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Chapter 4 ATMOSPHERIC STABILITY

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Chapter 4 ATMOSPHERIC STABILITY Chapter 4 ATMOSPHERIC STABILITY Wildfires are greatly affected by atmospheric motion and the properties of the atmosphere that affect its motion. Most commonly considered in evaluating fire danger are surface winds with their attendant temperatures and humidities, as experienced in everyday living. Less obvious, but equally important, are vertical motions that influence wildfire in many ways. 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. In turn, the indraft 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 behavior. 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 behavior. Subsidence occurs in larger scale vertical circulation as air from high-pressure areas replaces that carried aloft in adjacent low-pressure systems. This often brings very dry air from high altitudes to low levels. If this reaches the surface, going wildfires tend to burn briskly, often as briskly at night as during the day. From these few examples, we can see that atmospheric stability is closely related to fire behavior, and that a general understanding of stability and its effects is necessary to the successful interpretation of fire-behavior phenomena. ATMOSPHERIC STABILITY Atmospheric stability was defined in chapter I in which the parcel of air is embedded. These are: as the resistance of the atmosphere to vertical (1) The temperature lapse rate through the layer; motion. This definition and its explanation were (2) temperature of the parcel at its initial level; and based on the parcel method of analysis (3) initial dew point of the parcel. appropriate to a vertical temperature and moisture Adiabatically lifted air expands in the lower sounding through the troposphere. pressures encountered as it moves upward. This is This method employs some assumptions: (1) a cooling process, and the rate of cooling with The sounding applies to an atmosphere at rest; (2) increase in altitude depends on whether or not the a small parcel of air in the sampled atmosphere, if temperature reaches the dew point and consequent caused to rise, does not exchange mass or heat saturation. As long as the air remains unsaturated, across its boundary; and (3) rise of the parcel does it cools at the constant dry-adiabatic lapse rate of not set its environment in motion. We learned that 5.5°F. per 1,000 feet of rise. Rising saturated air lifting under these conditions is adiabatic lifting. cools at a lesser rate, called the moist-adiabatic Three characteristics of the sounding then rate. This rate averages about 3°F. per 1,000 feet, determine the stability of the atmospheric layer but, as we will see later, it varies considerably. STABILITY DETERMINATIONS 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 adiabatic rate. A temperature lapse rate less than the dryadiabatic rate of 5.5°F. per 1,000 feet for an unsaturated parcel is considered stable, because vertical motion is damped. A lapse rate greater than dry-adiabatic favors vertical motion and is unstable. 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. In the case of a saturated parcel, the same stability terms apply. In this case, however, the comparison of atmospheric lapse rate is made with the moist-adiabatic rate appropriate to the temperature encountered. Layers of different lapse rates of temperature may occur in a single sounding, varying from superadiabatic (unstable), usually found over heated surfaces, to dry-adiabatic (neutral), and on through inversions of temperature (very stable). In a saturated layer with considerable convective motion, the lapse rate tends to become Atmospheric stability of any layer is determined by the way moist-adiabatic. temperature varies through the layer and whether or not air in the layer it saturated. 50 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, adiabatic processes and stability determinations for either upward or downward moving air parcels make use of the appropriate dry- or moist-adiabatic lapse rates. The temperature structure of the atmosphere is always complex. As mentioned above, the moist-adiabatic lapse rate is variable-not constant as is the dry-adiabatic rate. Adiabatic Chart To facilitate making stability determinations, therefore, meteorologists analyzing upper-air observations use a thermodynamic diagram called an adiabatic chart as a convenient tool for making stability estimates. The basic portion of the chart is a set of gridlines of temperature and pressure (or height) on which the measured temperature and moisture structure of the atmosphere can be plotted. The moisture is plotted as dew-point temperature. Also printed on the chart is a set of dry-adiabatic and a set of moist-adiabatic lines. By referring to these adiabats, the lapse rates of the various layers or portions of the atmosphere can be compared to the dry-adiabatic rate and the moist-adiabatic rate. In later chapters we will consider other ways in which the adiabatic chart is used. Stability determinations from soundings in the atmosphere are made to estimate the subsequent motion of an air parcel that has been raised or lowered by an external force. In a stable atmosphere, the parcel will return to its original position when the force is removed; in an unstable atmosphere, the parcel will accelerate in the direction of its forced motion; and in a neutrally stable atmosphere, it will remain at its new position. Stability of Unsaturated Air We can illustrate use of the adiabatic chart to indicate these processes by plotting four hypothetical soundings on appropriate segments of a chart. We will first cons unsaturated air to which the constant dry-adiabatic lapse rate applies. Assume for simplicity, that each of our four soundings has a lapse rate indicated dia- To determine stability, the meteorologist plots tempeerture and moisture soundings on an adiabatic chart and compares the 51 lapse rates of various layers to the dry adiabats and moist adiabats. grammatically by a solid black line. Note also in the environment. At 1,000 feet, for example, the parcel accompanying illustration that each shows the temperature would be 61°F., but the temperature of temperature at 3,000 feet to be 50°F. For our the environment would be only 57°F. Buoyancy purposes, let us select a parcel of air at this point forces the parcel back up to its original level. The and compare its temperature with that of its damping action in either case indicates stability. environment as the parcel is raised or lowered by The parcel in (B) is initially in an inversion layer external forces. If it remains unsaturated, the where the temperature increases at the rate of 3°F. parcel will change in temperature at the per 1,000 feet of altitude. If the parcel is lifted, say dry-adiabatic rate indicated on the chart by red 1,000 feet, its temperature will decrease 5.5°F., arrows. while the temperature of the surrounding air will be The sounding plotted in (A) has a lapse rate of 3°F. higher. The parcel will then be 8.5°F. colder 3.5°F. per 1,000 feet. As the parcel is lifted and and will return to its original level as soon as the cools at its 5.5° rate, it thus becomes progressively lifting force is removed. Similarly, a lowered parcel colder and more dense than its environment. At will become warmer than the surrounding air and 5,000 feet, for example, its temperature would be will also return to its original level. Thus, inversions 39°F., but the temperature of the surrounding air at any altitude are very stable. would be 43°F. Gravity thus returns the parcel to Next, let us consider (C) where the parcel is its point of origin when the external force is embedded in a layer that has a measured lapse removed. Moved downward, the parcel warms at rate of 5.5°F. per 1,000 feet, the same as the dry adiabatic rate and becomes warmer than its In unsaturated air, the stability can be determined by comparing the measured lapse rate (solid black lines) to the dry-adiabatic lapse rate (dashed black lines). The reaction of a parcel to lifting or lowering may be examined by comparing its temperature (red arrows for parcel initially at 3,000 feet and 50°F.) to the temperature of its environment. the dry-adiabatic rate. If moved upward or sense. A stable lapse rate that approaches the dry- downward in this layer, the parcel will change in adiabatic rate should be considered relatively temperature at the same rate as that of its unstable. environment and, therefore, will always be in temperature equilibrium with the surrounding air. Warming of the lower layers during the daytime The parcel will come to rest at its new level when by contact with the earth's surface or by heat from external forces are removed.
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