Why does the stability of the atmosphere vary through the atmosphere?
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.
What is the importance of stability and instability in atmospheric science?
Each of these concepts can be applied to motions of air parcels in the atmosphere. The topic of stability in atmospheric science is important because the formation of clouds is closely related to stability or instability in the atmosphere.
How does evaporation affect the stability of the atmosphere?
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. Atmospheric stability varies with local heating, with wind speed, surface characteristics, warm- and cold air advection, and many other factors.
What happens when the atmosphere becomes unstable?
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.
How to determine the degree of stability of an atmospheric layer?
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 dry adiabatic 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.
Why is lapse rate unstable?
A lapse rate between the dry- and moist-adiabatic rates is conditionally unstable, because it would be unstable under saturated conditions but stable under unsaturated conditions. The temperature of a parcel raised from near the surface will follow the dry-adiabatic rate until saturation, then follow the moist-adiabatic rate. At the level where the parcel temperature exceeds the environment temperature, the parcel will begin free ascent.
How to determine stability of a meteorological chart?
To determine stability, the meteorologist plots temperature and moisture soundings on an adiabatic chart and compares the lapse rates of various layers to the dry adiabats and moist adiabats.
How does gravity affect the temperature of a parcel?
As the parcel is lifted and cools at its 5.5° rate, it thus becomes progressively colder and more dense than its environment. At 5,000 feet, for example, its temperature would be 39°F., but the temperature of the surrounding air would be 43°F. Gravity thus returns the parcel to its point of origin when the external force is removed. Moved downward, the parcel warms at the dry adiabatic rate and becomes warmer than its environment. At 1,000 feet, for example, the parcel temperature would be 61°F., but the temperature of the environment would be only 57°F. Buoyancy forces the parcel back up to its original level. The damping action in either case indicates stability.
What are the characteristics of the sounding?
These are: (1) The temperature lapse rate through the layer; (2) temperature of the parcel at its initial level; and (3) initial dew point of the parcel.
What is subsidence in a fire?
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.
What is atmospheric stability?
Atmospheric stability was defined in chapter 1 as the resistance of the atmosphere to vertical motion. This definition and its explanation were based on the parcel method of analysis appropriate to a vertical temperature and moisture sounding through the troposphere.
What is atmospheric stability?
Stability (or atmospheric stability) refers to air's tendency to either rise and create storms (instability), or to resist vertical movement (stability). The simplest way to understand how stability works is to imagine a parcel of air having a thin, flexible cover that allows it to expand but prevents ...
Why does the temperature of a balloon decrease with altitude?
Since air pressure decreases with altitude, the balloon will relax and expand, and its temperature will therefore decrease. If the parcel were cooler than the surrounding air, it would be heavier (since cool air is denser than warm air); and if allowed to do so, it would sink back down to the ground.
How to determine atmospheric stability?
The easiest way to determine atmospheric stability at a glance is by using an atmospheric sounding.
What is the measure of air temperature?
They can arrive at the same answer simply by measuring the actual air temperature at various heights; this measure is called the environmental lapse rate (the term "lapse" having to do with temperature's decline). If the environmental lapse rate is steep then one knows the atmosphere is unstable.
What would happen if we lifted an imaginary balloon?
On the other hand, if we lifted our imaginary balloon and the air within it was warmer, and hence, less dense than its surrounding air, it would continue to rise until it reached a point where its temperature and that of its surroundings were equal . This type of air is classified as unstable.
Is the lapse rate of an atmosphere stable?
But if the lapse rate is small, meaning there's relatively little change in temperature, it's a good indication of a stable atmosphere.
What is the lapse rate of air?
When a parcel of air is forced to move upward, it cools at dry adiabatic lapse rate (10°C per 1000m or 5.5°F per 1000 feet) whereas normal lapse rate is 6.5°C per 1000m. After rising to certain height the air becomes saturated and latent heat of condensation is added to the rising air so the rising air cools at wet adiabatic lapse rate (5°C per 1000m) whereas the normal lapse rate (6.5°C per 1000m) is greater than it.
What happens when the dry adiabatic lapse rate of an ascending dry air is higher than the normal?
When dry adiabatic lapse rate of an ascending dry air is higher than the normal lapse rate and if it is not saturated and does not attain dew point it becomes colder than surrounding air at certain height with the result it becomes heavier and descends. This process causes stability of atmospheric circulation due to which vertical circulation of air is resisted.
What is the relationship between adiabatic change and lapse rate?
The stability and instability depend on the relationships between ‘normal lapse rate’ and ‘adiabatic change of temperature’. Adiabatic rate is always constant whereas normal lapse rate of air temperature changes. When the normal lapse rate is higher than dry adiabatic rate, the air being warmer rises and becomes unstable. On the other hand, when the normal lapse rate of temperature is lower than dry adiabatic rate, the air being cold descends and becomes stable.
What is the atmospheric condition called when the lapse rate is less than the adiabatic lapse rate?
In such conditions if the normal lapse rate is less than wet adiabatic lapse rate even at condensation point, further vertical motion of air is stopped and thus such air is said to be absolutely stable and such atmospheric condition is called absolute stability.
What is the temperature of air at a height of 1000m?
For example, if the temperature of a certain parcel of air at ground surface is 40°C, the dry adiabatic and normal lapse rates are 10°C and 11°C per 1000m respectively, then the temperature of ascending air at the height of 1000m (one kilometre) would be 30°C (40°-10° = 30°C) while the temperature of the atmosphere at that height would be 29°C (40°-11°C = 29°C).
When does conditional instability occur?
Thus, the rising air becomes warmer and unstable, Conditional instability may occur only when the normal lapse rate ranges between dry adiabatic and wet adiabatic lapse rates. In other words, conditional instability occurs when normal lapse rate is greater than dry adiabatic lapse rate but less than wet adiabatic lapse rate. ...
When the rate of cooling of rising air (dry adiabatic lapse rate) is lower than the normal?
In other words, atmospheric instability is caused when the rate of cooling of rising air (dry adiabatic lapse rate) is lower than the normal lapse rate.
What is the difference between dry and wet adiabatic rate?
So, there is a difference between the dry adiabatic rate, wherein a parcel of air cools at a rate of about 10 degrees Celsius per 1000 meters, and the wet adiabatic rate, wherein a parcel of air cools at a rate of about 6 degrees Celsius per 1000 meters. The less the air parcel cools with altitude, the greater the difference between the parcel's temperature and the surrounding air temperature, and the greater the atmospheric instability.
How is instability determined?
Instability is determined by the relative difference in a rising parcel of air and the atmosphere around it, the latter of which naturally cools with altitude due to its environmental lapse rate. Air parcels cool with altitude at a dry adiabatic rate or a wet adiabatic rate, depending on the humidity of the parcel.
What is the term for the movement of air up the side of a mountain?
The water vapor in that parcel of air condenses, forming clouds. This is called orographic lifting, wherein the movement of air up the side of a mountain causes condensation and precipitation. The lateral movement of air can also cause lifting to occur.
How does temperature affect stability?
Stability in the atmosphere is determined by how much warmer a parcel of air is than the surrounding air. It's all relative. The atmosphere itself decreases in temperature with increased altitude due to the environmental lapse rate. A rising parcel of air will also change temperature with altitude because the outside air pressure is also decreasing. Less pressure means that the parcel of air gradually expands, which also impacts its temperature. So basically, the temperature of the air parcel relative to the surrounding air determines stability, and both temperatures are changing with altitude.
Why do airplanes have turbulence?
Turbulence is a result of instability in the atmosphere surrounding the aircraft. Unstable air is a parcel of air that is warmer than the air around it, causing it to rise.
What is the cause of the Earth's weather?
Because Earth's surface radiates much of the solar heat that it absorbs (land more so than water), the rising warm air and the instability it causes in the atmosphere is what generates all weather experienced on Earth. The troposphere, the lowest layer of the atmosphere, from the ground up to about 11 miles high, is the site of all of Earth's weather and most of the atmospheric instability.
What is atmospheric instability?
Atmospheric Instability refers to the difference in temperature, causing the rising and sinking of air. Learn types of lifting, how air instability is increased, and ways that air can be stabilized, noting the weather patterns attributed to this. Updated: 11/05/2021
How to calculate Showalter index?
The Showalter index is a dimensionless number computed by taking the temperature at the 850 hPa level which is then taken dry adiabatically up to saturation, then up to the 500 hPa level, which is then subtracted by the observed 500 hPa level temperature. If the value is negative, then the lower portion of the atmosphere is unstable, with thunderstorms expected when the value is below −3. The application of the Showalter index is especially helpful when there is a cool, shallow air mass below 850 hPa that conceals the potential convective lifting. However, the index will underestimate the potential convective lifting if there are cool layers that extend above 850 hPa and it does not consider diurnal radiative changes or moisture below 850 hPa.
How do thunderstorms develop?
Within an unstable layer in the troposphere, the lifting of air parcels will occur, and continue for as long as the nearby atmosphere remains unstable. Once overturning through the depth of the troposphere occurs (with convection being capped by the relatively warmer, more stable layer of the stratosphere ), deep convective currents lead to thunderstorm development when enough moisture is present. Over warm ocean waters and within a region of the troposphere with light vertical wind shear and significant low level spin (or vorticity), such thunderstorm activity can grow in coverage and develop into a tropical cyclone. Over hot surfaces during warm days, unstable dry air can lead to significant refraction of the light within the air layer, which causes inferior mirages.
What is the motion of an undular bore wave?
The approaching boundary will create a disturbance in the atmosphere producing a wave-like motion, known as a gravity wave. Although the undular bore waves appear as bands of clouds across the sky, they are transverse waves, and are propelled by the transfer of energy from an oncoming storm and are shaped by gravity.
What is the difference between CAPE and CIN?
CIN, convective inhibition, is effectively negative buoyancy, expressed B-; the opposite of convective available potential energy (CAPE), which is expressed as B+ or simply B.
What is the LI of the atmosphere?
The lifted index (LI), usually expressed in kelvins, is the temperature difference between the temperature of the environment Te (p) and an air parcel lifted adiabatically Tp (p) at a given pressure height in the troposphere, usually 500 hPa ( mb ). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable. Thunderstorms are expected with values below −2, and severe weather is anticipated with values below −6.
What is the meaning of CAPE?
Convective available potential energy (CAPE), sometimes, simply, available potential energy (APE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, ...
What are the effects of atmospheric instability?
Effects of atmospheric instability in moist atmospheres include thunderstorm development, which over warm oceans can lead to tropical cyclogenesis, and turbulence. In dry atmospheres, inferior mirages, dust devils, steam devils, and fire whirls can form. Stable atmospheres can be associated with drizzle, fog, increased air pollution, ...
Sections
Stability Determinations
Layer Stability
- Many local fire-weather phenomena can be related to atmospheric stability judged by the parcel method. Equally important, however, are weather changes that occur when whole layers of the atmosphere of some measurable depth and of considerable horizontal extent are raised or lowered. Here again, it is necessary to employ some assumptions with respect to conservation …
Lifting Processes
- A common process by which air is lifted in the atmosphere, as is explained in detail in the next chapter, is convection. If the atmosphere remains stable, convection will be suppressed. But we have seen that 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 pr…
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 a...
Subsidence
- Air that rises in the troposphere must be replaced by air that sinks and flows in beneath that which rises. Local heating often results in small-scale updrafts and downdrafts in the same vicinity. On a larger scale, such as the up-flow in low-pressure systems, adjacent surface high-pressure systems with their divergent flow normally supply the replacement air. The outflow at t…
Local Indicators of 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 a…
Summary
- In this chapter we have seen how 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 a…