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Term Paper on Atmospheric Stability
Term Paper Contents:
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- Term Paper on the Meaning of Atmospheric Stability
- Term Paper on Determining Atmospheric Stability
- Term Paper on Factors Affecting Atmospheric Stability
- Term Paper on Diurnal and Seasonal Variations in Stability
- Term Paper on Thermal Turbulence in Atmospheric Stability
- Term Paper on the Local Indicators of Atmospheric Stability
Term Paper # 1. Meaning of Atmospheric Stability:
Atmospheric stability is the resistance of the atmosphere to vertical motion. The distribution of temperature vertically in the troposphere influences vertical motion. A large decrease of temperature with height indicates an unstable condition which promotes up and down currents. A small decrease with height indicates a stable condition which inhibits vertical motion. Where the temperature increases with height, through an inversion, the atmosphere is extremely stable.
To determine stability conditions, temperature lapse rates are compared to dry- or moist-adiabatic lapse rates. Between stable and unstable lapse rates we may have a conditionally unstable saturated. During condensation in saturated air, heat is released which warms the air and may produce instability; during evaporation, heat is absorbed and may increase stability.
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Several different lifting processes contribute to atmospheric stability, such as local heating, with wind speed, surface characteristics, warm-and cold-air advection, and many other factors. As a result, atmospheric stability varies with these factors. Atmospheric stability also varies diurnally and seasonally. We can use type of cloud, wind-flow characteristics, occurrence of dust devils, and other phenomena as indicators of stability.
Subsidence is the gradual lowering of a layer of air over a broad area. When it begins at high levels in the troposphere, the air, which has little initial moisture, becomes increasingly warmer with resulting lower relative humidity as it approaches the surface. If some mechanism is present by which this warm, dry air can reach the surface, a very serious fire situation can result.
Atmospheric stability is closely related to fire behaviour, and a general understanding of stability and its effects is necessary to the successful interpretation of fire-behaviour phenomena. Atmospheric stability may either encourage or suppress vertical air motion. The heat of fire itself generates vertical motion, at least near the surface, but the convective circulation thus established is affected directly by the stability of the air.
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In turn, the in-draft into the fire at low levels is affected, and this has a marked effect on fire intensity. Also, in many indirect ways, atmospheric stability will affect fire behaviour. For example, winds tend to be turbulent and gusty when the atmosphere is unstable, and this type of airflow causes fires to behave erratically.
Thunderstorms with strong updrafts and downdrafts develop when the atmosphere is unstable and contains sufficient moisture. Their lightning may set wildfires, and their distinctive winds can have adverse effects on fire behaviour.
Term Paper # 2. Determining Atmospheric Stability:
The degree of stability or instability of an atmospheric layer is determined by comparing its temperature lapse rate, as shown by a sounding-, with the appropriate dry- or moist-adiabatic lapse rates.
The dry adiabatic rate is used for air that is not saturated and the moist-adiabatic rate is used for saturated air. The adiabatic process is reversible. Just as air expands and cools when it is lifted, so is it equally compressed and warmed as it is lowered. Hence, stability determinations for either upward or downward moving air parcels make use similar comparisons with the appropriate adiabatic lapse rates.
i. Stable:
A temperature lapse rate less than the dry adiabatic rate of 5.5°F per 1,000 feet for an unsaturated parcel are considered stable, because vertical motion is damped.
ii. Unstable:
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A lapse rate greater than dry-adiabatic (a super-adiabatic lapse rate) favours vertical motion and is unstable. Since it is unstable, the air tends to adjust itself through mixing and overturning to a more stable condition. Super-adiabatic lapse rates are not ordinarily found in the atmosphere except near the surface of the earth on sunny days. When an unsaturated layer of air is mixed thoroughly, its lapse rate tends toward neutral stability.
iii. Neutral:
In the absence of saturation, an atmospheric layer is neutrally stable if its lapse rate is the same as the dry-adiabatic rate. Under this particular condition, any existing vertical motion is neither damped nor accelerated.
Warming of the lower layers during the daytime by contact with the earth’s surface or by heat from a wildfire will make a neutral lapse rate become unstable. In an atmosphere with a dry-adiabatic lapse rate, hot gases rising from a fire will encounter little resistance, will travel upward with ease, and can develop a tall convection column. A neutrally stable atmosphere can be made unstable also by advection; that is, the horizontal movement of colder air into the area aloft or warmer air into the area near the surface.
Once the lapse rate becomes unstable, vertical currents are easily initiated. Advection of warm air aloft or cold air near the surface has the reverse effect of making the atmosphere more stable. The term “neutral” stability sounds rather passive, but we should be cautious when such a lapse rate is present. The temperature structure of the atmosphere is not static, but is continually changing.
Any warming of the lower portion or cooling of the upper portion of a neutrally stable layer will cause the layer to become unstable, and it will then not only permit, but will assist, vertical motion. Such changes are easily brought about. Thus, we should consider the terms stable, neutral, and unstable in a relative, rather than an absolute, sense. A stable lapse rate that approaches the dry-adiabatic rate should be considered relatively unstable.
Saturated Air:
So far we have considered adiabatic cooling and warming and the degree of stability of the atmosphere only with respect to air that is not saturated. Rising air, cooling at the dry-adiabatic lapse rate, may eventually reach the dew-point temperature. Further cooling results in the condensation of water vapour into clouds, a change of state process that liberates the latent heat contained in the vapour. This heat is added to the rising air, with the result that the temperature no longer decreases at the dry-adiabatic rate, but at a lesser rate which is called the moist-adiabatic rate.
On the average, this rate is around 3°F per 1,000 feet, but it varies slightly with pressure and considerably with temperature. The variation of the rate due to temperature may range from about 20°F per 1,000 feet at very warm temperatures to about 5°F per 1,000 feet at very cold temperatures. In warmer air masses, more water vapour is available for condensation and therefore more heat is released, while in colder air masses, little water vapour is available.
To determine the degree of stability or instability for a saturated parcel, the same stability terms apply as for unsaturated but the comparison of atmospheric lapse rate is made with the moist-adiabatic rate appropriate to the temperature encountered.
i. Stable:
A temperature lapse rate less than the moist adiabatic rate are considered stable.
ii. Unstable:
A lapse rate greater than moist-adiabatic is unstable.
iii. Neutral:
In the absence of saturation, an atmospheric layer is neutrally stable if its lapse rate is the same as the dry-adiabatic rate. Under this particular condition, any existing vertical motion is neither damped nor accelerated.
Term Paper # 3. Factors Affecting Atmospheric Stability:
i. Conditional Stability:
An atmosphere that has a lapse rate lying between the dry and moist adiabates is said to be conditionally unstable. It is stable with respect to a lifted air parcel as long as the parcel remains unsaturated, but it is unstable with respect to a lifted parcel that has become saturated.
A saturated parcel in free convection loses additional moisture by condensation as it rises. This, plus the colder temperature aloft, causes the moist-adiabatic lapse rate to increase toward the dry-adiabatic rate. The rising parcel will thus eventually cool to the temperature of the surrounding air where the free convection will cease. This may be in the vicinity of the tropopause or at some lower level, depending on the temperature structure of the air aloft.
ii. Layer Stability:
Atmospheric stability is affected by vertical movement of both parcels of air or whole layers of considerable horizontal extent of the atmosphere. When an entire layer of stable air is lifted it becomes increasingly less stable. The layer stretches vertically as it is lifted, with the top rising farther and cooling more than the bottom. If no part of the layer reaches condensation, the stable layer will eventually become dry-adiabatic. Occasionally, the bottom of a layer of air being lifted is moister than the top and reaches its condensation level early in the lifting.
Cooling of the bottom takes place at the slower moist-adiabatic rate, while the top continues to cool at the dry-adiabatic rate. The layer then becomes increasingly less stable at a rate faster than if condensation had not taken place. A descending (subsiding) layer of stable air becomes more stable as it lowers. The layer compresses, with the top sinking more and warming more than the bottom. The adiabatic processes involved are just the opposite of those that apply to rising air.
iii. Lifting Processes:
Lifting processes can cause air to be lifted into the atmosphere – convection, orographic lifting, frontal lifting, turbulence, and convergence.
iv. Convection:
A common process by which air is lifted in the atmosphere is convection. If the atmosphere remains stable, convection will be suppressed. But surface heating makes the lower layers of the atmosphere unstable during the daytime. Triggering mechanisms are required to begin convective action, and they usually are present. If the unstable layer is deep enough, so that the rising parcels reach their condensation level, cumulus-type clouds will form and may produce showers or thunderstorms if the atmospheric layer above the condensation level is conditionally unstable.
At times, the fire convection column will reach the condensation level and produce clouds. Showers, though rare, have been known to occur.
Layers of air commonly flow in response to pressure gradients. In doing so, if they are lifted up and over mountains, they are subjected to what is called orographic lifting. This is a very important process along our north-south mountain ranges in the western regions and the Appalachians in the East, because the general airflow is normally from a westerly direction. If the air is initially stable, and if no condensation takes place, it sinks back to its original level after passing over a ridge.
If it is neutrally stable, the air will remain at its new level after crossing the ridge. In an unstable atmosphere, air given an initial uplift in this way keeps on rising, seeking a like temperature level, and is replaced by sinking colder air from above. If the condensation level is reached in the lifting process, and clouds form, initially stable air can become unstable. In each case, the internal depth and lapse rate of the layer will respond as indicated above.
v. Frontal Lifting:
Warmer, lighter air layers frequently flow up and over colder, heavier air masses. This is referred to as frontal lifting and is similar in effect to orographic lifting. Stable and unstable air masses react the same way regardless of whether they are lifted by the slope of topography or by the slope of a heavier air mass.
Term Paper # 4. Diurnal and Seasonal Variations in Stability:
Stability frequently varies through a wide range in different layers of the atmosphere for various reasons. Layering aloft may be due to an air mass of certain source-region characteristics moving above or below another air mass with a different temperature structure. The inflow of warmer (less dense) air at the bottom or colder (more dense) air at the top of an air mass promotes instability, while the inflow of warmer air at the top or colder air at the surface has a stabilizing effect.
At lower levels, stability of the air changes with surface heating and cooling, amount of cloud cover, and surface wind all acting together. We will consider first the changes in stability that takes place during a daily cycle and the effects of various factors; then we will consider seasonal variations.
Diurnal Variations in Stability:
Diurnal changes in surface heating and cooling produce daily changes in stability, from night inversions to daytime super-adiabatic lapse rates that are common over local land surfaces. During a typical light-wind, fair-weather period, radiation cooling at night forms a stable inversion near the surface, which deepens until it reaches its maximum development at about daybreak. After sunrise, the earth and air near the surface begin to heat, and a shallow super-adiabatic layer is formed.
Convective currents and mixing generated in this layer extend up to the barrier created by the inversion. As the day progresses, the unstable super-adiabatic layer deepens, and heated air mixing upward creates an adiabatic layer, which eventually eliminates the inversion completely. This usually occurs by mid or late morning. Active mixing in warm seasons often extends the adiabatic layer to 4,000 or 5,000 feet above the surface by midafternoon. The super-adiabatic layer, maintained by intense heating, is usually confined to the lowest few hundreds of feet, occasionally reaching 1,000 to 2,000 feet over bare ground in midsummer.
As the Sun sets, the ground cools rapidly under clear skies and soon a shallow inversion is formed. The inversion continues to grow from the surface upward throughout the night as surface temperatures fall. The air within the inversion becomes increasingly stable. Vertical motion in the inversion layer is suppressed, though mixing may well continue in the air above the inversion. This mixing allows radiational cooling above the inversion to lower temperatures in that layer only slightly during the night.
This diurnal pattern of nighttime inversions and daytime super-adiabatic layers near the surface can be expected to vary considerably. Clear skies and low air moisture permit more intense heating at the surface by day and more intense cooling by radiation at night than do cloudy skies. The lower atmosphere tends to be more unstable on clear days and more stable on clear nights.
Strong winds diminish or eliminate diurnal variations in stability near the surface. Turbulence associated with strong wind results in mixing, which tends to produce a dry-adiabatic lapse rate. Mechanical turbulence at night prevents the formation of surface inversions, but it may produce an inversion at the top of the mixed layer. During the day, thermal turbulence adds to the mechanical turbulence to produce effective mixing through a relatively deep layer. Consequently, great instability during the day and stability at night occur when surface winds are light or absent.
Stability in the lower atmosphere varies locally between surfaces that heat and cool at different rates. Thus, dark-coloured, barren, and rocky soils that reach high daytime temperatures contribute to strong daytime instability and, conversely, to strong stability at night. Areas recently blackened by fire are subject to about the maximum diurnal variation in surface temperature and the resulting changes in air stability. Vegetated areas that are interspersed with openings, outcrops, or other good absorbers and radiators have very spotty daytime stability conditions above them.
Topography also affects diurnal changes in the stability of the lower atmosphere. Air in mountain valleys and basins heats up faster during the daytime and cools more rapidly at night than the air over adjacent plains. This is due in part to the larger area of surface contact and in part to differences in circulation systems in flat and mountainous topography.
The amount of air heating depends on orientation, inclination, and shape of topography, and on the type and distribution of ground cover. South-facing slopes reach higher temperatures and have greater instability above them during the day than do corresponding north slopes. Both cool about the same at night.
Instability resulting from superheating near the surface is the origin of many of the important convective winds. On mountain slopes, the onset of daytime heating initiate’s upslope wind systems. The rising heated air flows up the slopes and is swept aloft above the ridge-tops in a more-or- less steady stream.
Over level ground, heated surface air, in the absence of strong winds to disperse it can remain in a layer next to the ground until it is disturbed. The rising air frequently spirals upward in the form of a whirlwind or dust devil. In other cases, it moves upward as intermittent bubbles or in more-or-less continuous columns. Pools of superheated air may also build up and intensify in poorly ventilated valleys to produce a highly unstable situation. They persist until released by some triggering mechanism which overcomes inertia, and they may move out violently.
Seasonal Variations in Stability:
The amount of solar radiation received at the surface during the summer is considerably greater than in the winter. This is due to the difference in solar angle and the duration of sunshine. Temperature profiles and stability reflect seasonal variation accordingly. In the colder months, inversions become more pronounced and more persistent, and super-adiabatic lapse rates occur only occasionally.
In the summer months, super-adiabatic conditions are the rule on sunny days. Greater variation in stability from day to day may be expected in the colder months because of the greater variety of air masses and weather situations that occur during this stormy season.
In addition to the seasonal effects directly caused by changes in solar radiation, there is also an important effect that is caused by the lag in heating and cooling of the atmosphere as a whole. The result is a predominance of cool air over warming land in the spring, and warm air over cooling surfaces in the fall. Thus, the steepest lapse rates frequently occur during the spring, whereas the strongest inversions occur during fall and early winter.
Term Paper # 5. Thermal Turbulence in Atmospheric Stability:
Surface winds often vary considerably in both speed and direction over short intervals of time. They tend to blow in a series of gusts and lulls with the direction fluctuating rapidly. This irregular air motion is known as turbulence, which may be either mechanical or thermal in nature. At the surface, turbulence is commonly identified in terms of eddies, whirls, and gusts; aloft it is associated with “bumpy” flying.
The depth of the air layer through which the frictional force is effective also varies with the roughness of the surface; it is shallower over smooth surfaces and deeper over rough topography. The depth may also vary with the stability of the lower atmosphere. A low inversion will confine the frictional effect to a shallow surface layer, but a deep layer can be affected if the air is relatively unstable.
These effects vary widely both with time and between localities. Usually the friction layer is considered to be about 2,000 feet deep. The top of the friction layer is the gradient wind level above which the wind-flow tends to parallel the isobars or pressure-surface contours.
Thermal turbulence is associated with instability and convective activity. It is similar to mechanical turbulence in its effects on surface winds, but extends higher in the atmosphere. Since it is the result of surface heating, thermal turbulence increases with the intensity of surface heating and the degree of instability indicated by the temperature lapse rate.
It therefore shows diurnal changes, and is most pronounced in the early afternoon when surface heating is at a maximum and the air is unstable in the lower layers. It is at a minimum during the night and early morning when the air is more stable. Mechanical and thermal turbulence frequently occur together, each magnifying the effects of the other.
Thermal turbulence induced by the combination of convection and horizontal wind is the principal mechanism by which energy is exchanged between the surface and the winds aloft. Unstable air warmed at the surface rises to mix and flow along with the winds above. This turbulent flow also brings air with higher wind speeds—greater momentum—from aloft down to the surface, usually in spurts and gusts. This momentum exchange increases the average wind speed near the surface and decreases it aloft. It is the reason why surface winds at most places is stronger in the afternoon than at night.
Eddy formation is a common characteristic of both mechanical and thermal turbulent flow. Every solid object in the wind path creates eddies on its lee side. The sizes, shapes, and motions of these eddies are determined by the size and shape of the obstacle, the speed and direction of the wind, and the stability of the lower atmosphere. Although eddies may form in the atmosphere with their axes of rotation in virtually any plane, it is usual to distinguish between those which have predominantly vertical or horizontal axes.
A whirlwind or dust devil is a vertical eddy, as are eddies produced around the corners of buildings or at the mouths of canyons with steep sides. Large, roughly cylindrical eddies that roll along the surface like tumbleweeds are horizontal eddies.
Eddies associated with individual fixed obstructions tend to remain in a more-or-less stationary position in the lee of the obstruction. If they break off and move downstream, new ones form near the obstruction. The distance downwind that an obstacle, such as a windbreak, affects the wind-stream is variable. For most obstructions, the general rule of thumb is that this distance is 8 to 10 times the height of the obstacle.
Rotation speeds in eddies are often much greater than the average wind speeds measured with mechanical anemometers. These higher speeds are often of short duration at any point, except where stationary are found, but are still significant in fire behaviour. Whirlwinds, for example, develop speeds capable of lifting sizable objects. Eddies moving with the general wind-flow account for the principal short-term changes in wind speed and direction known as gustiness.
Term Paper # 6. Local Indicators of Atmospheric Stability:
The continent-wide network of weather stations that make regular upper-air soundings gives a broad general picture of the atmospheric structure over North America. These soundings show the major pressure, temperature, and moisture patterns that promote stability, instability, or subsidence, but they frequently do not provide an accurate description of the air over localities at appreciable distances from the upper-air stations.
We need, therefore, to supplement these observations with local measurements or with helpful indicators. Clouds, wind-flow characteristics, occurrence of dust devils, and other phenomena can be useful indicators of stability.
At times, it may be possible to take upper-air observations with portable instruments in fixed-wing aircraft or helicopters. In mountainous country, temperature and humidity measurements taken at mountaintop and valley-bottom stations provide reasonable estimates of the lapse rate and moisture conditions in the air layer between the two levels. In areas where inversions form at night, similar measurements indicate the strength of the inversion.
The heights of surface or low-level inversions can be determined by traversing slopes that extend through them. The height at which rising smoke flattens out may indicate the base of a low-level inversion. The tops of clouds in the marine layer along the Pacific coast coincide with the base of the subsidence inversion. The height of the cloud tops provides a good estimate of the height of the inversion.
Other visual indicators are often quite revealing. Stability in the lower layers is indicated by the steadiness of the surface wind. A steady wind is indicative of stable air. Gusty wind, except where mechanical turbulence is the obvious cause, is typical of unstable air. Dust devils are always indicators of instability near the surface. Haze and smoke tend to hang near the ground in stable air and to disperse upward in unstable air. Cloud types also indicate atmospheric stability at their level.
Cumulus-type clouds contain vertical currents and therefore indicate instability. The heights of cumulus clouds indicate the depth and intensity of the instability. The absence of cumulus clouds, however, does not necessarily mean that the air is stable. Intense summer heating can produce strong convective currents in the lower atmosphere, even if the air is too dry for condensation and cloud formation.
Generally, though, the absence of clouds is a good indication that subsidence is occurring aloft. Even if scattered cumulus clouds are present during the day and are not developing vertically to any great extent, subsidence very likely is occurring above the cumulus level. Stratus-type cloud sheets indicate stable layers in the atmosphere.
In mountainous country, where fire lookouts on high peaks take observations, a low dew-point temperature may provide the only advance warning of subsidence. Hygrothermograph records and wet- and dry-bulb temperature observations show a sharp drop in relative humidity with the arrival of subsiding air at the mountaintop. Early morning dew-point temperatures of 20° F or lower in summer or early fall may signal the presence of subsiding air, and provide a warning of very low humidities at lower elevations in the afternoon.