Atmospheric Physics

Passenger aircraft cabins are usually pressurised so that they are equivalent to the atmospheric air pressure at about 5,000ft. Calculate the percentage reduction in available oxygen at this pressure compared to sea level.

% reduction in oxygen = % reduction in pressure = 100.(p0 - p)/p0 = 100.(p0 - p0e-gz/RT )/p0 = 100(1- e-gz/RT ) = 100(1-0.84) = 16% using data below Z = 5000ft = 1524m (approximation eg 1500m accepted), R = 287 Jkg-1K-1, T = anything sensible eg 200C (293K) given aircraft cabins kept at a reasonable temperature, g= 9.81 ms-2

Explain, with reference to the lapse rate of the environment and the adiabatic lapse rates, what is meant by stability and instability

A rising parcel of air will cool on either the dry or saturated adiabatic lapse rate, depending on whether it is saturated or not. If, as the parcel rises and cools, it moves into air that is cooler than the parcel it will continue to rise because it is less dense than the surrounding air. This will happen if the environmental lapse rate is greater (more negative) than the adiabatic lapse rate. This situation is said to be unstable. If the rising parcel encounters air that is warmer than the parcel then it will be more dense than its surroundings and will stop rising / tend to sink. This happens when the environmental lapse rate is less than the adiabatic lapse rate, and this condition is known as stable - that is ascent/descent is suppressed. (Diagrams accepted as part of answer).

Venus is the brightest planet we see in the sky, having a planetary albedo of 0.76. It is also the most similar in size to Earth. Explain what is meant by planetary albedo, and the main factors that contribute to this albedo

Albedo is the surface reflectivity of solar radiation. It is the ratio of solar radiation reflected to incident solar radiation. Different surfaces have different reflectivities. The planetary albedo is the total time and space averaged reflectivity of the Earth and its atmosphere. Clouds are highly reflective (up to 90% for deep convective clouds), as is snow and ice. Other terrestrial surfaces have lower albedos, and water albedo (covering much of the surface) is 3-10% except at very large zenith angles

Explain what is meant by an adiabatic change, and write down the first law of thermodynamics for an adiabatic change, in terms of work done. For a parcel of air in the atmosphere explain the physical consequences of work being done on or by the parcel in this way. Hence explain what is meant by adiabatic lapse rate

An adiabatic change is one in which no heat is exchanged between the system and its surroundings. dQ = 0 Thus, dU = dW = -PdV [3] As a parcel of air moves up (or down) in the atmosphere it expands (dV positive) or contracts (dV negative) as pressure decreases (increases). If no heat enters or leaves the parcel then the work done by (on) the parcel has to be balanced by a change in internal energy. Internal energy is simply a function of T, therefore the T decreases (increases). [5] The adiabatic lapse rate is the change in temperature with height of a parcel of air undergoing an adiabatic ascent/descent

Describe the Coriolis force and write down the equation that defines it.

An imaginary force that accounts for the Earth's rotation in our fixed observation frame. It acts perpendicular to the motion, affecting wind direction but not speed, and acts to the R in the NH (L in SH). Fc = 2Ωv sinΦ where Ω is Earth's rotation rate, v is windspeed and Φ is latitude.

The greatest ozone depletion occurs in the Antarctic spring. Describe the conditions in the Antarctic winter that prime the stratosphere for ozone depletion by chlorine (originating from CFCs), and then describe what happens when the Antarctic spring arrives and the ozone hole develops and then dissipates.

Antarctic vortex traps air over the continent, isolating it from warmer air equatorwards. Temperature decreases during the polar night, becoming cold enough for PSCs to form, providing reaction surfaces that enable Cl2, originating from CFCs, to be released. When spring arrives and sunlight once more reaches the Antarctic stratosphere, the Cl2 is photdissociated into Cl atoms which act as very effective catalysts in destroying ozone, leading to massive ozone depletion within the vortex - the so-called ozone hole. [6] Sunlight also warms the atmosphere, driving atmospheric motion and breaking down the vortex so that ozone-rich air from outside the vortex mixes with the ozone depleted air and ozone values revert to near-normal. Pockets of ozone-poor air can drift over other regions of the SH during this time.

Choose one other standard temperature measurement. Describe what is measured and how, and explain the importance of your chosen measurement to one aspect of the economy.

Any suitable example e.g. concrete minimum - transport (road gritting) Soil temperatures - agriculture. Full description of technique Application

Why do you think it was possible to get such strong international agreement and compliance for the Montreal Protocol, but the Kyoto Protocol (on reducing greenhouse gases in general) has been less successful?

Any well reasoned argument e.g. ozone loss a direct threat to all life, while T increase is not so obviously catastrophic. Replacements available for CFCs and did not require major changes in lifestyle (in the developed world, arguably temporarily detrimental to development due to capture and replacement costs - as is now the argument for HFCs). Serious reductions in CO2 require major changes to fundamentals of modern life such as energy supply and transport, and hence lifestyles/costs.

Explain what is meant by the surface inter-tropical convergence zone (ITCZ), and the wind conditions you would expect to find there. Describe how, and why, the ITCZ changes position throughout the year. Explain why the ITCZ is not a simple zonal line (does not lie along a single line of latitude) but changes shape with time.

Area of low pressure where the tradewinds from each hemisphere meet. Little PG, low windspeeds, the doldrums.Also area of uplift so tropical rainfall, storms possible. The pressure patterns are controlled by differential surface heating, the main driver of which is the sun. The maximum heating is where the sun appears directly overhead. This is at the equator at the equinoxes and at the Tropics in the N/S hemisphere at the solstices. The ITCZ moves north and south of the equator following the sun. The ITCZ would be more zonal if the surface were uniform, but land and sea heat at different rates, given the same solar input, so the surface temperatures are not zonally consistent leading to waves in the surface T and P and the ITCZ. More pronounced in the NH, where there is more land mass, than in the SH.

Consider air rising up the side of a 3 km high mountain. The air is initially at a temperature of 9oC and a pressure of 1005 mb and then becomes saturated during its ascent, forming a cloud 900 m above the surface, and remains saturated. The air then descends 300 m on the leeward side of the mountain where it becomes sub-saturated. Calculate the temperature of the air once it reaches the surface on the leeward side, clearly stating any estimates you use in the calculation

Assuming the DALR = 9.8 o km-1 [1 Mark] and SALR is 6.8o km-1 (SALR ranged from 6-7 acceptable). [1 Mark] Temperature change is: ascent: 9-(0.9 x 9.8) - (2.1x6.8) = 9 - 8.82 - 14.28 = -14.1 [2 Marks] descent: -14.1 +(0.3*6.8)+(2.7*9.8) = -14.1+2.04+ 26.46 Air at surface is = 14.4oC

Explain why, at high levels close to the tropopause, the flow is always in a westerly direction

At altitude the pressure is always greater at a given height near the equator than it is at the Poles because of the differential heating, air T/densities.Planes of equal pressure slope upwards towards equator. PGF acts from equator towards pole and CF turns flow to right, moving from west to east ie westerly

Explain how surface pressure is measured, to enable synoptic charts to be marked as in part (b) above. Include a discussion of corrections which must be made to the basic measurement.

Barometers are used to measure pressure. They work with having mercury in a tube and use hydrostatic approximation, as mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing the mercury higher. Aneroid barometer is the other and uses deformation of unfixed membrane wall of a capsule magnified mechanically to move lever. Allows for temperature correction, but has wind errors

With respect to part c), explain the dual benefits of banning CFCs, and how one of those advantages has been lost

CFCs destroy ozone, and are also strong GHGs, contributing to global warming. The replacements for CFCs are less damaging to the ozone layer, but remain GHGs, sometimes even stronger than the CFCs they replaced.

Explain what is meant by convergence and divergence in the atmosphere

Convergence is air moving in / towards a point from all directions. Divergence is air moving out / away from a point

What typical values of lapse rate might you expect to observe in the lower troposphere?

DALR -9.8 K/km; SALR around 6-7 K/km

In mid-latitudes typical raindrop radii are ~ 0.5 - 1.0 mm, yet cloud droplets are only ~ 10μm in radius, or less for non-rain clouds. Briefly explain, with reference to growth and loss mechanisms, why water clouds generally only produce drizzle at our latitudes. What type of clouds are required for heavier rainfall?

Droplets can grow by Diffusion and Condensation, but this is not very efficient and the rate of growth reduces as droplet size increases (dR/dt α 1/R2 for constant mass change). Also, dm/dt likely to reduce as available water vapour used up. More efficient is Collision and Coalescence. Fall speed (terminal velocity) of drops is determined by size. Variously α R2, R, √R in different size regimes subject to laminar / turbulent flow. Regardless, large drops fall faster than small drops, and large drops will "collect" smaller, slower drops in the volume they fall through. This is a much more efficient process, even though not all drops coalesce - for larger falling drops some small droplets may be swept around the large drop by airflow, or even bounce off the large drop. As drops leave the cloud base they move into unsaturated air and will begin to shrink by evaporation. Cloud droplets evaporate almost immediately (distinct cloud base). For typical mid-latitude cloud depths for water clouds, the drops formed by C+C are not large enough to reach the ground as rain. They are reduced by evaporation to drizzle. To achieve rain we require the presence of sublimation nuclei and ice ie ice or more usually mixed phase clouds.

Given a windspeed of 10 ms-1, calculate the Coriolis force at a latitude of 45S over the Pacific Ocean, and a latitude of 45N over the city of Ottawa, Canada. Explain any differences in the force, and describe any further differences in influences on surface wind that you might expect at these two locations

Fc = 2Ωv sinΦ = 2 x 7.29 x 10-5 x 10sin45 = 1.031 x 10-3ms-2 (Force per unit mass) (Earth's rotation rate is given in table of constants on exam paper) Magnitude of force is the same, but it acts to the R over Canada and to the L over the Pacific. Near the surface friction will also affect windspeed, slowing the motion and therefore reducing the CF. This means the wind direction will turn more to the left (in the NH; R in the SH) with increasing friction. Friction over the city expected to be more than that over the ocean, so compared to the free atmosphere the wind over the city will have less velocity and be deflected more (cross the isobars) than that over the ocean.

Two common types of fog are radiation fog and advection fog. Explain the general conditions that lead to fog. For each of the two types explain how the fog forms, giving an example of the typical synoptic situation and weather conditions under which it might be experienced.

Fog forms when air at ground level reaches saturation, causing condensation and droplet formation (part b)). This can happen in a number of ways. [1] Radiation fog often occurs in winter when the skies are clear and conditions are calm and stable with light winds i.e. anticyclonic. The ground cools radiatively overnight, then cooling the air above it. If the air is cooled to dew point then condensation occurs and fog can form.The fog will often dissipate after sunrise as the ground is warmed, and then the air also warmed from below. Typically localized and short-lived, but can persist all day in winter when solar heating is diminished. [3] Advection fog occurs when moist air moves over a cold surface and is cooled from below to the point at which condensation occurs. It is common near coasts either when warm, moist air moves over a cold land surface, or over colder water (sea fog) and is then blown inland. It can also occur behind a warm front as it passes over cold land eg over a snow covered region in winter. Advection fog is more likely to be associated with a noticeable wind than radiation fog, it can also cover large areas and be advected significant distances, and occur at any time of day.

Derive a simple energy balance equation for Venus and thus calculate the planetary radiative temperature. Start with Stefan's law, and take the temperature of the Sun as 5780K. Venus diameter 12,200 km Avg distance from sun 108.2 million km

For planetary equilibrium, energy in (solar radiation) = energy out (terrestrial radiation) [1 mark] Assume sun and venus emit as black bodies. [1 mark] Total solar output = σTs4(Wm-2) x 4πRs = 3.85 x 1026W [2 marks] At Venus-sun distance (d) the sun is considered a point source and the incident radiation at this distance is So = Solar output / 4πd2 = 2617 Wm-2 [2 marks] The radiation is intercepted by the cross-sectional area of Venus, and since Venus rotates it is time and space averaged, so average incident radiation is So π Rv2/ 4πRv2 = 654 Wm-2. [1 mark] Albedo of Venus is 0.76, so absorbed solar energy = 0.24 x 654 = radiative emission of Venus (for equilibrium) = σTv4. [2 marks] Tv ~ 229K

Describe, in principle, a system that you might use to measure albedo, either of a surface or a planet.

For specific surface, two solarimeters, one facing up and one facing down, mounted above the surface (at sufficient height to avoid self shadowing). For planet - remote sensing i.e. satellite. Know e-t solar radiation and measure reflected solar radiation from view of earth - then average over space and time

Explain how freezing fog occurs, and describe the characteristic surface feature associated with freezing fog

Freezing fog occurs when the air is cooled below freezing and the droplets become super-cooled (but remain liquid). Droplets that are deposited on surface features freeze, to produce rime (feathery ice crystals) that build up on the windward side of vertical surfaces as the air blows past them and supercooled droplets impact on the cold surface

HFCs do not cause ozone depletion, however they contribute to global warming as strong greenhouse gases. Explain how the greenhouse effect works to warm the Earth's surface. Name three other major greenhouse gases.

GHGs (mainly in the troposphere) absorb longwave (IR) radiation emitted by the earth's surface. This warms the gases and they then emit (more) longwave radiation in all directions, including downwards towards the earth's surface. They thus increase the radiation received and absorbed by the surface, increasing warming of the surface and the air immediately in contact with it. [5] Carbon dioxide, water vapour, methane, or any other correct answer

Explain how global warming (increased temperatures near the surface) would affect the temperature of the stratosphere. Hence explain the potential implications for the ozone layer

GHGs, concentrated in the troposphere, absorb long wave radiation (emitted most strongly at the surface) and re-radiate it in all directions, some back to the surface, where it is again absorbed, increasing surface temperatures. If GHGs increase they absorb more of the outgoing longwave radiation, returning it to the surface and causing "global warming". However, the longwave radiation reaching higher levels in the atmosphere eg the stratosphere where it is absorbed by ozone, is reduced. This leads to a cooling of the stratosphere. [6] Cooling the stratosphere, especially in the polar regions where it is already coldest, would lead to increased occurrence of PSCs that greatly enhance the abundance of ozone depleting Cl, Br etc. through reactions that occur on the PSCs. This would lead to additional ozone depletion, and further cooling of the stratosphere. The greatest effect is likely to be in the Arctic where PSCs are currently less frequent than in the Antarctic.

Explain what is meant by homogenous and heterogenous nucleation with respect to the formation of cloud droplets, and why homogenous nucleation is generally not important in the atmosphere

Homogenous nucleation is the formation of pure water drops through the condensation of pure water vapour. For a drop to form (and then grow) condensation must exceed evaporation. However, surface tension causes the liquid to try and minimize its area, giving a curved surface of small radius, which binds individual water molecules less tightly than for a planar surface. This means that the molecules evaporate more easily, so to achieve drop formation and growth the supersaturation must be greater than that over a planar surface in the same conditions. The larger the drop radius, the lower the required supersaturation, but by default homogenous nucleation must begin with very small droplets. Heterogenous nucleation is the formation of droplets through condensation of water vapour onto a nucleus of some other material (eg aerosol particles). A wettable particle allows the water to spread across its surface, and then appears like a large drop of pure water, with a relatively low equilibrium supersaturation, thus favouring growth through condensation. Hygroscopic particles are even more effective if they absorb and dissolve in the vapour. The supersaturation of the resulting drop of solution is less than that of pure water, favouring growth. The supersaturations required to grow cloud droplets through homogenous nucleation are not generally observed in the atmosphere. Heterogenous nucleation, with existing CCN that already have a reasonable size and require lower supersaturations, are for more efficient and droplets can grow in atmospheric supersaturations. Thus homogenous nucleation is not of great importance

Describe and explain the wind direction at the base of each Hadley cell

In NH, air flows from sub-tropical high towards the equator, and is turned to the right by the coriolis force to give north easterlies. In the SH air also flowing towards the equator is turned to the left by the CF to give south easterlies. Near the equator/ITCZ pressure gradients are slack and winds light - "the doldrums".

With the aid of a sketch show how the pressure gradient is represented on synoptic charts, and illustrate a region with a strong pressure gradient and a region with a weak pressure gradient.

Isobars are lines of equal pressure and a strong PGF is seen as straight lines close together and weak PG is circles spaced in one another

Explain what is meant by latent heat.

Latent heat is the energy involved (required or released) when a substance changes state (at constant temperature) e.g. liquid to gas, latent heat of evaporation (J/kg)

When surface pressure is measured it is corrected to a reference level. What is the reference level and why is the correction made?

Mean sea level. If the correction were not made then the surface pressure patterns would to a large extent represent the topography of the surface, not the atmospheric dynamics.

explain how you can use a barometer to measure height above a given surface. Used in this way the instrument is an altimeter. If you were flying an aircraft, which 3 reference surface levels might you be interested in, and why?

Measure a value for p0, then measure p at the unknown level, and knowing H will give z. Pressure settings for flight: pressure at airfield/runway (QFE), useful for landing; regional sea level pressure (QNH) used locally for all aircraft under a given air traffic control; standard atmospheric pressure (1013 hPa) used for flight levels

A surface low pressure centre results in convergence at the surface. Explain this fully in terms of the forces acting on a parcel of air close to the surface. You may use diagrams to illustrate your answer

Need to consider pressure gradient force, Coriolis force and drag/friction. PGF acts from H to L pressure i.e. in towards the centre of the LP. [2] Once the air starts to move under the PGF, the Coriolis force will act to the right of motion, and proportional to windspeed, turning air to the right (in NH). [2] Away from the surface the two forces reach a steady state for angular motion (anticlockwise), with air flowing parallel to the curved isobars and PGF>CF. [2] Close to the surface friction acts to slow the motion and hence reduce the CF, but PGF remains constant in magnitude and always acts inwards towards the central LP. [2] The air motion therefore turns to the left (balancing the loss of force to the right from CF) and surface flow crosses the isobars at an angle determined by the surface features and the drag that they produce. The surface flow is then an inward, anticlockwise spiral.

In fact, the temperature on the surface of Venus is over 700K, and the atmosphere of Venus is composed primarily of carbon dioxide. Explain the radiative transfer processes that produce such an extreme surface temperature, and illustrate with the aid of a diagram how this changes the simple calculation in part b

On the LHS is the situation in the calculation of (a). Some solar energy is reflected as a result of the planetary albedo, and there is a balance between absorbed solar energy and emitted terrestrial energy. With an atmosphere in place, solar energy reaching the surface is little changed (the albedo of the atmosphere was included in the initial calculation), but most LW radiation emitted by the surface is absorbed by gases in the atmosphere causing warming. The atmosphere also emits according to Stefan's law (in all directions ie equally up and down), increasing the net downward radiation which further warms the planet's surface, and also emitting to the layer above. This process is repeated through successive layers of the atmosphere. This is the so-called greenhouse effect, to which we owe a warmer surface temperature than the basic calculation suggests. Carbon dioxide is a well known GHG, so this process dominates the atmosphere of Venus and essentially traps most of the energy within the atmosphere, leading to such high temperatures at the surface

Explain how ozone influences the temperature structure of the stratosphere

Ozone absorbs UV radiation, both in the production and destruction phase of the ozone cycle. This leads to warming in the regions where the ozone exists and offsets the cooling observed with decreasing height/pressure in the troposphere

Explain why it is important to protect the ozone layer

Ozone absorbs high energy e-m radiation from the sun (eg UVB and UVC wavelengths). These are damaging to life on earth eg damage DNA. Essentially, ozone allows life to exist otherwise unprotected on the surface of the earth.

On a synoptic chart places of equal surface pressure are connected by isobars. Explain how the isobars determine the pressure gradient force. Describe the weather you would expect i) when the isobars are very close together ii) in the middle of a large region enclosed by an isobar that is labeled with the highest pressure in the vicinity

PGF = (1/ρ)dp/dx where dp/dx is the change in pressure over a distance dx. The isobar spacing is therefore proportional to PGF. i) strong winds / gales ii) large region of (uniform) HP means little pressure gradient, low winds often clear and sunny weather (stable, descending air).

Explain why the trade winds that blow either side of the equator are easterlies, and why there is an area of typically weak winds known as the Doldrums in sub-tropical regions.

PGF at surface directs air from H to L pressure, that is towards the equator (strictly ITCZ). Coriolos force acts to the R in the NH, and to the L in SH - in both hemispheres this turns the equatorwards motion towards the west (ie the wind is an easterly). Note Coriolis force is small at low latitudes and zero at the equator. In subtropical regions, at the edge of the Hadley cell, air is descending (slowly) giving HP and divergence at the surface. Stable conditions, suppresses convection, air moving out of HP system, from a calm centre. Gives low windspeeds.

Write down an expression for the horizontal pressure gradient force and explain its significance for weather forecasting?

Pressure Gradient force, Fpg = Δp / ρax Fpg is pgf per unit mass, Δp is pressure difference between two places a distance x apart, ρa is air density. [2] Pressure gradient force, a horizontal force, acts on the air causing motion which we experience as wind. Pressure differences, caused by unequal heating at the Earth's surface, thus govern the movement of air both horizontally, and due to convergence and divergence, vertically. A strong PGF means strong winds blowing from H to L pressure, while a weak PGF implies low winds. More complete isobaric patterns allow cyclones and anticyclones (and associated weather) to be identified on synoptic charts.

Explain, in simple terms, what is meant by atmospheric pressure

Pressure exerted by the weight of all the air in a column above the point of interest.

Explain how rainfall radar works. Describe the provision of rainfall data by this method in the UK, and its use in forecasting

Radar works by sending out pulses of EM radiation and measuring the time (giving distance) and direction (dish rotates and scans up and down) from which return signals are received from "targets". The targets are precipitation sized drops/A wavelength of 5.6cm is used./15 radar in UK and Ireland./Range 75km/Resolution of composite picture is 5km, and 15 minutes/Captures rain, hail, snow. Drizzle too small./Provides snapshot picture. Identifies increasing or decreasing intensity as systems move./Time series of snapshots can be used for nowcasting and short-term forecasting inc. potential flood warnings.

Describe a Stevenson screen and its purpose. Then describe each thermometer, the temperature it measures, and explain how this is achieved. You may use diagrams as part of your answer.

Stevenson screen - wooden box with louvred sides. Painted white (reflects solar radiation and reduces internal heating) and mounted (traditionally) over a grass surface at 1.25m. Keeps direct sun off thermometers (door to N) while allowing air to flow through so true air temperature is measured. [4] Simple (dry bulb) air temperature. Vertically mounted MiG thermometer (or other liquid appropriate to local climate), temperature read by observer at specified time(s). [2] Wet bulb thermometer, measures Tw, the temperature the air would have if cooled to saturation by evaporation. As for dry bulb, but the bulb of the thermometer is covered by a muslin tube, the other end of which is in a reservoir of distilled water so that the muslin is always wet. Continuous evaporation of the water reduces the air temperature round the bulb, keeping it saturated. [3] Daily maximum - Thermometer is horizontal and has a restriction in its bore that keeps the Hg in its maximum position (or has index ahead of meniscus that remains when Hg constricts back down the tube). [2] Daily minimum - Alcohol in glass, with a small index inside the liquid. This is pulled towards lower temperatures by surface tension as liquid constricts, but remains in place when liquid flows the other way as T increases. [2] Both max and min taken at 0900 and have to be reset each day.

Explain how a night-time inversion forms, explain the associated weather that you might expect around dawn, and sketch the vertical temperature profile at dawn

Strong radiative cooling of the surface (eg on a clear night) cools the air from below (from the surface upwards). A lack of mixing results in a stratified vertical profile with cold air close to the surface, and warmer air above, giving an inversion. The inversion tends to isolate the surface from the rest of the boundary layer, giving calm conditions on the surface. Depending on time of year (T) expect dew or frost and mist or fog with an otherwise clear sky. Solar heating will warm the surface / dissipate mist, create mixing and convection which breaks down inversion

An aircraft is flying from Manchester to Edmonton, Canada (both at approximately 53.50N). It is cruising at 850 km/h IAS (that is speed through the air), when it enters the jet stream. Use your knowledge of the jet stream to estimate the aircraft's speed over the ground when in the centre of the jet stream. State all assumptions you make about the motion of air in the jet stream

The aircraft is flying east to west at 850 km/h airspeed. [1] The jet stream is a predominantly westerly flow (i.e. from the west, in opposition to the aircraft motion), with speeds of up to 400 km/h. Assume westerly wind at 250km/h (or any other sensible scenario). [2] Ground speed = airspeed + windspeed (actually velocity, account for direction) = 850-250 = 600 km/h.

Explain what is meant by adiabatic lapse rate

The change in temperature with height during an adiabatic ascent/descent i.e. one where there is no exchange of heat with the environment (dQ=0).

Explain, in principle, how wet and dry bulb temperatures are used to determine relative humidity

The difference between the wet bulb and dry bulb temperatures gives a measure of RH. If the air is saturated (has reached 100% RH) then no further evaporation is possible from the wet bulb thermometer and wet and dry bulb temperatures are equal. The greater the difference between the two temperatures, the lower the RH, though the relationship depends on pressure and temperature. Use a psychometric chart, or tables, to find value of RH

Describe and explain the differences between the dry adiabatic lapse rate and the saturated adiabatic lapse rate.

The dry adiabatic lapse rate is a constant in the troposphere (~ 9.8 K km-1) where reducing pressure with height causes expansion (work) and this is balanced by loss of internal energy (hence T reduction). When air reaches saturation any further expansion and cooling results in condensation of water vapour. This releases latent heat energy which warms the air, offsetting the cooling. The air therefore cools less rapidly than dry air. The SALR varies with altitude (and T) because the amount of latent heat released depends on the water vapour content of the air. At higher altitudes the wv content is lower and the SALR eventually tends towards the DALR

Would you expect to experience the thickest fog in a polluted region, or in a region of clean air? Fully explain your answer in terms of droplet formation processes

Thickest fog in polluted regions. [1] Homogenous nucleation is the formation of pure water drops through the condensation of pure water vapour (in completely clean air). For a drop to form (and then grow) condensation must exceed evaporation. However, surface tension causes the liquid to try and minimize its area, giving a curved surface of small radius, which binds individual water molecules less tightly than for a planar surface. This means that the molecules evaporate more easily, so to achieve drop formation and growth the supersaturation must be greater than generally occurs in the atmosphere. Not effective. Heterogenous nucleation is the formation of droplets through condensation of water vapour onto a nucleus of some other material (eg aerosol particles). A wettable particle allows the water to spread across its surface, and then appears like a large drop of pure water, with a relatively low equilibrium supersaturation, thus favouring growth through condensation. Hygroscopic particles are even more effective if they absorb and dissolve in the vapour. The supersaturation of the resulting drop of solution is less than that of pure water, favouring growth. [6] Polluted regions, by their nature, have many more CN in the air and therefore allow the formation of many more droplets and hence denser fogs form e.g. "pea soupers" of London in 20th century

Fog is the result of tiny water droplets suspended in air. Give the official definition of fog in terms of visibility. Comment on its applicability for transport.

Visibility < 1000m. [1] This is appropriate for aviation and the speeds at which aircraft approach a runway. For driving, visibility of 1000m is not a great hazard and traffic flow is more likely to be influenced at shorter visibility distances eg in thick (200m) or dense (50m) fog.

With reference to parts b) and c), explain the difference between the dry adiabatic lapse rate and the saturated adiabatic lapse rate. Sketch both lapse rates on a diagram and explain the limit of the saturated adiabatic lapse rate as height increases.

When air is dry (that is unsaturated), it rises, expands and cools according to the first law of thermodynamics. With no heat exchange, the DALR is a constant (at -dT/dz is 9.8 K/km - see part e). As the air cools (retaining its water vapour content) it will eventually become saturated. At this point any further rising and cooling will lead to condensation. Latent heat is released by the parcel and this serves to warm it (or consider it as offsetting some of the cooling caused by ascent). As the heat is supplied by the parcel itself the process is still considered adiabatic Plot T on x axis, z on y axis. DALR is a straight line with negative gradient. SALR is curved and tends towards the DALR at altitude as the water vapour available for condensation decreases as T decreases and wv has already condensed out - therefore less LH is released and the associated warming effect is reduced.

Explain the effect of convergence high in the troposphere on the surface pressure and surface flow. What kind of weather would you expect in this situation?

When there is convergence high in the atmosphere it increases the air in the column above that surface region, creating surface high pressure. The air moves down away from the region of convergence, giving descending air (warms and dries) and then divergence at the surface. Dry, clear, sunny weather

Explain the effect of divergence high in the troposphere on surface pressure and surface flow. Now what kind of weather might you expect? If divergence at altitude increases how will the weather be affected? If divergence at altitude decreases how will the weather be affected?

When there is divergence high in the troposphere are is moving out of the top of the column, reducing the air mass in the column and so leading to surface low pressure. Air rises to replace that lost air and thus air is drawn into the low pressure surface region: surface convergence. Rising air, expect cloud formation, likely with precipitation If divergence increases the low pressure will get lower (deepen), increasing surface winds/convergence/ascent - weather will get worse, eg strong winds and heavy rain. If divergence decreases the low pressure will "fill" ie become less low. Winds at the surface and ascent will weaken, precipitation will slow/cease.

Write down the first law of thermodynamics

dU = dQ +dW (changes in internal energy, heat supplied to/from system, work done on/by system)

The vertical pressure gradient force is balanced by gravity. Write down the hydrostatic equation and then use the Ideal Gas Law to define the exponential scale height

dp/dz = - ρg Use p= ρRT, gives dp/p = -gdz/RT Integrate to get p = p0e-gz/RT =p0e-z/H where H is exponential scale height

Briefly describe and explain the UK weather you would expect to be associated with the following pressure systems i) A large high pressure system centred over the Midlands in summer ii) A deep low pressure system centred over NW Scotland in winter

i) Large HP over UK in summer will bring stable weather with low winds. Descending air with give dry conditions and probably clear skies (therefore sunny and warm during the day, cooling at night leading to dew), although the visibility will decline with time as pollutants get trapped. Possibility of thunderstorms in pm. ii) Deep low over NW Scotland in winter. Strong winds, certainly over Scotland and probably the rest of the UK. Frontal systems associated with the low will bring cloud and rain across the country. Weather for any particular region will depend on where it is in relation to the fronts: Ahead of warm front (cloud, drizzle, more persistent rain), warm sector (increase in temperature, decrease in cloud cover and rain) or behind the cold front (temp. drops, heavy rain showers and squalls, then clearing).

At upper levels, divergence must occur. Explain what happens to the surface low pressure system, and its associated weather, if i) Divergence aloft exceeds surface convergence ii) Surface convergence exceeds divergence aloft

i)D>C. Air leaving the column above the surface low is greater than air entering the column. Surface P decreases further i.e. the low deepens, PGF increases, windspeeds and uplift increase, providing energy to the system through LH release, and generally expect more severe weather and storms. ii)) C>D. Air entering the column above the surface low exceeds air leaving the column at altitude. The surface pressure increases, the low "fills", decreasing the PGF, reducing windspeeds and uplift, the severity of the weather decreases and the system eventually dies out

Hypoxia is a condition where the body is deprived of a sufficient supply of oxygen. The table below shows how a person's time of useful consciousness (time s/he is able to do anything useful, like fly an aircraft) decreases with altitude if s/he is only able to breathe air at the allocated pressure level eg in an open cockpit without a supply of oxygen. a) Start with the hydrostatic equation and then use the Ideal Gas Law and Dalton's law of partial pressures to explain why the time of useful consciousness decreases so rapidly above about 15,000ft.

then p = ρRT dp/dz = -pg/RT, dp/p =( -g/RT)dz Integrate from surface (p0, z=0) to get pressure profile and In(p/p0) = -gz/RT P = p0e-gz/RT ie pressure decreases exponentially with height. [6 marks] Dalton's law says that the total pressure of a gas is the sum of the partial pressures. The proportion of oxygen in the air stays the same with height but the oxygen pressure reduces as the air pressure reduces. Therefore gain less oxygen with every lung full of air as ascent occurs. In the lower part of the atmosphere there is still sufficient oxygen for full body function. Above about 10,000ft the effects of reducing oxygen begin to become apparent. As altitude increases the TUC reduces with the oxygen partial pressure ie ~ exponentially.