Which Of The Following Lapse Rates Represents The Greatest Temperature Change Over Altitude?

Introduction

A big corporeality of cloud formation and much of the dynamic behaviour of the temper depends upon vertical movements of air. The tendency of air masses to move up or down is termed its stability. Unstable air masses are prone to vertical movements, while stable air resists vertical motion. The stability of air is a function of its buoyancy with respect to the surrounding air, which is in turn dependent on their relative densities. In Lecture two we saw that density is related to both pressure and temperature: for constant temperature, density increases along with pressure; and for constant pressure level density decreases with increasing temperature. In other words, warm air will tend to expand and become less dense, making it more than buoyant than cooler, denser air, and causing it to ascension. Conversely, cool air will tend to contract and get more dumbo, decreasing its buoyancy and causing information technology to sink. A huge amount of atmospheric behaviour follows from these uncomplicated relations.

Lapse rates

To understand more than most the stability of air masses, we need to examine the temperature changes undergone by an air mass as information technology rises or sinks, which in turn relate to pressure level changes. The simplest example concerns unsaturated air, i.eastward. air which carries all the available wet in gas form (Lecture 3). Relationships between force per unit area and temperature atomic number 82 to a simple linear relation between temperature and distance for rising or sinking air. This is known as the Dry Adiabatic Lapse Charge per unit (DALR), and is equal to 9.8°Thousand km^-i. Air lifted upward will absurd at this rate due to reduction in pressure, air sinking volition warm at this rate due to pressure increases. This was demonstrated in the pressure vessel (Lecture ii). The word 'adiabatic' is derived from the Greek word for 'impassable', and it refers to a system which does not lose or gain energy. Thus, rising air is said to cool or warm adiabatically when its temperature changes are due entirely to pressure changes. In reality, some degree of energy exchange volition always accept place, but these are by and large pocket-sized on brusque timescales.

When condensation or evaporation occur in the air, however, lapse rates of rising or falling air differ from this value. Every bit we have seen in Lecture 1, latent heat is released by condensation and consumed by evaporation (2,500 J g^-i). This alters the adiabatic lapse charge per unit: because energy is released by condensation, ascent air volition cool more than slowly if condensation is occurring. Thus, at that place is a smaller change in temperature with height than would be the case for unsaturated air. The modified lapse rate is termed the Saturated Adiabatic Lapse Rate (SALR). The varying amounts of water vapour that tin be held in air at dissimilar temperatures means that the SALR is non linear. The SALR is lowest at high temperatures, considering of much higher saturation mixing ratios: i.e: greater amounts of free energy are released at the vapour/droplet transition, therefore temperature changes with altitude are reduced. At low temperatures, the SALR is more than similar to DALR: smaller amounts of moisture are bachelor for condensation, then the modification of the lapse charge per unit is less.

Adiabatic Lapse Rates are ordinarily different to the existent vertical change in temperature, known as the Environmental Lapse Charge per unit (ELR). The ELR is influenced by patterns of heating, cooling and mixing, and the past history of an air mass. Actual vertical temperature gradients in the atmosphere are thus highly variable, and tin can even testify an increase in temperature with height, a situation known equally a temperature inversion.

Atmospheric condition Determining Air Stability

The stability of air masses depends on the relative values of the ELR and the appropriate Adiabatic Lapse Rate.

Stability
Air is stable if the ELR less than the ALR. If, for whatsoever reason, a parcel of air is uplifted, it will cool to lower temperatures than its new surroundings along the ALR. Hence the air parcel will exist denser than its environs and will tend to fall dorsum to its original level. This situation is encouraged by a small ELR or a temperature inversion.

Instability
In that location are two cases:

Absolute instability
In this case, the ELR is greater than both DALR and SALR. Uplifted air cools relatively slowly, and will thus be warmer and less dumbo than its new environment. It will therefore tend to continue to rise.

Provisional instability
In this instance, the ELR is less than the DALR but greater than the SALR. Air will be stable unless forced to rise to distance where condensation occurs, whereupon spontaneous uplift volition occur.

Air Stability and Potential Temperature

In Lecture ii, we introduced the idea of potential temperature. Potential temperature is denoted q(Greek theta), and can be defined every bit:

the temperature that an air mass would have if information technology were moved dry out adiabatically to a level at which pressure level is g hPa.

The equivalent concept for saturated air is the wet-bulb potential temperature (q _w), which is the temperature that air would have if information technology moved to a level where the pressure is 1000 hPa, along the Saturated Adiabatic Lapse Rate.

Potential temperature is an extremely important concept, because it allows us to directly compare air masses regardless of their distance or pressure level, and thus allows u.s.a. to predict how air masses volition interact. To illustrate the concept, permit us re-examine the atmospheric condition for air stability, from the point of view of potential temperature. For simplicity, we will consider only the example of dry air.

Stability
A stable atmosphere is i in which potential temperature increases with altitude. That is, if the environmental lapse rate is such that potential temperature increases with altitude, then the atmosphere volition be stable. This is the aforementioned as proverb that the ELR is less than the ALR. Examples of this state of affairs are when the lower levels of the air are cooled by a cold ground surface, or if warm air is advected over absurd air. It is too the example in the stratosphere, where the air is heated from above by UV battery: this is why the stratosphere is then stable.

Instability
An unstable temper is one in which potential temperature decreases with distance. In this case, the lowest levels have the highest potential temperature: this upsets the hydrostatic equilibrium (Lecture ii), and the lower air will thus tend to rise. This is the situation in which air is heated from below by longwave emission from the ground surface. It is equivalent to the case where the ELR is greater than the ALR.

Neutrality
A situation nosotros have not yet considered is a neutral atmosphere: in this example, potential temperature is constant with distance. This is equivalent to saying that the ELR = ALR. This situation is quite mutual in windy, well mixed conditions in the lower troposphere. Air heated at the ground is rapidly mixed upwards by convection and turbulent winds, thus equalising potential temperature.

Representing Air Stability

One of the most versatile and useful means of representing air stability is based on plots of actual temperature against potential temperature for vertical transects through the atmosphere. Such temperature-potential temperature diagrams are known as T f diagrams or Tephigrams. (This is because potential temperature can exist regarded as equivalent to entropy, which is denoted by the Greek alphabetic character f (Phi)). On the basic Tephigram, temperature is plotted on the vertical axis, and potential temperature on the horizontal axis. Air pressure then plots as a serial of gently curving diagonal lines slanting upward from bottom right to top left. Because air pressure level decreases with altitude, it is useful to rotate such diagrams until the pressure level isolines are approximately horizontal, with the highest pressures at the lesser (1050 hPa) and the lowest at the peak (commonly 200 hPa) so that the diagram and then appears as a vertical slice through the atmosphere. The shape of temperature profiles and then shows at a glance whether the air is stable or not. Furthermore, these diagrams likewise allow the exact calculation of the behaviour of air masses.

Above: Tephigram showing lines of equal temperature (rising from left to right), potential temperature (ascension from right to left), pressure (sub-horizontal curved lines), and saturated idiabats (steep dashed curved lines). Also shown are temperature curves derived from soundings over Northern Ireland (red) and the sahara (blueish). The Irish curve closely follows a saturated adiabat through most of the atmosphere, characteristic of a well-mixed, cloudy temper. The precipitous change in management simply below 300 mbar is the tropopause: the abrupt modify in thermal characteristics of the atmosphere between the troposphere nad stratosphere. The Sahara line (blue) is parallel to a dry out adiabat (line of equal potential temperature: this is characteristic of a dry atmopshere well mixed by convection.

Important concepts to notation in connexion with Tephigrams:

• For dry air, rising or falling air changes temperature along the dry adiabatic lapse rate. This ways it will follow lines of equal potential temperature, which are marked as diagonal lines on the diagram. These are known every bit dry out adiabats.
• For saturated air, rising or falling masses will follow the saturated adiabatic lapse charge per unit. Examples of these cooling/warming curves are shown on Tephigrams as curved lines, kickoff most vertical at the bottom of the diagram, then gradually curving into parallel with the dry out adiabats. These are known as saturated adiabats.
• Lines representing the ecology lapse rate can thus be compared at a glance with the gradient of the dry out and saturated adiabats, thus providing a rapid impression of air stability at all levels of the atmosphere.
• The lifting condensation level: this is the altitude at which condensation volition occur for a given air mass raised adiabatically. It coincides very closely with the deject base of operations. Below that altitude, ascension or falling air will follow a dry adiabat, in a higher place it, a saturated adiabat.
• Dew point: this is a related concept to the lifting condensation level. It is the temperature at which condensation occurs (for constant force per unit area).

Consequences of Instability: Convection and formation of cumulus clouds.

Virtually of the heating of the atmosphere is achieved by longwave radiation from ground or water surfaces. This means that, in the troposphere, maximum energy receipts are commonly at the everyman levels. This volition raise temperatures (and potential temperatures) there, upsetting the hydrostatic balance and creating instability. This one fact accounts for a huge amount of atmospheric behaviour. It explains why vertical motions are and so prevailent in the troposphere: the atmosphere is constantly mixed to evacuate free energy from lower levels to the upper troposphere, where it can lose energy by longwave radiations into space. Thermally-driven vertical mortions are known as convection.

Convection is initiated by heating at a footing or water surface. The vertical dimension of convective cells is determined by the temperature profile of the atmosphere, and the moisture content of the air. The temperature profile (or, as we have seen, the vertical potential temperature gradient) is the ultimate determinant of stability. Limited convection tin can occur in a mostly stable temper if excess heating occurs most the ground. In this instance, energy can exist gently lofted upwards in dry out thermal cells. Such thermals tin be hundreds or thousands of metres high in some warmed dry atmospheres, providing ideal conditions for parapenting and other aeriform pursuits.

The virtually vigorous convective cells involve the formation of cumulus clouds. This is because cloud formation involves the release of latent heat which, every bit we have seen, provides an extra source of energy during condensation. Indeed, latent heat release provides the bulk of the energy involved in large cumulus systems.

Modest cumulus are the visible portion of small convective cells, which can form in the lower part of the troposphere due to heating from below. Small cumulus are preferentially developed over land, due to greater heating compared with water surfaces where latent heat is consumed during the evalopration of h2o. Cumulus have a cellular form, either with the deject in the centre of the cell (closed cells) or around cell boundaries (open cells). In the latter example, articulate air sinks in the middle of the cell, and rises between cells. The type of jail cell pattern relates to air properties and rates of energy exhange.

Open up convection cells seen from infinite

Deject streets are elongated convection cells which class when at that place is horizontal ship of a convecting air mass. Cloud streets are most mutual where a cool airstream blows over a warm surface; e.m. northerly winds in the n. hemisphere mid latitudes.

Cloud streets formed where cold air blows off pack ice over warmer h2o. Note the series of vortices formed downwind of January Mayen Island (Karman vortex train)

Consequences of Stability: Lee moving ridge clouds

We have seen that for stable air (where potential temperature increases with height), air that is forced to rise will return to its original altitude. One of the most beautiful consequences of this behaviour is lee waves or mount waves downwind of large obstacles to the menstruation. As air is blown against a mountain, information technology is forced to ascent. If it is stable, then on the lee side of the mount information technology will fall once more. All the same, its momentum is such that it will shoot past its original altitude and go lower than it was before. It then is forced to rise again by its disequilibrium with hydrostatic conditions. Information technology will again overshoot, this time going too high, and then on. If the resulting moving ridge intersects the condensation level of the air, clouds will form at the crests of the waves. These clouds, amongst the almost beautiful in nature, are commonly seen downwind of mountain ranges in stable, windy conditions. They resemble slap-up plates or lenses. Await out for them in the wide skies of St Andrews Bay: they are quite common. If the wet content of the air varies with distance, such lee wave clouds tin can grade vertical stacks of lenses, chosen piles des assiettes, or 'piles of plates'.

Solitary wave cloud, Aberdeenshire

Lee waves over Corsica from space
The basic mathematical description of these waves is very elegant. Consider beginning the vertical movements of the wave. The air oscillates up and down at a characteristic frequency, which is controlled by the temperature contour of the air. In item, it depends on the vertical gradient in potential temperature. If potential temperature increases up (stability), disturbed air will oscillate up and down at the Brunt – Vaisala Frequency N / iip, where

N = ((g/q) x (Dq/Dz))^1/2

yard is gravity
q is the potential temperature, and
Dq/Dz is the vertical slope of potential temperature;
the exponent ^1/2 means 'the square root'.

The units of N / iip is sec^-1, or cycles per second. The equation shows that the oscillation frequency is greatest for larger gradients in potential temperature. This clearly makes sense: the steeper the potential temperature gradient, the greater the disequilibrium experienced by displaced air, therefore it volition tend to rising or autumn with higher velocity.

Now consider the horizontal movement. The air moves horizontally with a wind speed Five. For a moving ridge, the wavelength l is equal to the velocity divided by the frequency:

l = V / f

Since the wave frequency is Due north / 2p,

l =  2p Five / N

This shows that the wavelength (spacing) of lee wave clouds will be greatest for loftier current of air speeds and slightly stable air (minor increase of potential temperature with altitude). Closely spaced waves will result from low wind speeds and loftier potential temperature gradients. Withal, the waves volition non propagate forever. Eventually, friction between the aquiver layer and the surrounding air volition damp the oscillation, and the lee wave train will eventually die out.

Fifty-fifty if you don't completely follow the maths, you have to acknowledge lee waves are pretty cool clouds.

Source: https://tallbloke.wordpress.com/2012/01/31/back-to-basics-2-lapse-rates-and-atmospheric-stability/

Posted by: lopezthapt1997.blogspot.com

Share this post