L.8: Atmospheric Stability

Temperatures in a rising parcel of air are shown at various altitudes in the image above. Given that the dry adiabatic lapse rate is 10° C / 1 kilometer and the moist adiabatic lapse rate is 6° C / 1 kilometer, the data indicates that a cloud must have formed at:

1 km @ 10c

The Moist Adiabatic Lapse Rate occurs at what rate?

6.0 degrees C per 1000 meters

The atmosphere is _______________ when the environmental lapse rate is smaller than the than the moist adiabatic lapse rate, or G < Gs < Gd.

Absolutely Stable

The atmosphere is _______________ when the environmental lapse rate is greater than the than the moist adiabatic lapse rate, or Gd > G > Gs.

Absolutely Unstable

Which of the environmental temperature profiles above indicates the <em>most stable</em> atmosphere?

D (right)

As an air parcel reaches its coolest temperature, it begins to sink at what rate?

Dry Rate

Dry adiabatic lapse rate

During an adiabatic process, the rate of cooling remains the same as long as the parcel is unsaturated. The rate at which an adiabatically rising, unsaturated air parcel would cool, is called the dry adiabatic lapse rate. The dry adiabatic lapse rate is 9.8° C per 1000 meters, or more simply, 10° C per 1000 meters.

The rate at which the actual air temperature changes with increasing height above the surface is referred to as the:

Environmental Lapse Rate

Environmental lapse rate

Environmental lapse rate is the rate of temperature decrease with height in the environment surrounding the parcel. The environmental lapse rate can be determined by launching a radiosonde balloon that records temperature data as the balloon goes up into the atmosphere.

Snow is the only form of precipitation that doesn't form thanks to atmospheric instability.

F

A neutral atmosphere always has clear conditions

False

Air parcels cool as they rise at the same rate at the same elevations.

False

Stability is a property of an air parcel, not of the atmosphere.

False

Conditionally unstable atmosphere

For one last time, we will imagine that we sent a radiosonde into the atmosphere to take measurements. The radiosonde returns temperature readings that decrease by 7° C for each 1000-meter increase in height. You know by now that this is the environmental lapse rate, which is lower than the dry adiabatic lapse rate (10° C / 1000 meters), but greater than the moist adiabatic lapse rate (~6° C / 1000 meters). According to this example, a moist air parcel at the ground with temperature at 30° C, when lifted to 1000 meters, will have a temperature of 24° C, which will be warmer than the environment temperature at 1000 meters (23° C). Therefore, if released, the parcel will continue to rise in the atmosphere. However, a dry air parcel at 30° C, when lifted to 1000 meters, will have a temperature of 20° C, which will be colder than the environment temperature at 1000 meters (23° C). If released, the parcel will fall back to its original position. When the environmental lapse rate is between the dry and the moist adiabatic lapse rate (Gs < G < Gd), the atmosphere is conditionally unstable. By conditional, WE MEAN THAT THE ATMOSPHERE IS UNSTABLE FOR SATURATED AIR, BUT STABLE FOR UNSATURATED AIR.

Air parcel

Hypothetically, air parcels are balloon-like bubbles of air, defined by their homogeneity. The basic properties of an air parcel, such as TEMP AND MOISTURE, are the SAME THROUGHOUT THE PARCEL. No part of the parcel is warmer or cooler or moister or dryer than another. Like a balloon, they are flexible, but their surfaces are impermeable--mass and energy cannot pass through an air parcel.

Absolutely unstable atmosphere

If a parcel is less dense than the environment, its temperature must be higher than its surrounding or Tp - Te > 0. Since the parcel is lighter than its surroundings, it will experience positive buoyancy and rise, and the atmosphere is said to be unstable. Notice here we say that the atmosphere is stable or unstable--stability is a property of the atmosphere, not of the parcel. In summary, one way atmospheric stability can be determined is by comparing a parcel's temperature with that of its environment. Tp - Te > 0: unstable Tp - Te < 0: stable Tp - Te = 0 : neutral ************************* Once again, let us imagine that we sent a radiosonde into the atmosphere to take measurements. The radiosonde returns temperature readings that decrease by 11° C per 1000-meter increase in height. As you may notice, this environmental lapse rate is faster than the dry adiabatic lapse rate (10° C / 1000 meters), and the moist adiabatic lapse rate (~6° C / 1000 meters). Therefore, a moist air parcel at the ground with a temperature of 30° C, when lifted to 1000 meters, will have a temperature of 24° C (30 - 6 = 24). This moist air parcel will be warmer than the environment at that elevation, which is at a temperature of 19° C (30 - 11 = 19). Similarly, a dry air parcel of 30° C, when lifted to 1000 meters, will have a temperature of 20° (30 - 10 = 20), which is also warmer than 19° C. In both cases, because the temperature of the parcel is greater than the temperature of the environment, it is less dense and more buoyant, and will continue to rise if released. When these circumstances occur, the atmosphere is absolutely unstable. As we learned in the previous section of this lesson, we can also determine stability with the environmental lapse rate. The atmosphere is absolutely unstable when the environmental lapse rate (in this example, 11° C / 1000 meters) is greater than the dry adiabatic lapse rate (10° C / 1000 meters; which is always greater than the moist adiabatic rate): G >Gd> Gs. This situation, which is usually results from surface heating and is confined to a shallow layer near the surface, is usually SHORT LIVED. Vertical mixing can eliminate it.

Absolutely stable atmosphere

In general, we determine the stability of air by comparing a rising parcel's temperature to that parcel's surroundings. An air parcel will rise in the atmosphere when its density is less than the density of its surroundings. !!DENSITY= However, density is not measured directly, so we need to convert this concept to something that is easier to measure, which is PRESSURE. The pressure of the parcel is assumed to be the same as the pressure of the environment. When pressure is the same, density is inversely proportional to temperature: warmer air is less dense, colder air is more dense. So, the difference between the density of an air parcel and the density of the environment can be expressed by the difference in their TEMPERATURES. ************************ If a parcel is more dense than the environment, its temperature must be lower than its surroundings, or Tp - Te < 0. The parcel will experience negative buoyancy and resist upward motion, and the atmosphere is said to be stable. ************************** Suppose for a moment that we are taking measurements of the atmosphere, and we release a radiosonde on a balloon. The radiosonde sends back the measurements as it is carried into the atmosphere. For example, it measures the temperature. Notice in the figure above that the temperature decreases 4° C per 1000 meters--this is the environmental lapse rate (symbolized as "G"), as it is measuring the change in temperature of the environment as it increases in altitude. In this example, the environmental lapse rate is less than the moist adiabatic lapse rate (6° C / 1000 meters, symbolized as "Gs"). A moist air parcel at the ground, with a temperature of 30° C, when lifted to 1000 meters, will have a temperature of 24° C (30 - 6 = 24), symbolized as "Tp." A dry air parcel at the ground, with a temperature of 30° C, when lifted to 1000 meters, will have a temperature of 20° C (30 - 10 = 20), as it loses temperature according to the dry adiabatic lapse rate (10° C / 1000 meters, symbolized as "Gd"). This dry air parcel is now colder than the environment temperature ("Te") at 1000 meters (26° C, per the graphic, as determined by the environmental lapse rate). Recall the equation from earlier in this lesson, which describes how atmospheric stability can be determined by comparing the parcel's temperature with that of its environment (Tp - Te). In this example, Tp for the moist air parcel = 24, and Te = 30. Tp - Te = 24 - 30 = -6, which is less than 0. When Tp - Te < 0, the atmosphere is stable. In other words, when a lifted parcel of air is colder (and, therefore, heavier) than the air surrounding it, the atmosphere is absolutely stable. Notice that this works for the dry air parcel as well (20 - 30 = -10, which is less than 0). The atmosphere is absolutely stable when the environmental lapse rate is less than the moist adiabatic lapse rate (~ 6° C / 1000 meters; which is always less than the dry adiabatic rate): G < Gs < Gd. [Note: You may be wondering what happens if the environmental lapse rate is somewhere between the moist and the dry lapse rates. Hold that thought, we will return to it shortly.]

Sounding

Information from weather balloons, including temperature, dew point, wind speed, et cetera, describe environmental properties, which are reported in diagrams called soundings.

Lapse Rate

Lapse rate is defined as the RATE OF TEMPERATURE DECREASE WITH HEIGHT. There are two types. As mentioned earlier, an air parcel's lapse rate can be determined by looking at whether the parcel is saturated or unsaturated. Because you now know the lapse rates for saturated and unsaturated parcels of air, the missing piece in determining the stability of the atmosphere is knowing the environmental lapse rate, so that you can compare the environmental lapse rate to the parcel lapse rate. Thus to determine atmospheric stability, all we need to know is the environmental lapse rate.

The atmosphere is __________________ if an air parcel stays still.

Neutral

When the atmosphere is stable, what is happening?

Parcels of air are not rising

When the atmosphere is unstable, what is happening? Parcels of air are:

Rising

What do you need for the moist adiabatic lapse rate?

Saturated Air

If an air parcel rises and cools at the same rate, or slower than the environmental lapse rate, what is going on in the atmosphere?

Stability

We talked a lot about atmospheric stability in Lesson 8. Which of these weather conditions is likely if we had an "absolutely unstable" atmosphere on a hot and super humid day in mid-July at Disney World (Orlando, Florida)?

Strong thunderstorms

Adiabatic warming occurs when an air parcel sinks.

T

Air parcels are 'bubbles of air' that (1) are flexible, (2) have impermeable surfaces, and (3) have a uniform temperature and moisture content.

T

Instability can be seen as tall thunderclouds.

T

Positive buoyancy of an air mass can lead to instability.

T

The atmosphere can be stable for saturated air, but not for unsaturated air at the same time.

T

The environmental lapse rate is calculated outside of a parcel of air.

T

The shape of clouds forming is a sign of stability or instability in the atmosphere.

T

A parcel's buoyancy is related to:

Temperature

What causes a stable atmosphere?

The atmosphere is more stable if the air at higher elevations is warmed by an influx of warmer air, or the air closer to the surface cools. Air closer to the surface can cool in a variety of ways--for example, in summer, warm air moves over the cool Great Lakes and vertical motion of air is therefore suppressed.

What causes an unstable atmosphere?

The atmosphere is more unstable if the air aloft (higher in elevation) is cooled by an influx in cold air or a rising motion. The atmosphere is also more unstable if the air near the ground is warmed by solar heating, an influx of warm air, or if it moves over warmer surfaces (such as cold air moving over a warmer lake surface during winter, resulting in lake effect snow).

Environment

The environment refers to the ATMOSPHERE SURROUNDING AN AIR PARCEL that is not affected by the air parcel. Its properties, such as temperature, are determined by large scale motions of the atmosphere.

Parcel lapse rate

The parcel lapse rate refers to the process of an air parcel decreasing in temperature with increasing height as it rises up into the atmosphere. This rate of temperature decrease depends on whether the parcel is unsaturated or saturated.

What if the temperature of a parcel of air is the same as the temperature of its environment?

The parcel stays still

Moist adiabatic lapse rate

The rate at which an adiabatically rising saturated air parcel cools is called, understandably, the moist adiabatic lapse rate. This rate is not constant, rather, air cools at a slower rate when the parcel contains more water vapor. But on average, the moist adiabatic lapse rate is approximately 6° C per 1000 meters.

Thermal

Thermals = heated air parcels During the day, thermals rise up in the atmosphere. As you know, rising air expands and cool, while sinking air compresses and warms.

Atmospheric Stability

This lesson will look at atmospheric stability, which is an important concept in meteorology because it affects the vertical motion of air parcels. This rising motion leads to the development of clouds and precipitation. You will learn that stability in the atmosphere is determined by comparing the temperature of a rising air parcel to the temperature of the surrounding air. You will also learn about the processes that cause air to be more or less stable and how we see these processes in phenomena such as clouds and fog. ******************************* If an air parcel is displaced from its original height (that is, the initial perturbation in the figure below), and it: (1) returns to its original height, then the atmosphere is stable; (2) accelerates upward because it is buoyant, then the atmosphere is unstable; or (3) stays at the place where it was originally displaced, then the atmosphere is neutral.

SUMMARY

Tp - Te < 0: stable Tp - Te > 0: unstable Tp - Te = 0 : neutral Comparing a parcel's lapse rate to the environmental lapse rate. The atmosphere is: (1) absolutely stable when the environmental lapse rate is slower than the moist lapse rate (therefore, also the dry), (2) absolutely unstable when the environmental lapse rate is faster than dry lapse rate (therefore, also the moist), or (3) conditionally unstable when the environmental lapse rate is between the moist and dry lapse rates.

Equation for comparing a parcels temperature with that of its environment

Tp-Te Ex: Recall the equation from earlier in this lesson, which describes how atmospheric stability can be determined by comparing the parcel's temperature with that of its environment (Tp - Te). In this example, Tp for the moist air parcel = 24, and Te = 30. Tp - Te = 24 - 30 = -6, which is less than 0. When Tp - Te < 0, the atmosphere is stable. In other words, when a lifted parcel of air is colder (and, therefore, heavier) than the air surrounding it, the atmosphere is absolutely stable. Notice that this works for the dry air parcel as well (20 - 30 = -10, which is less than 0).

What do you need for the dry adiabatic lapse rate?

Unsaturated Air

The atmosphere is __________________ if an air parcel accelerates upward.

Unstable

What does atmospheric stability concern?

Vertical motion in the atmosphere

Adiabatic process

When an air parcel expands and cools or compresses and warms without interchanging heat with the environment, it underwent an adiabatic process.

Stability in the atmosphere is determined by comparing the temperature of __________ to the temperature of __________.

a rising air parcel; its surrounding environment

Which of the following sets of conditions would produce a cumulus cloud with the lowest base?

air temperature 90° F, dew point temperature 60° F

As you rise up from the ground, you start as the _____________________ rate and reach the _____________________ rate after a certain altitude.

dry; moist

As you rise up from the ground, you start as the _____________________ rate and reach the _____________________ rate after a certain altitude.

dry;moist

Which of the following conditions would be described as the most unstable?

environmental lapse rate is 11° C per kilometer BC: 30-11= 19 and 30-6= 24

What do you need for the dry adiabatic lapse rate?

unsaturated air

An adiabatic process is one that occurs:

without transfer of heat or matter between a system and its surroundings.