Textbook chapter 2, p. 33-36 chapter 3, p. 74-79

chapter 4, p. 122-123

Stability and Cloud Development

Why does the atmosphere sometimes produce stratus clouds while at other times cumulus, and/or cumulonimbus clouds form?

The Development of Clouds

what caps the smoke plume and clouds?

Temperatures of Rising and Sinking Air Parcels

Recall that in the troposphere, the temperature typically decreases at an average lapse rate of 6.5C per kilometer *.

This observed rate of temperature decrease is called the Environmental Lapse Rate (ELR).

ELR = -T/z

* this equals 3.6 F/1000 ft

Temperatures of Rising and Sinking Air Parcels

Rising air parcels undergo decompression as they move upwards into regions of lower pressure. Decompression causes adiabatic cooling.

For unsaturated (RH < 100%) parcels, the rate of cooling, known as the Dry Adiabatic Lapse Rate (DALR), is about 10C per kilometer.

Similarly, sinking air parcels warm adiabatically as they are compressed at lower altitudes.

Note that rising parcels do NOT cool because they are moving into regions of lower temperature! Cooling results from decompression of the air.

LCL and MALR

As a rising air parcels cools, its relative humidity increases, eventually reaching 100% when the temperature has declined to the parcel’s dew point temperature. This level is called the lifting condensation level.

Above the LCL, condensation takes place as the rising parcel continues to cool adiabatically.

Condensation releases latent heat, which mitigates against the adiabatic cooling.

Thus above the LCL, the parcel cools less rapidly. This lower cooling rate is the Moist Air Lapse Rate (MALR).

Its value depends on the latent heat release rate. It usually lies between 4C/km and 9C/km.

Dry vs moist adiabatic ascent

Below the LCL, the parcel cools at the DALR.

Above the LCL, the parcel cools at the MALR.

Moist adiabatic processes

• High supersaturation is not observed. Instead, RH in a cloud is never far from 100%.

• This means that T and Td remain essentially equal to one another as the air continues to ascend.

• As water molecules condense from gas to liquid, the vapor mixing ratio decreases.

Stability and Instability

Stability and Instability in the Atmosphere

In a stable atmosphere, vertically displaced air parcels tend to return spontaneously to their original position.

In an unstable atmosphere, vertically displaced air parcels tend to move spontaneously further away from their original position.

In a neutrally stable atmosphere, vertically displaced air parcels neither return to nor continue to move away from their original position.

How can we determine the stability condition of the atmosphere?

The stability condition of the atmosphere can be determined by comparing the density of an air parcel with that of the surrounding air at the same pressure (altitude) level.

Parcels that are more dense than their surroundings will sink, whereas those that are less dense will rise.

Warmer parcels are less dense and tend to rise, while colder parcels are more dense and tend to sink.

Atmospheric Stability and Instability

At z = 3 km, the temperature (Te) is 10C. At this altitude:

A parcel with a temperature (Tp) of 30C will rise.

A parcel with a temperature (Tp) of -10C will sink.

A parcel with a temperature (Tp) of 10C will neither rise nor sink.

Atmospheric Stability and Instability

Atmospheric stability or instability can be determined by comparing an air parcel’s temperature with that of the surrounding air at the same pressure.

Our comparison requires that the temperature profile be known. This profile, known as the environmental lapse rate (ELR), is measured twice per day at 00:00 and 12:00 UTC, at numerous observing stations around the world.

An Absolutely Stable Atmosphere

Lifted unsaturated air parcels cool adiabatically at the DALR (d) (green line).

Lifted saturated parcels cool at the MALR (m) (red line).

When the ELR is small, rising parcels are always colder than the surrounding air, so they tend to resist lifting and sink down.

An Absolutely Stable Atmosphere

An inversion is especially stable.

Stability at the surface inhibits vertical mixing and fog dispersion

An Absolutely Unstable AtmosphereLifted unsaturated air parcels cool adiabatically at the DALR (d) (green line).

Lifted saturated parcels cool at the MALR (m) (red line).

When the ELR is large, rising parcels are always warmer than the surrounding air, so they are buoyant and rise spontaneously.

Conditional Instability

Lifted unsaturated air parcels cool adiabatically at the DALR (d) (green line).

Lifted saturated parcels cool at the MALR (m) (red line).

When the ELR lies between the DALR and the MALR, dry parcels will sink, while saturated parcels will spontaneously rise.

An Example of Conditional Instability

Consider an atmosphere with an ELR of -8C/km.

An unsaturated parcel rises from the surface. The initial parcel temperature is 30C, with a dew point temperature of 14C.

An Example of Conditional Instability

As the parcel cools towards the dew point temperature, its relative humidity increases.

Note that the parcel is being lifted in a stable atmosphere.

The parcel becomes saturated (RH = 100%) at 2 km. This is therefore the LCL.

An Example of Conditional Instability

Above the LCL the parcel cools less rapidly, at the MALR. Recall that this is due to latent heat release from condensation partially offsetting adiabatic cooling.

Above the LCL, the relative humidity of the parcel is 100%.

An Example of Conditional Instability

At an altitude of 4 km, the parcel temperature becomes equal to the ambient temperature.

Up to this altitude, the parcel has been lifted through a stable atmosphere.

Above 4 km, the parcel becomes warmer than its environment. It therefore now rises spontaneously because it has entered a region of the atmosphere that is unstable for saturated parcels.

The altitude where the parcel becomes buoyant is the level of free convection (LFC).

An Example of Conditional Instability

Cloud base is found at the LCL, where the RH first becomes 100%.

An Example of Conditional Instability

Cloud top occurs at the altitude where the parcel is no longer buoyant (9 km in this example).

Note that above the LFC, cloud height is determined by the depth of the unstable layer.

Day-to-night change in lapse rate

near sunrise near noon

Growing a thunderstorm

try this applet