Chapter #6

What processes in the atmosphere can cause the atmosphere to become more stable?

1) Warm air advection above the surface 2) Downward movement of air above (subsidence) that will cause the air temperatures above to warm due to compression 3) Cold air advection on and/or near the surface 4) Overnight radiational cooling on the surface 5) Air moving over a colder surface which will cause the surface air temperature to decrease

What mechanisms can cause the atmosphere to become more unstable?

1)Cooling aloft a)Cold air advection above the surface b)Radiational cooling off of top of clouds 2)Warming at the surface a)Daytime solar heating b)Warm air advection on the surface c)Cool air moving over a warmer surface

Name the four mechanisms that are responsible for the development of the majority of the clouds we observe.

1)Surface heating and free convection 2)Topography (orographic lift) forced ascent 3)Widespread ascent (convergence of surface air) Example: Low pressure system 4)Uplift along weather fronts, air masses of different density interacting with one another

List the (3) lapse rates, and their numerical values, that are used for a rising or descending parcel of air.

1.Dry Lapse Rate (10°C/km) 2.Dew-Point Lapse Rate (2°C/km) 3.Moist Lapse Rate (6°C/km)

Define 'stable' and 'unstable' when relating the terms to the atmosphere.

Stability in the atmosphere relates to a comparison of temperatures between a parcel of air and the air surrounding the parcel. When a parcel of air is colder than the surrounding air, it will not be able to rise because it is more dense or heavier than the surrounding air. If the parcel is forced to rise, it will return to its place of origin once the lifting mechanism dissipates because the temperature of the parcel is still colder than the surrounding air. (Stable) When a parcel of air is warmer than the surrounding air, it will have buoyance because it is less dense or lighter than the surrounding air and it will be able to rise on its own freely. (Unstable)

Explain the difference between the 'dry lapse rate' and the 'environmental lapse rate'. How do we obtain the environmental temperatures in order to calculate the 'environmental lapse rates'?

The 'dry lapse rate' applies to the cooling or warming of a parcel of air that is rising or descending through the atmosphere. The 'dry lapse rate' has a constant value. The 'environmental lapse rate' indicates the overall vertical temperature profile of the atmosphere. These temperatures are measured twice daily by the release of radiosonde balloons at the various National Weather Service Offices. The 'environmental lapse rates' can and do change regularly and can even change from one layer of the atmosphere to the next layer of the atmosphere.

What conditions cause an unstable atmosphere?

The 'environmental lapse rate' is large (steep) when unstable conditions prevail. In other words the difference in the air temperature from the surface to those aloft is relatively large. Therefore we need cold air aloft and warm air at the surface to steepen the 'environmental lapse rate'.

What conditions cause a stable atmosphere?

The 'environmental lapse rate' is small when stable conditions prevail. In other words the difference in the air temperature from the surface to those aloft is relatively small. Therefore we need warm air aloft or cool air at the surface or a combination of both to achieve this relatively small temperature decrease with height.

To answer this question and the next question correctly, you will need to know the various lapse rates and how to use them. Question #17: The surface temp of a parcel of air is 24°C and the dew-point temp is 16°C. Using the adiabatic process lift a parcel of air to the 2 km level. What is the temp and dew-point temp of the air parcel at the 2 km height?

Initial Temp = 24°C and Dew-Point Temp = 16°C (The air parcel is unsaturated due to the spread in the Temp and Dew-Point Temp of 8°C). To start, let's just move the parcel up 1 km initially. The new parcel Temp will be 14°C (24 - 10 = 14) and the new parcel Dew-Pt Temp will be 14°C (16 - 2 = 14) at the 1 km level. At the 1 km height the parcel is now saturated since the Temp and Dew-Pt Temp are equal at 14°C. So we must use the Moist LR for both the Temp and Dew-Pt Temp for the last km rise from 1 km to the 2 km level. The new Temp at the 2 km level will be 8°C (14 - 6 = 8) and the new Dew-Pt Temp will be 8°C (14 - 6 = 8). Therefore the correct answer is Temp 8°C and Dew-Pt Temp 8°C at the 2 km height.

Calculate the stability of a parcel of air at the 1 km and 2 km level. The starting temp for the parcel is 30ºC and the dew-pt temp is 22ºC. The environmental temps are 22ºC at the 1 km level and 12ºC at the 2 km level. To answer this question you will need to first find the parcels temp at the 1 km and 2 km level, then compare the parcel's temp to the environmental temps to make the decision if the parcel is stable or unstable at the two different levels.

Initial Temp = 30°C and Dew-Point Temp = 22°C (The air parcel is unsaturated due to the spread in the Temp and Dew-Point Temp of 8°C). Let's just move the parcel up 1 km initially. The new parcel Temp will be 20°C (30 - 10 = 20) and the new parcel Dew-Pt Temp will be 20°C (22 - 2 = 20) at the 1 km level. At the 1km height the parcel is now saturated since the Temp and Dew-Pt Temp are equal at 20°C. So we must use the Moist LR for both the Temp and Dew-Pt Temp for the last km rise from the 1 km to the 2 km level. The new Temp at the 2 km level will be 14°C (20 - 6 = 14) and the new Dew-Pt Temp will be 14°C (20 - 6 = 14). Now compare the parcel temp at 1 km and 2 km level to the environmental temps at those two levels. 1 km Environment temp 22°C, Parcel temp 20°C (Stable) 2 km Environment temp 12°C, Parcel temp 14°C (Unstable)

Why do we use 10˚C/km for the 'dry lapse rate' and only 6˚C/km for the 'moist lapse rate'?

Once the air parcel becomes saturated (100% RH), the latent heat of condensation (600 cal/gr) will add sensible heat to the parcel which in turn will offset the cooling due to expansion and slow the rate of cooling within the parcel. Hence the cooling rate will decrease from 10°C/km (dry) to 6°C/km (moist)

Explain why rain shadows form on the leeward side of mountains.

Precipitation falls on the windward side of the mountain. Lets take a closer look at the flow of an air parcel up and over a mountain.

Use your new found knowledge on the topic of stability to answer these two questions. 1.On a typical summer day, how does atmospheric stability change from the daytime to nighttime? 2.What kind of stability is needed for the development of a thunderstorm. In other words, what kind of atmospheric stability will allow the thunderstorm to grow in the vertical?

1.During the daytime, incoming solar radiation will cause the earth's surface and the lowest region of the troposphere to warm causing the atmosphere to become more unstable. During the overnight hours, radiational cooling will cause the earth's surface and lowest region of the troposphere to cool causing the atmosphere to become more stable. 2.In order for a thunderstorm to grow vertically, the temperature within the thunderstorm mass must be warmer than the surrounding environmental air. Under this condition the thunderstorm is able to build in the vertical because the atmosphere in the region of thunderstorm development is unstable.

What are the necessary conditions needed to determine the correct use of the various lapse rates for a parcel of air?

1.The dry lapse rate is used for a parcel's temperature when the parcel of air is unsaturated (RH < 100%). -10°C/km in a rising parcel and +10°C/km in a sinking parcel. 2.The dew-point lapse rate is used for a parcel's dew-point temperature when the parcel of air is unsaturated(RH < 100%). -2°C/km in a rising parcel and +2°C/km in a sinking parcel. 3.The moist lapse rate is used for the parcel's temperature and dew-point temperature when the parcel of air reaches saturation (RH = 100%) -6°C/km for a rising parcel. Note: The moist lapse rate is not used in sinking air because the parcel will warm and RH values will fall to values below 100%

What type of cloud formations are typically associated a warm front and cold front?

Air that is forced to ride up and over the gentle slope of a warm front will produce layered clouds in the stratus category because the atmosphere is mainly stable. Air that is forced upward along a more aggressive cold front will tend to build vertically and be in the cumulus category because the atmosphere is mainly unstable.

The temperature at the surface is 25˚C and the dew-point temperature is 15˚C. Cumulus clouds form in the afternoon. What will be the approximate base of these cumulus clouds?

Cloud Base = (T-Td)/2.5C x 1000 = (25-15)/2.5 x 1000 Cloud Base = 4,000 ft AGL

List the lapse rate used for the vertical temperature profile of the atmosphere.

This lapse rate is called the 'Environmental Lapse Rate'. This lapse rate is not a constant number. Because the atmospheric temperatures are always changing, the calculation of the 'environmental lapse rate' will vary. To calculate the 'environmental lapse rate' we use the temperature data provided by a upper-level sounding that was collected during the ascent of a radiosonde balloon.

Define "Conditionally Unstable"?

This occurs when the 'environmental lapse rate' value is found between the numerical value of the 'moist' and 'dry lapse rate'. Conditionally Unstable means, if unsaturated air could be lifted to a level where it becomes saturated, instability would result. In other words, the higher the moisture content in the parcel, the quicker the parcel can reach saturation when lifted. Therefore the level where the parcel becomes unstable is conditioned on the initial water content of the parcel before lifting.

What is an absolutely stable atmosphere?

When a parcel of air remains colder than the surrounding air while being lifted, we say the air is stable. In other words, when the 'environmental lapse rate' is less than the 'moist adiabatic lapse rate' (6°C/km) the atmosphere is absolutely stable.

What is an unstable atmosphere?

When a parcel of air remains warmer than the surrounding air while being lifted, we say the air is unstable. In other words, when the 'environmental lapse rate' is greater than the 'dry lapse rate' (10°C/km) the atmosphere is absolutely unstable.

Define what an adiabatic process is?

When a rising parcel of air expands and cools or when a sinking parcel of air compresses and warms with no exchange of heat with the surrounding air outside of the parcel, this situation is called an adiabatic process.

What kind of clouds are associated with a stable atmosphere?

When stable air is forced to rise it will have a tendency to spread out. The clouds that form under this condition will also spread out forming layers. Therefore clouds with this characteristic will be either cirrostratus, altostratus, stratus or nimbostratus. (stratus → 'layered')