Humidity and Stability AOS 101 Section 301 March 23, 2009

A Brief Aside

Humidity Humidity is a measure of the water vapor content of the air Humidity is important because the latent heat involved in the evaporation and condensation of water vapor plays a large role in our day-to-day weather

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways:

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Vapor Pressure The portion of atmospheric pressure exerted by water vapor alone

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Vapor Pressure The portion of atmospheric pressure exerted by water vapor alone The maximum (saturation) vapor pressure increases as the temperature increases

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Dewpoint The temperature the air has to be cooled to in order to reach saturation

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Dewpoint The temperature the air has to be cooled to in order to reach saturation The air becomes saturated because the cooled air reaches a saturation vapor pressure equal to its actual vapor pressure

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure

Measurements of Humidity You can measure the humidity of the atmosphere in a number of ways: Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure This measure doesn’t actually say anything about how humid the air is, since the water vapor content of the air is limited by its temperature

Vapor Pressure TIME

Vapor Pressure What is going on here? TIME

Vapor Pressure Relative Hum. TIME

Vapor Pressure Relative Hum. What is the relative humidity here? TIME

Vapor Pressure Relative Hum. Sat. Vapor Pres. TIME

Vapor Pressure Relative Hum. Actual vapor pressure less than saturation vapor pressure = unsaturated Sat. Vapor Pres. Actual vapor pressure equal to saturation vapor pressure = saturated TIME

Vapor Pressure Relative Hum. Sat. Vapor Pres. Temperature TIME

Vapor Pressure Relative Hum. Steep drop in temperature while air is unsaturated Sat. Vapor Pres. Temperature Air cools more slowly after reaching saturation: Why? TIME

Vapor Pressure Relative Hum. Sat. Vapor Pres. Temperature Dewpoint TIME

Temperature This “bend” in the temperature line after the air reaches saturation is incredibly important when discussing atmospheric stability… TIME

Atmospheric Stability Stability is a measure of the tendency of a system to return to its original state once it has been disturbed In the atmosphere, we are concerned with whether a “parcel” of air, when forced to rise, will return to its original altitude (stable) or continue to rise (unstable)

Stability: A Cartoon If the red ball is displaced slightly from its location…

Stability: A Cartoon If the red ball is displaced slightly from its location…

Stability: A Cartoon If the red ball is displaced slightly from its location… The ball returns to its original location because of the effect of gravity. Here, gravity is behaving as a restoring force – a force that restores the system back to its original state. This is a stable system.

Stability: A Cartoon Let’s change the system:

Stability: A Cartoon If the red ball is displaced slightly from its location...

Stability: A Cartoon If the red ball is displaced slightly from its location...

Stability: A Cartoon If the red ball is displaced slightly from its location...

Stability: A Cartoon If the red ball is displaced slightly from its location... Gravity now accelerates the ball away from its original position. This is an unstable system.

Atmospheric Stability When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

Atmospheric Stability When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

Atmospheric Stability When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

Atmospheric Stability When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude: Stable atmospheres are typified by clear skies

Atmospheric Stability When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

Atmospheric Stability When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

Atmospheric Stability When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

Atmospheric Stability When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward: Unstable atmospheres are typified by clouds and precipitation

Humidity and Stability Humidity plays a role in the stability of the atmosphere, because the stability of the air is related to the temperature of a lifted air parcel compared to the temperature of its surroundings The latent heat released in condensation will change how the air cools as it rises (the “bend”), which changes the stability for a saturated air parcel compared to an unsaturated one

Stability: Buoyancy Force In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward

Stability: Buoyancy Force In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force.

Stability: Buoyancy Force In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force. Once the parcel rises and cools to a level where its temperature matches the temperature of its environment, it no longer experiences a buoyancy force, and stops rising.

Thermodynamic Diagram 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram 1000 Environmental Temperature Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force. 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram Once the parcel rises and cools to a level where its temperature matches the temperature of its environment, it no longer experiences a buoyancy force, and stops rising. 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram So far, we haven’t considered the effect of latent heat release. Let’s see what happens when we add humidity to the air and allow it to saturate by cooling as it rises: 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram Start with a humid (but not saturated) parcel of air at the same temperature as before: 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram The air parcel continues on the same path as before, rising and cooling at the same rate, until it saturates. 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram The saturated parcel will continue to cool as it rises, but will do so at a slower rate, because the latent heat of condensation works to keep the air parcel warmer. Here is the “bend” from earlier. 1000 Height 100 50 10 -20 -10 Temperature 10 20

Thermodynamic Diagram The saturated parcel will continue to cool as it rises, but will do so at a slower rate, because the latent heat of condensation works to keep the air parcel warmer. Here is the “bend” from earlier. 1000 Height 100 50 10 -20 -10 Temperature 10 20

Lapse Rates The rate at which the temperature decreases with height is known as the lapse rate:

Lapse Rates The rate at which the temperature decreases with height is known as the lapse rate: Dry Adiabatic Lapse Rate – The rate at which a dry parcel of air cools as it rises = 9.8 C/km Moist Adiabatic Lapse Rate – The rate at which a saturated parcel of air cools as it rises = 6 C/km

Lapse Rates An environmental lapse rate that is less than the moist adiabatic lapse rate is considered absolutely stable. This is because even a saturated parcel of air cannot rise and find itself to be warmer than the environmental temperature. LR < 6 C/km

Lapse Rates An environmental lapse rate that is more than the dry adiabatic lapse rate is considered to be absolutely unstable. This is because even a dry air parcel will find itself to be warmer than its environment when it is lifted slightly. LR > 9.8 C/km

Lapse Rates An environmental lapse rate that is between the dry and moist adiabatic lapse rates is considered to be conditionally unstable; a dry air parcel will be stable in that environment, but a saturated air parcel will not. 9.8 C/km > LR > 6 C/km