. Maybeck, Meteorologist

Plymouth State College Plymouth, New Hampshire

Preface: This paper is a brief discussion of atmospheric temperature inversions and their recognition and application in lighter-than-air flight. The majority of lighter-than-air flight today is in "hot air balloons". While the application of atmospheric temperature inversions is applicable to this type of ballooning, this paper is basically intended for those Aeronauts who venture forth in "gas" balloons. Pilots of both free-lift gas balloons and non-rigid powered balloons (blimps) may benefit from this information. While this information is also applicable to rigid airships (dirigibles), their practical non-existence in today's world of flight makes this paper quite academic with regard to dirigibles. The efforts put forth in this paper were inspired by the dedication to that beautiful art of ballooning that I perceived from the many members attending the Granite State Balloon Association's, "Safety Seminar", that I had the good fortune to attend and speak at, in Hillsboro, New Hampshire, in February, 1996.

Abstract: This is a brief paper describing the meteorological parameters of, and the effects upon lighter-than-air gas balloons, of subsidence inversions. A reversal of the normal decrease in temperature with altitude may occur in the real atmosphere. This atmospheric layer where the temperature increases with an increase in altitude is called a temperature inversion. Temperature inversions are layers of pronounced atmospheric stability, while the layers below and above the inversion may be basically atmospherically unstable. Upper air inversions caused by the sinking of air that is then warmed adiabatically are called "subsidence inversions". The basic meteorological functions associated with subsidence inversions that affect gas ballooning are the differences in atmospheric stability and the differences in wind across the inversion. There are many ways to visually recognize subsidence inversions, including cloud formations, haze and smoke layers, and the soaring of birds in flight. The use of the various flight instruments on board the aerostat may also be used to instrumentally recognize, and calculate the intensity of, a subsidence inversion.

Inversions

The "normal" lapse rate in the troposphere is about 3.5°F per 1000 feet. This "normal" lapse rate is really just the average rate at which the temperature decreases with increase in altitude. The actual lapse rate in any parcel of air may vary considerably dependent upon its water vapor content, e.g., its saturation. Under varying conditions, a reversal of the normal lapse rate may develop, where a layer of air may have an increase in temperature with increase in altitude. This is called an "inversion". Inversions frequently develop at or near the earth's surface during clear, calm nights due to the cooling caused by the earth's radiation. Warm air advected over a cooler surface may also form surface inversions. These surface or very low level inversions, due to the advection of warmer air over the cold surface air, will be diluted due to the mixing of the surface layer, if the wind becomes stronger. In these cases, the inversion may be found at some distance above the ground. Well above the earth's surface, inversions may also form due to the descending, or subsiding air in the atmosphere associated with high-pressure areas. This descending air then spreads out horizontally above the surface layer. The inversion is therefore a product of the adiabatic heating of the descending air. The layer just above these "subsidence inversions" is then usually relatively dry. In ballooning, one of the most important features of the air in the layer of the increasing temperature, is pronounced stability. Mixing, caused by the turbulent action of convection, is hampered by an inversion, since the rising air currents are suppressed from moving in the vertical by the stability of the air within the inversion layer. Whereas the mixing caused by turbulent air tends to spread moisture vertically, the relative humidity below the top of the inversion may be quite high compared to the relatively dry air just above the inversion layer. Many times this layer of higher moisture content will develop stratus or stratocumulus clouds just below the top of the inversion. The top of dust, haze, and smoke layers is also indicators of the top of a low level inversion. Figure 1 - Conditional Stability(Instability)

The vertical movement of air in the atmosphere undergoes a temperature change. Downward moving air is heated by adiabatic compression, while upward moving air is cooled by adiabatic expansion. Inversions aloft frequently have distinct cloud formations associated with them. The anvil-shaped tops of thunderstorms are due to a temperature inversion at very high altitude through which the convection within the cumulonimbus cannot penetrate. Stratocumulus clouds are almost always capped by a temperature inversion. Strong cumulus clouds with sufficient convective energy to "bust" through an inversion are frequently girdled by a layer of altostratus at the top of the inversion.

Subsidence Inversions

The slow sinking of air in areas of high pressure is an important factor in air mass modification. This slow sinking or "subsiding" is then responsible for the development for a large number of the inversions that form in the free atmosphere, well above the earth's surface. These subsidence inversions are formed by the slowly sinking air being heated by adiabatic compression. These subsiding layers are more stable than they were at their original higher altitudes. Subsiding air almost never continues downward to the earth's surface. Near the earth's surface there is always, however slight, some turbulent mixing taking place. This slow sinking is almost always counteracted by the turbulent mixing. Therefore, since subsiding air almost never reaches the ground, "subsidence inversions" are always found well above ground level.

Summary

Subsidence inversions are usually only about two or three thousand feet thick. The base of subsidence inversions may vary from about 6,000 feet to around 18,000 feet, but usually range in the 8,000 to 12,000 foot range. The layer of air beneath the base of the inversion layer and the layer of air above the top of the inversion layer are usually quite unstable, while the layer of air within the inversion layer is very stable.

Subsidence Inversions and Gas Ballooning

There are basically two meteorological functions associated with subsidence inversions that can affect gas ballooning. The first is stability of the air, and the second is wind. As shown, the air within the temperature inversion layer is very stable, while the air above the inversion layer can be quite unstable. Either a hot-air or gas balloon attempting to descend through a subsidence inversion will find, first, a possible bump or hesitancy of the balloon to "bust" through the top of the inversion. Once through the top of the inversion, the balloon may require much more lift than was required above the inversion top to maintain the same rate of descent. Likewise, attempting to ascend through the top of a subsidence inversion usually requires much more lift below the inversion top than above the inversion top to continue the same rate of ascent. Sometimes, the air may be so very stable just below the top of the inversion that ascending balloons find it difficult to "bust" through the inversions, and descending balloons find this stable layer at the top of the inversion causes a platform that may be difficult to push through. This is frequently found with low-powered blimps. Descending balloons and blimps may actually tend to bounce off the top of the inversion if insufficient ballasting, venting, or in the case of blimps, insufficient down trim is used. A conclusion may be made that balloons can many times float in near equilibrium just above the top of a subsidence inversion using a reduced amount of lift. The unstable air just above the top of the inversion affords the extra lift needed for prolonged lighter-than-air flight without the use of added lift either from hot-air or from gas. Since subsidence inversions are usually above the altitudes of the hot-air balloonists, the recognition of subsidence inversions can be a boon to gas balloonists regardless of whether the aerostat is a helium, hydrogen, or ammonia balloon. The wind below and above a temperature inversion may be radically different. The wind within an inversion layer is usually quite light (even calm). The wind just below the base of, and just above the top of a subsidence inversion may have a radically different wind direction than the wind within the inversion layer. The wind speed is frequently moderately stronger just below the base of the inversion layer and also moderately, to many times more than moderately, stronger just above the top of the inversion layer, than within the inversion layer itself.

Visually Recognizing Subsidence Inversions

Although subtile, there are many ways to visually recognize a subsidence inversion. Finding the base or lower extremity of the inversion is difficult, but the top of the temperature inversion is frequently quite obvious. If the sky is clear, the top of a haze or smoke or dust layer may point out the top of a subsidence inversion. By midday, small cumului that are forming may tend to have a flattened top indicating the top of a subsidence inversion. Many times this flattened top may spread out slightly, forming what early air-mail pilots referred to as "pancake cumulus". As noted above, strong cumulus clouds with sufficient convective energy to "bust" through an inversion are frequently girdled by a layer of altostratus at the top of the inversion. As a subsidence inversion becomes stronger, and continues to sink, the tops of cumulus clouds may be depressed into the cloud form known as "stratocumulus formed by the spreading out of cumulus" (low cloud Cl 4). Whether they are formed by the flattening out of cumulus or have formed on their own...stratocumulus type clouds are almost always capped or topped by the upper limit of a moderate to strong temperature inversion. When a large high pressure system dominates the area, subsidence inversions once formed during the day, tend to sink during the night to, or near, the level of the lower extremity of a moist air layer. This is usually very close to the previous day's convective condensation level, which would be near the base of yesterday's cumulus clouds. Since subsidence inversions are usually only a few thousand feet in vertical depth, this clue would give a good "estimate" to the "probable" top of the subsidence inversion during early morning hours before any visible haze layer or cumulus clouds may have developed. Visible observations of birds soaring and gliding without flapping their wings may give another clue. During midday and late afternoon, the birds may well be soaring around in the uplift just above the top of a "thermal". These thermals usually top out if they encounter any stable layer in the atmosphere, such as a subsidence inversion. During early morning flight, before the ground has heated up and any thermals have had a chance to generate, soaring birds, such as hawks and vultures, frequently maintain lift in flight by using the additional lift afforded by the unstable layer just above a subsidence inversion. Figure 2 - Typical Subsidence Inversion

Instrumentally Recognizing Subsidence Inversions

A subsidence inversion can be recognized in flight using the flight instruments usually found on board the aerostat. A temperature indicating device designed to indicate the ambient air temperature of the atmosphere outside of the gondola or basket is very helpful. An increase in this free air temperature during ascent would indicate that the balloon is entering the base or bottom of an inversion layer. Conversely, a decrease in the free air temperature, after there had been an increase in temperature, would indicate that the balloon has now passed through the top of the inversion layer. During descent, the temperature should normally continue to increase. If a decrease in free air temperature is noted during descent this would indicate the the balloon has passed through the top of an inversion. Once the free air temperature begins to increase again, the balloon has now passed through the base or bottom of the inversion layer. By noting the altitude on the altimeter corresponding to these various temperature changes, the temperature change rate, or lapse rate, can be easily calculated. If the lapse rate within the inversion layer indicates a temperature increase with increase in altitude of more than 2 or 3 degrees Fahrenheit per one thousand feet, then the inversion is steep enough to indicate very stable air within the inversion layer. This increase in stability induces a great reduction in lift. If a variometer is included in the aerostats instrument cluster, a "change" in the ascent rate or the sink rate may be noted. A decrease in the ascent rate is an indication that the aerostat has entered the bottom of an inversion layer, and the subsequent increase in the ascent rate again, indicates that the balloon has now passed through the top of the inversion layer. During descent, an increase in the sink rate indicates entering the top of the inversion layer, and the subsequent decrease in the sink rate again indicates the balloon has passed through the bottom of the inversion layer.

Note: ALL ALTITUDES REFERED TO IN THIS PAPER ARE "AGL". HEIGHT ABOVE GROUND LEVEL

Conclusion

Although this paper is brief, it is hoped that it may give some insight into the vagaries of "subsidence inversions" and how they may affect gas balloonists. Although, akin to hot air ballooning, those Aeronauts who venture aloft in gas balloons, are subject to a different set of meteorological rules. Archimedes' principle of buoyancy does apply, but... the application of modern weather forecasting can, and does, make gas ballooning, not only safer, but extremely more enjoyable.

Basic Definitions

Adiabatic Process, n. In meteorology, a thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. In an adiabatic process, "compressions" always results in warming, "expansions" in cooling. In meteorology, the adiabatic process is often also taken to be a "reversible process".

Adiabatic Lapse Rate, n. In meteorology, a lapse rate of temperature, defined as the rate of decrease of temperature with height of a parcel of air lifted adiabatically through an atmosphere in "hydrostatic equilibrium".

  • For dry air this rate is approximately 5.4°F per 1000 feet.
  • For moist air, owing to the release of latent heat, this lapse rate is less than, and for very moist air, considerably less than, the dry adiabatic lapse rate, and the differential equation representing the process must be integrated numerically.
  • The normal (standard or average) lapse rate in the atmosphere below the tropopause is approximately 3.5°F per 1000 feet. (See Lapse Rate below).

Inversion, n. Act of inverting; state or position of being inverted. In meteorology, a departure from the usual decrease or increase with altitude of the value of an atmospheric property; also, the layer through which this departure occurs (the "inversion layer"), or the lowest altitude at which the departure is found (the "base of the inversion"). This term is basically applied to a temperature inversion.

Lapse Rate, n. In meteorology, the decrease of an atmospheric variable with height, the variable being temperature, unless otherwise specified. The normal (standard or average) lapse rate in the atmosphere below the tropopause is 3.5°F per 1000 feet.

Subsidence, n. Act of subsiding. In meteorology, a descending motion of the air in the atmosphere, usually with the implication that the condition extends over a rather broad area.

Sudsidence Inversion, n. In meteorology, a temperature inversion produced by the adiabatic warming of a layer of subsiding air. This inversion is enhanced by vertical mixing in the air layer below the inversion.


Other articles by this author:

This site was first published July 12, 1996

Source URL: http://www.maybeck.com/inversions/


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