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S216 Environmental Science > Unit 2.1 Air > Flashcards

Unit 2.1 Air Flashcards

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0
Q

List three trace constituents of the atmosphere and explain their importance.

A
  • Water vapour in the atmosphere is a stage in the water cycle. It also absorbs infrared radiation, contributing to the greenhouse effect.
  • Carbon dioxide is used in photosynthesis, but also absorbs infrared.
  • Ozone scatters harmful ultraviolet radiation.
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1
Q

Describe the two main constituents of air.

A
  • Nitrogen (N2) makes up 78.1% of the atmosphere. It consists of two nitrogen atoms held together with a very strong triple bond, and so it is very stable.
  • Oxygen (O2) makes up 20% of the atmosphere.
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2
Q

Define and use the concept of mixing ratio.

A
  • Mixing ratio is the ratio of the number of particles of a given gas to the total number of particles, within a set volume.
  • e.g. Mixing ratio of nitrogen in atmosphere is 0.781
  • e.g. Mixing ratio of carbon dioxide in atmosphere is 365ppmv
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3
Q

Name at least three electromagnetic wavelength ranges.

A
  • Visible light
  • Ultraviolet
  • Infrared
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4
Q

State the perfect gas law.

A

p = (n / v) kT

Pressure (Pa) = number density x Boltzmann constant (1.38 x 10^-23 J K^-1) x temperature (K)

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5
Q

Explain the perfect gas law in terms of how it relates to the concepts of pressure, density and temperature.

A

p = (n / v) kT

The pressure of a volume of gas is the amount that it ‘pushes’ it’s surroundings. This ‘push’ increases when there are more molecules of gas within the volume (number density) or when they are moving faster (temperature).

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6
Q

State the definition of force.

A

“a force measures the amount by which an object is pushed or pulled”

F (N) = m (kg) x a (m s^-2)

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7
Q

Define pressure in terms of force.

A

“pressure is defined as the force per unit area”

P (Pa) = F (N) / A (m^2)

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8
Q

State Newton’s three laws.

A
  1. If no force is applied, then the speed and direction of an object’s motion does not change.
  2. Force = mass x acceleration
  3. For every action (force) there is an equal and opposite reaction (opposing force).
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11
Q

Define ‘adiabatic’

A

“without heat exchange”

Air which is cooling adiabatically is not exchanging heat with surrounding air, but is expanding.

Conversely, adiabatic warming occurs through contraction.

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12
Q

How does temperature change with height, within the lower atmosphere?

A

Within the troposphere, temperature decreases with height at a rate of 6 degrees C per km. At the tropopause (10km), temperature becomes stable until a height of 20km, before increasing through the remainder of the stratosphere.

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13
Q

How does pressure change with height, within the lower atmosphere?

A

Pressure decreases exponentially with height, halving every 5500m.

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14
Q

Explain why pressure falls of steadily with height.

A

In a column of air, pressure at any point is caused by the weight of the air above. At a lower point, there is more air above, and therefore higher pressure; conversely air pressure is lower at a higher point, as the weight of the air above is less.

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15
Q

Calculate the mass of air above a given height from the pressure.

A

The upwards pressure is equal to the weight of the air above (mg).

Upwards force (N) = pressure (Pa) x area (m^2) - weight (m x g)

Mass = (pressure x area) / gravitational acceleration

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16
Q

Explain how temperature is measured.

A

Drybulb thermometer: expansion of mercury (or alcohol) within a narrow tube. Indices for maximum and minimum temperature.

Thermograph: expansion and contraction of a bimetallic strip; produces a thermogram.

9.00 UTC in UK.

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17
Q

Explain how baromic pressure is measured.

A

Mercury barometer: sealed column of mercury immersed in an open vessel of mercury. As pressure increases, mercury in the open vessel is pushed down and up the column.

Aneroid barometer: expansion and contraction of an aneroid cell.

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18
Q

Explain how wind speed and direction are measured.

A

Speed: anenometer.
Estimated at sea using Beaufort Scale (1 - 12)

Direction: vane.
000 is calm, 360 is north. Veering is change clockwise, backing is change anticlockwise.

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19
Q

Explain how humidity is measured.

A

Specific humidity: mass mixing ratio, g kg^-1. Not dependent on temperature or pressure.

Dewpoint: cooling a sample of air until water begins to condense.

Relative humidity: ratio of actual humidity mass mixing ratio to saturation value of mass mixing ratio. Affected by temperature. hair hygrometer.

Wetbulb thermometer.

Vapour pressure (p44).

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20
Q

Explain vapour pressure.

A

A measurement of humidity.

Within a unit of air, the maximum (saturated) pressure of the water vapour within a unit of air minus the actual pressure of the water vapour.

Saturation deficit = ps - pv

Relative humidity = pv / ps

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22
Q

Explain how clouds are measured.

A

Low (to 2000m), middle (2000-5000m), high (5000m+)

Ten basic genera.

Amount: oktas (eights). Sunshine hours.

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23
Q

Explain how precipitation is measured.

A

Gauge, with five inch diameter at 12 inches above the ground.

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24
Q

Explain how visibility is measured.

A

Synoptic code, 00 to 99.

00 to 50: poor to moderate visibility in steps of 100m (up to 5km).

51 to 75: steps of 1km (from 5km to 30km).

75 to 99: steps of 5km.

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25
Q

Explain how ‘present weather’ is measured

A

Code 00 to 99. Lower numbers represent less innocuous weather.

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26
Q

Explain why meteorologists analyse basic pressure features on weather maps, and why the pattern of such features is related to wind speed.

A

Synoptic charts show areas of relative high and low pressure. To maintain equilibrium, air moves from areas of high pressure to low.

These charts can predict wind strength and speed. Wind blows parallel to isobars, and its strength is greatest across a large pressure gradient.

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27
Q

Describe a seeder-feeder mechanism.

A

Caused by the interaction of two layers:

  • high (2km) precipitating stratiform ‘seeder’ cloud
  • fast-flowing moist low level wind

As the low moist air rises to cross mountains, it produces a water rich ‘feeder’ cloud. Rain from the seeder cloud washes rain through the feeder cloud, and replenishes it.

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28
Q

Explain why it is necessary to adjust a barometer reading to mean sea-level.

A

Pressure is influenced by altitude, as well as the weather conditions which we want to study. Therefore, we must remove the influence of altitude by correcting our measurement to its equivalent at sea-level. This allows measurements taken at different altitudes to be compared.

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29
Q

Identify, with the aid of a chart, the nine basic cloud types which are the foundation of cloud observation.

A
  • stratocumulus (Sc)
  • stratus (St)
  • cumulus (Cu)
  • cumulonimbus (Cb)
  • altostratus (As)
  • altocumulus (Ac)
  • nimbostratus (Ns)
  • cirrus (Ci)
  • cirrostratus (Cs)
  • cirrocumulus (Cc)
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30
Q

Calculate the mass of liquid in a cloud, assuming a simple shape for it

A

Volume of hemisphere: 2/3 x pi x r^3
Volume of droplet: 4/3 x pi x r^3

In 1m^3 the volume of water is:
Number of droplets per 1m^3 x Droplet volume

Total volume of water is:
Volume in 1m^3 x total cloud volume.

Density of water: 1000kg per m^3

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32
Q

Explain why lapse rates mean that ground frosts are more common than air frosts.

A

Frosts occur at a drybulb temperature of 0.

Cooling at night is caused by radiation by the ground.

It takes time for this cooling to move up.

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33
Q

Describe how a polar-orbiting satellite covers the Earth.

A

The satellite orbits along a north-south direction.

However, the Earth below is orbiting east-west.

34
Q

Explain the nature of both visible and thermal infrared satellite images.

A

Visible images detect the light reflected back into space: the albedo. This is useful for mapping clouds, which have very high albedo.

Thermal infrared images measure the amount of radiation from the surface, and therefore the surface temperature.

35
Q

Explain briefly how radars sense precipitation and air motion and how radar mapping is useful.

A

Radars emit radio-waves, at such a wavelength that they are reflected back by precipitation-sized droplets.

Doppler radars can measure small shifts in the wavelength of reflected pulses, to detect the velocity (direction and speed) of the air.

In the UK radars provide unified picture of national rainfall pattern every 15 minutes.

36
Q

List the challenges of making, collecting and transmitting observations on the global scale.

A

Poorer coverage in poorer countries, especially in southern hemisphere.

A movement towards automation means that those variables which must be measured by eye are no longer collected.

37
Q

Give the formula for kinetic energy.

A

Ek = 1/2 x m x v^2

Kinetic energy = half x mass x velocity squared

38
Q

Give the formula for gravitational potential energy.

A

Ep = mgh

Potential energy = mass x gravitational acceleration x height

Gravitational acceleration: 9.81 m s^-2

39
Q

What are the respective temperatures and peak wavelengths of the Earth and the Sun?

A

Earth: 290 K, infrared

Sun: 5800 K, visible light

40
Q

Explain the distinction between absorption and scattering.

A

Absorption: energy is trapped within the bonds of the receiving molecule.

Scattering: the energy is not absorbed, but its velocity is altered by the receiving molecule.

41
Q

State Wein’s Law.

A

An object of temperature T (units of Kelvin, K) has its spectrum peaked at wavelength l (lambda; units of metres, m) given by:

l x T= 2.9 x 10^-3 m K

42
Q

Give the formula for thermal energy.

A

kT

Temperature (K) x Boltzmann constant

Boltzmann constant: 1.38 x 10^-23

43
Q

State the Stefan-Boltzmann Law.

A

The rate of energy emitted, R, by an object of surface area A, at temperature T is:

R = A x s x T^4

where s (sigma) is 5.67 x 10^-8 W m^-2 K^-4

44
Q

Explain the greenhouse effect in relation to why the Earth temperature thus calculated is far less than the surface temperature.

A

The Earth receives energy in the visible and ultraviolet spectra, and emits infrared radiation. The atmosphere allows visible light to reach the surface, but absorbs infrared, preventing it from escaping into space. As a consequence, energy is trapped in the atmosphere, raising the temperature.

45
Q

Explain the term steady state.

A

A steady state is one of overall balance between input and output.

For example, where incoming solar radiation is balanced by the emission of terrestrial radiation into space.

46
Q

Explain the terms positive and negative feedback.*

A

Positive feedback: a change in one variable, which causes change in another, in such a way as to push away from a steady state. E.g. runaway climate change.

Negative feedback: a change in either input or output, which alters the other, leading to a new steady state.

47
Q

Define the terms stable and unstable.

A

In stable conditions, the lapse rate of a packet of dry air (DALR) or moist air (SALR) is greater than that of the air around it (ELR), and so at some point the two will reach the same temperature, at which point movement ceases.

In unstable conditions, the lapse rate of the packet of air is less than that of the dry air around it. As a consequence, the temperature difference increases and the air packet accelerates upwards.

48
Q

Give the appropriate lapse rates for unstable and stable atmospheric conditions.

A

The packet of dry air cools at the DALR (or moist at SALR) while surrounding air cools at the ELR.

Stable conditions occur when the ELR is less than the DALR (9.8 degrees C per km) or SALR (variable with temperature).

Unstable conditions occur when the ELR is greater than the DALR or SALR. The ELR is said to be a superadiabatic lapse rate.

49
Q

Explain how heating of air in the atmosphere can lead to the storage of gravitational potential energy.

A

Perfect gas law: p = nKt / v

In a hypothetical column of air, the number of molecules above a single point does not change, and so pressure does not change. Any increase in temperature must therefore be balanced by an increase in volume: all of the molecules in the column move upwards, so gaining gravitational potential energy.

50
Q

Explain how the heating of air in the atmosphere can lead to convection.

A

When a parcel of air is heated, it expands, becoming less dense than the air around it. Being less dense, it ascends.

51
Q

Explain the typical temperature structure of the atmosphere in terms of the heating processes involved.

A

There are two sources of heat: the Sun and the surface of the Earth.

The Sun heats the stratosphere, as UV is absorbed by ozone. Therefore, temperature rises with altitude.

The troposphere is heated by convection of heat from the surface. Temperature falls with altitude. Convection can only occur below the tropopause, which acts as a thermal inversion layer.

52
Q

Explain seasonal variations, in terms of the Earth’s rotation and orbit around the Sun.

A

In its rotation of the Sun, the Earth is tilted at 23.5 degrees from its axis.

Each hemisphere is tilted towards the Sun for half of the year, during which it receives more solar radiation, and experiences summer.

53
Q

Explain latitudinal variations in terms of the Earth’s rotation and orbit around the Sun.

A

The angle between incoming solar radiation and the surface of the Earth: the angle of incidence.

In polar regions, the Sun lies much lower above the horizon. As a consequence, incoming solar radiation is spread over a much greater surface area, with each location receiving less energy.

54
Q

Describe how annual incoming solar radiation and outgoing terrestrial radiation vary with latitude.

A

Lower latitudes see the highest levels outgoing terrestrial radiation, but even higher levels of absorbed solar radiation: an energy surplus.

Higher latitudes see less outgoing terrestrial radiation, but even lower absorbed solar radiation: an energy deficit.

55
Q

Describe a cold anticyclone.*

A

Continental anticyclones are fed by intense cooling of the surface during winter. Feed cold dry air.

Siberian high.

56
Q

Describe how seasonal incoming solar radiation and outgoing terrestrial radiation vary with latitude.

A

The balance of absorbed and outgoing radiation remains broadly the same in lower latitudes.

Seasonal variations are much greater at higher latitudes: the amount of radiation relieved is far greater in summer, while outgoing radiation is especially high during long winter nights. E.g. The Siberian High.

57
Q

Describe a warm anticyclone.*

A

Subtropical anticyclones are semi-permanent features (always present, but they shift seasonally), in which air sinks gently from upper to lower depths, causing adiabatic compression, and so heating. Oceanic. Feed warm moist air.

Azores high.

58
Q

Explain the link between the regions of radiative excess and deficit and the associated poleward heat transport carried out by the atmosphere and oceans.*

A

The wind carries warmer air from areas of surplus to those of deficit (i.e. higher latitudes) through three cells. In Hadley Cells, warm moist air rises at the ITCZ and is carried to higher latitudes, at which point it sinks and returns to the equator as trade winds. Beyond Hadley Cells are mid-latitude Ferrell cells, and beyond this are Polar Cells.

Oceans also transport heat. Surface currents are driven by winds (gyres). The Gulf Stream transports warm water to Western Europe from the Gulf of Mexico.

59
Q

Explain the differences between cold and warm anticyclones.

A

Warm:

  • oceanic
  • feeds warm moist air
  • semi-persistent

Cold:

  • continental
  • feeds cold dry air
  • seasonal (winter)
60
Q

State typical rates of change of drybulb temperature upwards in atmosphere.

A

Environmental lapse rate: 6 degrees per km of altitude.

62
Q

Explain how cold and warm anticyclones act as source regions for airmasses.

A

In both warm and cold anticyclones, air sinks from upper to lower levels. Pushed by the stream of sinking air above, air at the lowest level spirals outwards as surface winds.

In a cold anticyclone, the sinking motion is caused by air in the lower levels cooling and becoming more dense. As such, the sinking column is relatively shallow. In a warm anticyclone, this sinking is a part of the Hadley cell system.

Both lead to significant temperature gradients: in warm anticyclones this stems from adiabatic compression of the sinking air; in cold anticyclones it stems from the intense cooling of the surface.

63
Q

Explain the seasonal variations of cold and warm anticyclones.

A

Cold anticyclones are caused by intense cooling of the surface during long winter nights. They do not occur through summer.

Warm anticyclones are semi-persistent. They are linked to sinking air within Hedley Cells, and follow the seasonal shifts of this system.

64
Q

Explain how frontal cyclones form.

A

Frontal depressions are travelling weather systems caused by air flowing out of areas of high pressure (‘source regions’) and into areas of low pressure (‘sinks’).

65
Q

Describe the basic nature of warm fronts.

A

A warm front is the leading edge of tropical maritime air, for example, that has streamed into the mid-latitude North Atlantic from the Azores High.

Warm fronts bring warm, moist air.

66
Q

Describe the basic nature of cold fronts.

A

Cold fronts are the leading edge of polar maritime air, for example, that has swept across the North Atlantic from a cold anticyclone in North America.

67
Q

Describe the basic nature of occluded fronts.

A

In an occluded front, the warm moist air of a maritime front is lifted over the cold dry air of a polar front. This commonly occurs over the mid Atlantic.

68
Q

Describe the evolution of a ‘classic’ frontal cyclone.

A

Frontal cyclones typically occur where warm and cold air are moving in opposite directions either side of the front. Being more dense, the colder air will undercut the warmer, leading to a shallow slope.

Along the front, a wave will develop. In time this will intensify into a strong cyclonic circulation.

Because the cold air moves faster than warm, it will eventually begin to close the wave, leading to occlusion.

Fig 4.10

69
Q

Describe and explain the basic types of weather associated with selected airmasses.

A

…

70
Q

Explain the basic link between barometric pressure and wind patterns, including the importance of horizontal thermal contrast.

A

Winds blow from areas of high pressure to those of low pressure. However, direction is often parallel with isobars.

The strength of wind is related to the pressure gradient: stronger winds are found at steeper gradients.

71
Q

Describe the nature of monsoons.

A

Monsoons are sub-continental-scale changes in the atmospheric circulation, related specifically to a seasonal reversal in wind direction.

Example: during the northern winter, Southern Asia is fed by dry northerly winds from the Siberian High. During northern summer, it is fed by winds from the ‘roaring forties’, which pick up moisture from the Indian Ocean. The ITCZ lies south of India for half of the year, and north for the other half.

72
Q

Describe the nature of the ITCZ.

A
  • Inter-Tropical Convergence Zone
  • meeting point of the northern and southern hemisphere wind systems (or Hadley Cells)
  • characterised by heavy precipitation, as the moist air of the Trade Winds rises
  • shifts seasonally, being further north during the northern summer.
73
Q

Describe the nature of the Trade Winds.

A

Moist air rises at the ITCZ, travelling towards higher latitudes and sinks at about 30 degrees. This sunken air then returns to the ITCZ as a surface wind.

These winds are so called as they allowed European merchant ships to sail to the tropics.

74
Q

Explain the physical / dynamical link between the three cell model and surface climate features.

A
  • ITCZ: moist air rising leads to tall thunderclouds and heavy rainfall.
  • 30 degree: sinking air leads to adiabatic compression and warm anticyclones

< shifting ITCZ and seasonal changes? >

75
Q

Explain the relationship between the pattern of zonally averaged annual mean evaporation and precipitation and the associated horizontal transport of water by the atmosphere.

A

… P130

76
Q

Describe the link between the Southern Oscillation and Il Niño.

A

Il Niño is the warming of oceans along the coasts of Ecuador and Peru. This occurs every year, but the warming is far more extreme during low-index phase of the southern oscillation.

77
Q

Describe the evolution of an ENSO event.

A

When the high pressure in the east reduces at the same time as the low pressure in the west increases, the pressure gradient reduces to the extent that the normal convective loop is disrupted.

78
Q

In the UK, why is winter typically associated with stronger winds than summer?

A

The regions of radiative excess and deficit force energy transport toward the poles by both atmospheric and oceanic motions, away from the hot tropics into the colder regions outside the tropics. The annual picture masks very significant seasonal variations, such that the latitudinal winter gradients of radiative excess/deficit are very much stronger than those in the summer. This means that the surface temperature difference between tropical and polar latitudes is approximately twice as large in winter as in summer. Consequently, wind speeds in the mid-latitudes are about twice as large in winter as in summer.

S216 Environmental Science (15 decks)

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