Unit 7: Atmospheric Stability and Instability
Upon completion of this unit you will be expected to:
- Define atmospheric stability (stable air), and discuss its general effects on fire behavior.
- Describe atmospheric instability (unstable air), and give three ways that it can contribute to possible extreme fire behavior.
- List and discuss the four lifting processes which can cause thunderstorm development.
- Describe thermal belts: when, where, and how they develop, and their effects on fire behavior.
- Discuss subsidence, and describe two situations due to subsidence which can increase fire behavior.
- Describe morning surface inversions and their effects on fire behavior.
- Determine the stability or instability at various levels of the atmosphere based on visual indicators and/or temperature readings at different elevations or altitudes.
In earlier weather units of this course, we discussed the facts that the earth's gaseous mantle, called "the atmosphere," is very fluid, with air constantly moving and mixing, and that air changes in temperature, moisture, pressure, and other properties. By now you know that air moves horizontally or vertically in response to the earth's rotation, to large and small scale pressure gradients, to various lifting mechanisms, and to gravity.
Unit 6 concentrated on the horizontal movement of air or wind. In this unit, we will discuss vertical air movement, what causes it, and what it means to firefighters. This vertical movement, either upward or downward, is generally influenced by the degree of stability or instability of the atmosphere at any particular time.
Atmospheric Stability and Instability
Atmospheric stability : The resistance of the atmosphere to vertical motion.
Temperature distribution and lapse rates were discussed in Unit 4, where you learned that temperatures normally increase as we get closer to the earth's surface. This is due in part to the greater molecular activity of denser, more compressed air at lower altitudes. These conditions change throughout a 24-hour period, as the daytime solar heating and nighttime heat loss to and through the atmosphere tend to modify the temperature distributions.
Temperature distribution of vertically moving air
The term "adiabatic process" was used in Unit 4, which simply means warming by compression, or cooling by expansion, without a transfer of heat or mass into a system. As air moves up or down within the atmosphere, it is affected by this process. (See figures below) This temperature difference will be 5-1/2 degree decrease per 1,000 feet increase in altitude. This is also termed the dry adiabatic lapse rate. The atmosphere may or may not have a temperature distribution that fits the dry adiabatic lapse rate. Usually it does not.
Unstable air encourages vertical movement of air and decreases fire activity.
The actual lapse rate may be greater or less than the dry adiabatic lapse rate and may change by levels in the atmosphere. This variation from the dry adiabatic lapse rate is what determines whether the air is stable or unstable. If the air is unstable, the vertical movement of air is encouraged, and this tends to increase fire activity. If the air is stable, vertical movement of air is discouraged, and this usually decreases or holds down fire activity. The importance of this atmospheric property will become evident by the time you have completed this unit.
Dry Lapse Rates
The actual temperature lapse rate in a given portion of the atmosphere could range from a plus 15° per 1,000 feet to a minus 15° per 1,000 feet. These would represent the extremes of very stable air to very unstable air. Rather than be concerned with all of these degrees of stability or instability, we usually describe the atmosphere as falling into one of five conditions.
The vertical air temperature distribution in the atmosphere is highly variable. For dry air it ranges as follows:
- Very stable : Temperature increases with increase in altitude. This is a "plus" temperature lapse rate, or an inversion.
- Stable : Temperature lapse rate is less than the dry adiabatic rate, but temperature decreases with altitude increase.
- Neutral : Temperature lapse rate is the same as the dry adiabatic rate of 5.5 degrees Fahrenheit per 1000 feet increase.
- Unstable : Temperature lapse rate is greater than the dry adiabatic rate. It may be 6 degrees Fahrenheit or more.
- Very unstable : Temperature lapse rate is much greater than the dry adiabatic rate, and is called super-adiabatic.
Vertical Air Movement and Diurnal Changes
Vertical movement of air in the atmosphere
We said that stable air tends to resist vertical air movement. If a horizontally moving parcel of air is lifted or forced to rise, as over a mountain, that parcel will tend to settle back to its original level. It is heavier than the air around it; therefore, it will sink back, if possible, to the level from which it originated.
If the atmosphere is neutral; that is, the actual temperature lapse rate equals the dry adiabatic lapse rate, a parcel of air that is lifted will be neither heavier nor lighter at a different altitude. As this parcel is forced up, it decreases in temperature at a rate of 5-1/2° per 1,000 feet. The surrounding air at the new altitude will have the same temperature, and it will remain neutral.
If the atmosphere is unstable, any parcel of air that is lifted will tend to rise like a hot air balloon. As this parcel rises, it decreases in temperature at a rate of 5-1/2° per 1,000 feet, and will continue to rise until it reaches a level in which its new temperature equals that of the surrounding air.
In Unit 4, we learned that as a parcel of air cools, its relative humidity increases. If the parcel cools enough, 100-percent relative humidity, or its dew point, will be reached. At that point clouds form. Saturated air that continues to rise gives up more and more of its bound moisture as it cools. This saturated air now cools at a lesser rate, about 3° per 1,000 feet, due to the released latent heat of condensation. Air rises and cools at the dry adiabatic lapse rate until it reaches its saturation point and then continues to rise and cool at the wet adiabatic lapse rate.
What causes these differences in temperature lapse rates in the atmosphere? Well, there are several factors that contribute to varying temperature distributions, but the most important is heating and cooling at the earth's surface. (See graphic above) As the earth loses its heat at night through radiation, the air in contact with the ground also cools. Conduction becomes an important process here, as air at lower levels cools faster than the air above. This creates a condition of cool, heavier air below warmer air. The layer of cool air above the surface deepens as the night progresses. In mountainous terrain, this condition is even more pronounced in the valleys, as cool air drainage from the slopes above helps to deepen the layer of cold air. The deepening of the cool air layer is also affected by the amount of cloud cover at night. Clear nights cool faster than cloudy nights. Warmer air just above a cool air layer creates a very stable air condition. Smoke or any other parcel of air that is forced to rise will stop when the warmer air is reached. This inversion layer discourages vertical air movement.
Conditions usually begin to reverse after sunrise. As the earth's surface is heated by solar radiation, it warms the air in contact with it and above it by conduction and convection. The stable air at lower levels warms until it is no longer colder than the air above, and the temperature lapse rate approaches the dry adiabatic rate. Inversions usually disappear sometime before noon as unstable conditions continue to develop. Air near the surface is not much warmer than the air above, thus making it buoyant and able to rise. Any lifted parcel of air will continue to rise until it reaches a level of equal temperature. A smoke column could rise many thousands of feet.
The lower atmosphere at night is usually always stable; whereas, during the daytime it is usually unstable. This is especially true if the weather is fair with mostly clear skies. Stable or unstable air conditions can develop under cloudy skies, but their degree of development is usually less.
Diurnal changes in temperature in the lower atmosphere occur due to heating and cooling at the earth's surface.
From the two graphics above, we can conclude that diurnal changes of temperature in the lower atmosphere occur due to heating and cooling at the earth's surface. We can also conclude that cooling from below promotes stability, while heating from below promotes instability. These diurnal changes in the lower atmosphere have a pronounced effect on fire behavior.