Atmospheric Stability

Question 1

What is a parcel?

Answer 1

Meteorologists use the term parcel to describe a body of air that has specific temperature and humidity characteristics. Think of an air parcel as a volume of air that is about 300 m in diameter or more.

Question 2

What are the two opposing forces that decide the vertical position of a parcel of air?

Answer 2

Two opposing forces decide the vertical position of a parcel of air: an upward buoyant force and a downward gravitational force. A parcel of lower density than the surrounding air is buoyant, so it rises; a rising parcel expands as external pressure decreases. In contrast, a parcel of higher density descends under the force of gravity because it is not buoyant; a falling parcel compresses as external pressure increases.

Question 3

How dose the temperature of the volume of air determine the density of an air parcel?

Answer 3

The temperature of the volume of air determines the density of the air parcel—warm air has lower density; cold air has higher density. Therefore, buoyancy depends on density, and density depends on temperature.

Question 4

What is Stability?

Answer 4

Stability refers to the tendency of an air parcel either to remain in place or to change vertical position by ascending (rising) or descending (falling). An air parcel is stable if it resists displacement upward or, when disturbed, tends to return to its starting place.

The condition of a parcel of air with regard to whether it remains where it is or changes its initial position. The parcel is stable if it resists displacement upward and unstable if it continues to rise.

Question 5

What is the difference between a stable and unstable air parcel?

Answer 5

An air parcel is stable if it resists displacement upward or, when disturbed, tends to return to its starting place. An air parcel is unstable if it continues to rise until it reaches an altitude where the surrounding air has a density and temperature similar to its own.

Question 6

What are the two temperatures that determine the stability or instability of an air parcel?

Answer 6

The stability or instability of an air parcel depends on two temperatures: the temperature inside the parcel and the temperature of the air surrounding the parcel. The difference between these two temperatures determines stability.

Such temperature measurements are made daily with radiosonde instrument packages carried aloft by helium-filled balloons at thousands of weather stations

Question 7

What is the normal lapse rate and the environmental lapse rate?

Answer 7

The normal lapse rate, introduced in Chapter 3, is the average decrease in temperature with increasing altitude, a value of 6.5 C°·1000 m−1. This rate of temperature change is for still, calm air, and it can vary greatly under different weather conditions. In contrast, the environmental lapse rate (ELR) is the actual lapse rate at a particular place and time. It can vary by several degrees per thousand meters.

Question 8

What two generalizations predict the warming or cooling of an ascending or descending parcel of air?

Answer 8

Two generalizations predict the warming or cooling of an ascending or descending parcel of air. An ascending parcel of air tends to cool by expansion, responding to the reduced pressure at higher altitudes. In contrast, descending air tends to heat by compression.

Question 9

What dose diabatic and adiabatic mean?

Answer 9

These mechanisms of cooling and heating are adiabatic. Diabatic means occurring with an exchange of heat; adiabatic means occurring without a loss or gain of heat—that is, without any heat exchange between the surrounding environment and the vertically moving parcel of air.

Adiabatic
Pertaining to the cooling of an ascending parcel of air through expansion or the warming of a descending parcel of air through compression, without any exchange of heat between the parcel and the surrounding environment.

Question 10

What are the two ways that adiabatic temperature changes are measured?

Answer 10

Adiabatic temperature changes are measured with one of two specific rates, depending on moisture conditions in the parcel: dry adiabatic rate (DAR) and moist adiabatic rate (MAR).

Question 11

What is Dry adiabatic rate?

Answer 11

Dry adiabatic rate (DAR)
The rate at which an unsaturated parcel of air cools (if ascending) or heats (if descending); a rate of 10 C° per 1000 m. (See Adiabatic; compare Moist adiabatic rate.)

Question 12

What is the moist adiabatic rate?

Answer 12

Moist adiabatic rate (MAR) (7)
The rate at which a saturated parcel of air cools in ascent; a rate of 6 C° per 1000 m. This rate may vary, with moisture content and temperature, from 4 C° to 10 C° per 1000 m. (See Adiabatic; compare Dry adiabatic rate.)

Question 13

What is MAR lower then DAR?

Answer 13

The cause of this variability, and the reason that the MAR is lower than the DAR, is the latent heat of condensation. As water vapour condenses in the saturated air, latent heat is liberated, becoming sensible heat, thus decreasing the adiabatic rate. The release of latent heat may vary with temperature and water-vapour content. The MAR is much lower than the DAR in warm air, whereas the two rates are more similar in cold air.

Question 14

What determines the stability of an atmosphere over an area?

Answer 14

The relationship of the DAR and MAR to the environmental lapse rate, or ELR, at a given time and place, determines the stability of the atmosphere over an area. In turn, atmospheric stability affects cloud formation and precipitation patterns, some of the essential elements of weather.

Question 15

What are the three conditions in the lower atmosphere that temperature relationships produce?

Answer 15

Temperature relationships in the atmosphere produce three conditions in the lower atmosphere: unstable, conditionally unstable, and stable.

Question 16

What is an example of an unstable condition?

Answer 16

The air parcel continues to rise through the atmosphere because it is warmer (less dense and more buoyant) than the surrounding environment. Note that the environmental lapse rate in this example is 12 C° · 1000 m−1. That is, the air surrounding the air parcel is cooler by 12 C° for every 1000-m increase in altitude. By 1000 m, the rising air parcel has cooled adiabatically by expansion at the DAR from 25° to 15°C, while the surrounding air cooled from 25°C at the surface to 13°C. By comparing the temperature in the air parcel and the surrounding environment, you see that the temperature in the parcel is 2 C° warmer than the surrounding air at 1000 m. Unstable describes this condition because the less-dense air parcel will continue to lift.

Question 17

What is an example of stable condition?

Answer 17

A condition in which the air parcel has a lower temperature (is more dense and less buoyant) than the surrounding environment. The relatively cooler air parcel tends to settle back to its original position—it is stable. The denser air parcel resists lifting, unless forced by updrafts or a barrier, and the sky remains generally cloud-free. If clouds form, they tend to be stratiform (flat clouds) or cirroform (wispy), lacking vertical development. In regions experiencing air pollution, stable conditions in the atmosphere worsen the pollution by slowing exchanges in the surface air.

Question 18

What is an example of conditionally unstable?

Answer 18

If the ELR is somewhere between the DAR and the MAR, conditions are neither unstable nor stable. In Figure 7.14b, the ELR is measured at 7 C° · 1000 m−1. Under these conditions, the air parcel resists upward movement, unless forced, if it is less than saturated. But if the air parcel becomes saturated and cools at the MAR, it acts unstable and continues to rise.