ATOC 1050 Midterm 1~ Ch.6 Atmospheric Stability

What is convection?

"Convection" describes buoyant vertical air currents within the atmosphere, often resulting in the formation of cumulus clouds and/or thunderstorms

4 ways that air parcels can be lifted?

(1) fronts, (2) sea breeze (or lake breeze), (3) topography, (4) convergence at low levels

List 5 ways that an air parcel can be brought to its level of free convection.

(1) lifting along a cold front, (2) lifting along a sea breeze, (3) lifting along topography, (4) low-level convergence, (5) solar heating of air near the surface.

Why does an air parcel cool as it rises?

Air parcels rising through the atmosphere expand because they encounter lower pressure. Expansion requires work, and the energy required to expand the air comes from the thermal energy of the air. With less thermal energy, the air's temperature is lowered.

What causes the state of being "conditionally unstable"?

An Environmental Lapse Rate between 6 C/km and 10 C/km.

What is an adiabatic process?

An adiabatic process is one in which a parcel of air does not mix with its environment or exchange heat energy with its environment. The temperature of an air parcel that undergoes an adiabatic process does change temperature however, due to changing in pressure it encounters and/or saturation effects (the release of latent heat for example, during saturation ascent).

What is meant by the term "air parcel"?

An air parcel is a small volume of air that is assumed to retain its identity as it moves through the atmosphere.

Describe the motion of a vertically displaced air parcel in a stable atmosphere, an unstable atmosphere, and a neutral atmosphere.

An air parcel when displaced vertically will return to its original position in a stable atmosphere, accelerate away from its original position in an unstable atmosphere, and remain at its new position in a neutral atmosphere.

Why are cumulonimbus clouds less common in winter than in summer in the US?

Atmospheric conditions are more stable during winter, in general. Surface air temperatures are relatively cold in winter compared to air aloft (environmental lapse rates are smaller) and it is more difficult for air to become buoyant. During summer, environmental lapse rates are larger, surface air parcels are warmed by solar heating in the daytime and air is more likely to become buoyant.

What do the inflation of a tire and the descent of an air parcel have in common?

Both involve compression, which warms air.

How does heating of the ground by sunlight affect the stability of the lower atmosphere?

Heating the ground warms the near-surface air and makes the lower atmosphere less stable (increases the environmental lapse rate...assuming the upper level air temps remain the same).

What does it mean if air is "stable" or "unstable"?

If air is displaced vertically it will return to its position if the environment is "stable" and it will continue to move vertically in the direction of the displacement if the environment is "unstable".

Explain how atmospheric stability at a particular location can change.

If the lapse rate of a layer of air changes, the stability of that air will change. Lapse rates can be changed by heating and/or cooling the bottom of a layer of air relative to the top of thelayer, or by warm or cold air moving into a region at the top or bottom of a layer.

What is the significance of instability with respect to thunderstorms?

Instability determines the location and intensity of convective storms in the atmosphere.

Why is the moist adiabatic lapse rate different from the dry adiabatic lapse rate?

Latent heat released during condensation in saturated air offsets some of the cooling due to decompression as air rises.

As the environmental lapse rate becomes larger, does the atmosphere become more stable or less stable? Why?

Less stable because the environmental temperature will decrease with height at a faster rate with a large lapse rate. This means that a rising parcel is more likely to be warmer than the environment and more likely to rise. For example, if the environmental lapse rate is 11ºC/km, than lifting an unsaturated air parcel (which cools at 10ºC/km) or a saturated air parcel (which cools at 6ºC/km) will both lead to parcels that are warmer and less dense that the surrounding environment.

Is the atmosphere likely to be more unstable in mid-afternoon or in early morning? Why?

Mid-afternoon. Incoming solar radiation heats air during the day near the surface, causing the air to become less dense and more buoyant, resulting in rising parcels and potentially cumulus clouds.

If the environmental lapse rate in the lower troposphere is approximately 5ºC/km, would you expect strong convection to develop? Why or why not?

No. A lapse rate of 5°C/km indicates a stable atmosphere. Rising air, saturated (6ºC/km) or unsaturated (10ºC/km), will always be colder than its surrounding environment and the air will return to its original position.

Which of the following is (are) a constant: dry adiabatic lapse rate, moist adiabatic lapse rate, environmental lapse rate?

Only the dry adiabatic lapse rate is constant (10°C/km). The moist adiabatic lapse rate and environmental lapse rate vary with altitude. For our intro class, we consider the moist adiabatic lapse rate to be 6ºC/km (so constant), but this isn't strictly true in the real world...

Would strong convection be expected to develop if the environmental lapse rate was 8ºC/km? Explain.

Possibly. A lapse rate of 8°C/km indicates a conditionally unstable atmosphere. If a saturated parcel is lifted in this environment, condensation willoccur during the ascent and the parcel will continue to rise on its own -a convective cloud will develop. An unsaturated parcel lifted in this environment would be cooler than the surrounding air, and would return to its original position.

WHat is the Lifted Index? How is the value of the Lifted Index related to the severity of storms?

The Lifted Index is a measure of instability of the atmosphere. The LI is calculated by subtracting the temperature of a parcel lifted to 500 mb (from the surface) from the temperature of the environment at 500 mb. The lower the value of Lifted Index (more negative), the more potential for severe thunderstorms. A positive Lifted Index means the parcel is colder than the surrounding air, and thus the air is stable.

What is the level of free convection?

The altitude at which a lifted air parcel first becomes buoyant and can rise on its own.

What is meant by "conditional instability"?

The condition required for instability is that the displaced parcel of air be saturated. (Unsaturated air parcels will be stable, saturated air parcels will be unstable.). The environmental lapse rate has to be between 6ºC/km and 10ºC/km for it to be conditionally unstable.

What is the approximate value of the dry and moist adiabatic lapse rates in the lower atmosphere?

The dry adiabatic lapse rate is 10°C/km and the moist adiabatic lapse rate, in the lower atmosphere, is approximately 6°C/km. This means that an unsaturated air parcel will cool at 10ºC/km during ascent, while a saturated air parcel will cool at 6ºC/km during ascent. The difference is caused by 4ºC/km of latent heat warming in the saturated air parcel that counteracts the 10ºC/km cooling (i.e., 10-4 = 6). Descending air parcels always warm at 10ºC/km (they never remain saturated during descent).

What is the name of the lapse rate obtained from rawinsonde measurements?

The environmental lapse rate. This is simply the actual measured change in temperature in the atmosphere, which varies day to night, day-to-day, season to season, etc.

An inversion is indicative of a stable layer in the atmosphere. Briefly explain why this is so.

The environmental temperature increases with an increase in altitude within an inversion. As a parcel of air rises upwards, its temperature always decreases due to decompression. In an inversion, the environmental temperature will increase with altitude while the air parcel temperature decreases. Under these conditions, if an air parcel rises, it will always become colder and thus more dense than the surrounding environment and will return to its original position (stable). This is why the tropopause is a very stable layer, and a lid on the world's weather.

What is the lifting condensation level?

The lifting condensation level is the altitude at which condensation first occurs in a lifted air parcel. I.e., where the air has cooled enough by ascent to reach its dew point.

What is the difference between the lifting condensation level and the level of free convection?

The lifting condensation level is the altitude where condensation first occurs when an air parcel is lifted from the surface (this would be the cloud bases). The level of free convection is the altitude where a rising air parcel becomes buoyant. The level of free convection typically is at a higher altitude than the lifting condensation level.

How is the density of an air parcel related to its temperature and to atmospheric stability?

Warm air is less dense than cold air. Stability is determined by comparing the density of air parcel to the environmental air surrounding the parcel. Air parcels displaced upward that become more dense than their environment will return to their original position indicating a stable environment. Air parcels displaced upward that become less dense than their environment will accelerate away from their original position indicating an unstable environment.

Rising unsaturated air parcels will remain _____ than surrounding air, and will _____________ (stable).

_cooler_ _sink_back_down_

Rising saturated air parcels will remain ____ than surrounding air, and will _________(unstable).

_warmer_ _rise_on_their_own_