The lapse rate is the rate of change in temperature observed while moving upward through the earth's atmosphere. There are two important lapse rates namely, environmental lapse rate and adiabatic lapse rate. In this blog, we have discussed each of these rates and their implications in detail.
Lapse Rate
As said, the lapse rate is the rate of change in temperature observed while moving upward through the earth's atmosphere. It is positive when the temperature decrease with elevation, zero when the temperature is constant with elevation, and negative when the temperature increase with elevation.
Environmental Lapse Rate
The environmental lapse rate also called normal lapse rate, is the lapse rate of non-rising air. It is highly variable. It is affected by radiation, convection, and condensation. Its value is approximately equal to 6.5°C per kilometre in the lower atmosphere.
Adiabatic Lapse Rate
An adiabatic condition is a condition where no heat exchange occurs between the given system and its surroundings. As said, the adiabatic lapse rate is the rate at which the temperature of an air parcel changes in response to the compression or expansion associated with elevation change under adiabatic conditions. Before getting into detail about the adiabatic lapse rate, it is important to know about the air parcel.
Air Parcels
As we know, mixing is a result of random motions of individual molecules. But at atmospheric levels, this molecular mixing is important only for a few metres near the surface and beyond turbopause. In intermediate levels, mixing is achieved by the exchange of air parcels or air bubbles. These parcels can vary in size from a few mm to several km, but there are certain assumptions to be considered regarding air parcels.
Assumptions for an Air Parcel
An air parcel is thermally insulated from the surroundings
Air parcel is at the same pressure as the environmental air at the same level
The kinetic energy of the process is a negligible fraction of its total energy. This implies that the parcel moves very slowly.
Now that we know about air parcels, let's get back to the adiabatic lapse rate discussion. There are two adiabatic lapse rates, namely,
Dry adiabatic lapse rate
Moist adiabatic lapse rate
1. Dry Adiabatic Lapse Rate
The dry adiabatic lapse rate for air depends only on the specific heat capacity of air at constant pressure and the acceleration due to gravity. The dry adiabatic lapse rate for the Earth’s atmosphere equals 9.8°C per kilometre. Thus, the temperature of an air parcel that ascends or descends 2 km would fall or rise 19.6 °C respectively.
2. Moist Adiabatic Lapse Rate
When an air parcel that is saturated with water vapour rises, some of the vapour will condense and release latent heat. This process causes the parcel to cool more slowly than it would if it were not saturated. Therefore, this rate would be lesser than the dry adiabatic lapse rate.
The moist adiabatic lapse rate varies considerably because the amount of water vapour in the air is highly variable. The greater the amount of vapour, the smaller the adiabatic lapse rate. As an air parcel rises and cools, it may eventually lose its moisture through condensation; its lapse rate then increases and approaches the dry adiabatic value.
Relation Between ELR and ALR
Case 1: ELR > ALR (Super adiabatic condition)
An air parcel going up will continue to go up as the air parcel becomes warmer.
An air parcel going down will continue to go down as the parcel becomes colder than the surrounding air.
The air parcel is highly unstable.
This condition can be compared with a ‘ball on the concave down surface’. The ball on the surface tends to move in either direction.
Case 2: ELR < ALR (Sub adiabatic condition)
It can be inferred from the plot that the ALR is steeper than the ELR. So,
If the parcel of air rises up, it becomes colder than the surrounding air. Hence, comes down to the original position.
If the parcel of air goes down, it becomes warmer than the surrounding air. Hence, rises up to the original position.
The air parcel is highly stable under this condition.
This condition can be compared with a ‘ball on the concave upward surface’. The ball on the surface tends to come back to the central original position.
Let us assume the ELR has a negative sign(temperature increases with elevation). It happens rarely in the atmosphere, so-called an extreme case. This is also called the case of temperature inversion, which we already have seen. The air parcel undergoing this condition is extremely stable.
Case 3: ALR = ELR (Neutral condition)
As the environmental air has the same temperature as the air parcel, the parcel does not go up or down. This is a neutral condition.
Atmospheric stability
The term stability comes up frequently in meteorological conversations. Stability and instability are the two terms to concentrate on. The term stability explains the temperature profile that isn't helpful for storm formation. At the point when bundles of air rise, they experience a climate that makes the air decelerate, stop and sink down before storm convection can form. Instances of stable layers are inversions and covering layers. Instability is just the opposite and refers to a troposphere that is helpful for rainstorm/thunderstorm formation. Unstable layers can be created by solar hitting the ground surface, cooling up high.
Assessing atmospheric stability
To know whether a parcel of air will rise or sink in the atmosphere, we should compare the temperature of the air parcel (Tp) with that of the environment (Te) at some altitude.
Generally, Te is always larger than Tsp and Tup at any level. So, the saturated or unsaturated air parcels are always cooler than the temperature of the environment, which makes them sink back to the surface. The condition for absolute stability is:
Ld > Lm > Le
Where,
Ld: Dry adiabatic lapse rate (9.8°C/km)
Lm: Moist adiabatic lapse rate (6°C/km)
Le: Environmental lapse rate
Tup: Temperature of an unsaturated parcel
Tsp: Temperature of a saturated parcel
The condition for absolute instability is:
Le> Ld> Lm
So, the saturated or unsaturated air parcels are always warmer than the temperature of the environment, which makes them ascend in the atmosphere.
The condition for conditional instability is:
Ld> Le> Lm
The unsaturated air parcel will be cooler than the environment, so it sinks back to the atmosphere.
The saturated air parcel will be warmer than the environment, so it continues to ascend in the atmosphere.
Example Problem
Following observations have been made for the temperature of the atmosphere with respect to elevation to find the stability of the atmosphere:
The atmosphere is-
Stable
Unstable
Neutral
Extremely stable
Explanation
ΔT/ΔZ = (15-14.5)/(40-10) = 0.5/30 = 16.7 degree C per km
ΔT/ΔZ = (14.5-13.5)/(100-40) = 6/40 = 16.7 degree C per km
So, ELR = 16.7 degree C per km
We know, ALR = 9.8 degree C per km
ELR > ALR (super-adiabatic condition)
So, the atmosphere is unstable.
Option B is correct.