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CHAPTER 8

ADIABATIC PROCESSES

AND STABILITY

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INTRODUCTION.

An adiabatic process changes the temperature of a gas within a defined system, without any transfer of heat energy across the boundaries of the system. The term

‘adiabatic’ literally means “an absence of heat transfer.” This chapter will explain the concept of the adiabatic processes and their fundamental link to atmospheric stability.

Thecompressionandexpansionof‘parcels’ofairwithintheatmosphereareexamples of adiabatic processes.

Figure 8.1 When air is compressed, its temperature rises adiabatically.

When a parcel of air is compressed, the potential energy of the air is increased, causing an increase in molecular activity, raising the temperature of the parcel of air.

The important point to note is that this temperature change is not caused by any external heat source, but as a result of the compression of the air.

In an

adiabatic process, no

heat energy

flows in or out of the system.

As a parcel of

air descends in the

atmosphere,

it is compressed and heats up adiabatically.

As a parcel of

air rises in the atmosphere, it

expands and cools adiabatically.

Figure 8.2 When air expands, its temperature falls adiabatically.

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The reverse process takes place if an air parcel expands. During expansion, the molecules lose kinetic energy, and the temperature of the air falls as a consequence.

The Dry

Adiabatic

Lapse Rate (DALR) is 3°C per 1000 feet.

ADIABATIC TEMPERATURE CHANGES.

Adiabatic temperature changes occur in the atmosphere as air rises and descends.

As you learnt in Chapter 5, pressure decreases with altitude and, so, if a parcel of air were to rise up through the atmosphere, the pressure of the surrounding air would decrease, and the parcel of air would expand. This expansion would cause the temperature within the parcel to fall because of “adiabatic cooling”. (See Figure 8.2.)

Conversely, when a parcel of air descends, the surrounding pressure increases, and the parcel is compressed, causing the temperature to increase, because of “adiabatic warming”. (See Figure 8.1.)

ADIABATIC LAPSE RATES.

The rate at which the temperature of a parcel of air rises or falls with height is known as the “adiabatic lapse rate”. The value of the adiabatic lapse rate is dependent on the moisture content of the air.

T h e Dr y A d i a b a t i c L a p s e R a t e .

If unsaturated air (that is, air at less than 100% humidity) is forced to rise or descend within the atmosphere, the temperature of the air parcel changes at a rate of 3°C per 1 000 feet. This is known as the “Dry Adiabatic Lapse Rate”. Figure 8.3 shows a graph of temperature against height representing the Dry Adiabatic Lapse Rate (DALR).

Figure 8.3 The Dry Adiabatic Lapse Rate is 3º C per 1000 ft.

T h e S a t u r a t e d A d i a b a t i c L a p s e R a t e .

If the air is saturated (that is, if the relative humidity of the air is 100%), the rate of change of temperature with height is lower than when the air is unsaturated. The “Saturated Adiabatic Lapse Rate” is 1.5°C per 1 000 feet. However, the Saturated

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Adiabatic Lapse Rate (SALR) is not actually constant, but it varies from 1.2°C per 1 000 feet, close to the Earth’s surface, to 2.8°C per 1 000 feet in the upper atmosphere.

Some text books on Meteorology use a figure of 1.8°C per 1 000 feet for the SALR, but, for the Private Pilot’s Licence examination, 1.5°C per 1 000 feet is used as a working average of the temperature change with height for saturated air.

Figure 8.4 The Saturated Adiabatic Lapse Rate is approximately 1.5º C per 1000 ft.

Figure 8.4, shows both the SALR and the DALR.

The SALR is lower than the DALR because of the action of latent heat.

When saturated air cools as it is rising, condensation occurs, and latent heat is released by the water vapour, thus reducing the rate at which the parcel of air cools. When saturated air warms up as it is descending, the water droplets evaporate, absorbing latent heat from the water vapour, thus reducing the rate at which the parcel of air warms up.

The Saturated

Adiabatic Lapse Rate

(SALR) is

1.5°C per 1000 feet.

The SALR

is less than the DALR

because of

the release and absorption of l a t e n .t h e a t

ATMOSPHERIC STABILITY.

The adiabatic processes you have just learnt about will help you understand atmospheric stability.

When we speak of the atmosphere as being stable, we are referring to the tendency of air, when displaced, to return to its original position when the force that initially displaced the air is removed.

Atmospheric stability lies behind the process of cloud formation, and, thus, an understanding of atmospheric stability helps us to make predictions about the weather.

If the atmosphere is unstable, air which has been displaced tends to be further displaced, even when the initial force causing the displacement is removed.

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Displaced air which is

cooler than its surroundings will tend to return to its original

position. In such conditions, the atmosphere is stable.

If the atmosphere is stable, air which has been displaced will tend to return to its original location.

A S t a b l e A t m o s p h e r e .

Atmospheric stability, then, is a relatively simple concept; if air, having being raised through the atmosphere is colder than its surroundings, it will naturally tend to descend to its original position when the lifting force is removed, because cold air is denser and heavier than warmer air.

Figure 8.5 If the parcel of rising air cools at a higher rate than the surrounding air, the atmosphere is stable.

Figure 8.5, shows a parcel of unsaturated air ascending, and cooling at the Dry Adiabatic Lapse Rate of 3° per 1 000 feet. The temperature of the air surrounding the parcel is decreasing at the slower ISA Environmental Lapse Rate (ELR) of 2° per 1 000 feet. So, if the air parcel had been at the same temperature as the surrounding air as it begins to rise, it quickly becomes cooler and more dense than its surroundings.

Consequently, if the force that was lifting the parcel is removed, the parcel of air will sink back towards the Earth’s surface again.

A n U n s t a b l e A t m o s p h e r e

If a parcel of air were to be raised through the atmosphere, and remains warmer than its surroundings, because the Environmental Lapse Rate is greater than the Dry Adiabatic Lapse Rate, as shown in Figure 8.6, the vertical displacement of the rising air will continue and increase, even when the lifting force is removed, because the parcel of air remains less dense, and, therefore, lighter than the surrounding air. In this situation, the atmosphere is said to be unstable.

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Figure 8.6 If the parcel of air cools a lower rate than the surrounding air, the atmosphere is unstable.

Basic adiabatic processes within the atmosphere, and, therefore, the degree of atmospheric stability are, thus, controlled by the prevailing, and variable, Environmental Lapse Rate (ELR), described in Chapter 7. It follows, then, that if we know the actual Environmental Lapse Rate for a given day, we can predict whether the atmosphere will be stable or unstable.

The levels of stability of the atmosphere are classified as Absolute Stability, Absolute

Instability and Conditional Stability.

If the

Environmental Lapse Rate

is known,

we can predict whether the atmosphere will be stable or unstable.

ABSOLUTE STABILITY.

A state of Absolute Stability is said to exist when the Environmental Lapse Rate (ELR) is less than the Saturated Adiabatic Lapse Rate (SALR). In other words, when the change of temperature of the environmental air with altitude is less than 1.5°C per 1 000 feet.

Examining the graph in Figure 8.7, we see that, given the prevailing ELR, the atmosphere is stable, because rising unsaturated and saturated air will always be cooler than the surrounding environmental air.

Figure 8.7 Absolute Stability will exist when the Environmental Lapse Rate (ELR) is lower than the SALR.

Absolute

Stability exists when the

Environmental

Lapse Rate is less than the Saturated Adiabatic Lapse Rate.

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Figure 8.8 In a stable atmosphere, rising unsaturated air will tend to return to its original position when the force causing the air to rise is removed.

If a parcel of unsaturated air were forced to rise to 1 000 feet, as depicted in Figure 8.8, the temperature of the air parcel would fall at the DALR, which is 3°C per 1 000 feet. Thus, in the atmospheric conditions represented by Figure 8.7, by the time the parcel of dry air, whose temperature at the surface was 20°C, had reached

1 000 feet, it would have cooled to a temperature of 17°C. But, at 1 000 feet, the temperature of the ambient or environmental air would have fallen to only 19°C, because the ELR, in this case, is only 1°C per 1 000 feet.

Consequently, if the displacement force were removed, the parcel of dry air would tend to return to the surface because it is denser and heavier than the environmental air. If the parcel were displaced to 2 000 feet, the difference in temperature between it and the environmental air would be even greater. Therefore, the tendency to return to the surface would be even more pronounced.

Saturated air, when displaced vertically upwards, falls in temperature at the SALR of 1.5°C per 1 000 feet. So, if a parcel of saturated air with a temperature at the Earth’s surface of 20° were to be displaced to 1 000 feet, as depicted in Figure 8.9, its temperature will reduce to 18.5°C. Just like the unsaturated air, in the atmospheric conditions represented by Figure 8.7, where the ELR is 1°C per 1 000 feet, this parcel of air would also be colder than the surrounding air, and would tend to return to the surface when the lifting force was removed.

Figure 8.9 In a stable atmosphere vertically displaced saturated air will also tend to return to its original position.

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Again, if the displacement were continued to 2 000 feet, the temperature difference between the parcel of air and its surroundings would increase, and the tendency to return to the surface would be more pronounced.

These examples show that whenever the Environmental Lapse Rate is less than 1.5°C per 1 000 feet the atmosphere is stable.

Is o t h e r m s a n d In v e r s i o n s a r e A s s o c i a t e d w i t h

There are two particular examples of ELR which have special significance in marking a stable atmosphere.

An isotherm occurs when temperature remains constant with increasing height. (See Figure 8.10.) An isotherm will always contribute to a stable atmosphere which is why the Tropopause limits vertical air movement and is, effectively, the ‘top of the weather’. As you have learnt, in the lower levels of the Stratosphere, the ELR is isothermal.

The other significant type of ELR, also depicted in Figure 8.10, is the inversion. An Inversion is characterised by temperature increasing with height. An inversion makes for a very stable atmosphere.

Figure 8.10 The pale green shaded area, in which EL R ares all less than the S A L, denotesR stable atmospheric conditions. In the shaded area are examples of special cases of EL R: an

i s o t hande anr mi n v e r. s i o n

T h e C h a r a c t e r i s t i c s o f a S t a b l e

The effect of a stable atmosphere is to suppress vertical displacement of air, so preventing air rising any significant distance through the atmosphere, even if convection is present. Astable atmosphere, then, prevents any significant cumuliform cloud formation. If any cloud is produced in a stable atmosphere, it is predominately stratiform-type cloud, although cumulus of small vertical extent may also form.

A stable atmosphere can also have a marked effect on surface visibility. Because a stable atmosphere suppresses vertical movement of air, any pollution, such as dust or smoke, will be trapped near the surface and accumulate, creating very hazy conditions with poor surface visibility.

A t m o s p h e r e .

Stable air

resists vertical displacement.

Therefore any

clouds formed will be stratiform rather than cumuliform, and visibility will be poor.

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ABSOLUTE INSTABILITY.

Absolute Instability is said to exist when the Environmental Lapse Rate (ELR), is greater than the DALR; in other words, when the rate of change of temperature with altitude is higher than 3°C per 1 000 feet. The graph at Figure 8.11 depicts atmospheric conditions in which the atmosphere is absolutely unstable.

Absolute Instability

exists when the ELR is

greater then the DALR.

Figure 8.11 Absolute Instability of the atmosphere. Absolute Instability exists when the ELR is higher than the DALR.

Figure 8.11 depicts an unstable atmosphere with an ELR of 4°C per 1 000 feet, and a surface temperature of 20°C. If, as shown in Figure 8.12, a parcel of unsaturated air were forced to rise to 1 000 feet, its temperature would fall by the DALR, which is 3°C per 1 000 feet. Therefore, by the time the air parcel reached 1 000 feet, the temperature of the parcel of air would be 17°C. But at 1 000 feet, in the unstable atmosphere that we are considering, the temperature of the environmental air would be 16°C.

Figure 8.12 Absolute Instability. Any displaced unsaturated air will continue to rise when the

ELR is greater than the DALR.

The parcel of displaced air, therefore, is now warmer than the environmental air. It follows, then, that even if the displacement force were to be removed, the parcel of air would continue to rise, because it is less dense and, therefore, lighter than the environmental air.

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