LEC.175 Atmosphere, Weather, and Climate Flashcards
1
Q
How much cloud cover is there across the globe?
A
~1/2
2
Q
What is the most abundant gas in the atmosphere (%)?
A
Nitrogen (78%)
3
Q
What do units ppmm and ppmv mean?
A
ppmm = parts per million by mass
ppmv = parts per million by volume
4
Q
By how much does CO2 increase per year in ppmv?
A
~20 ppmv/yr
5
Q
What are 5 reasons why the atmosphere is important?
A
- Warms Earth’s surface by ~33°C
- Mixes heat vertically by convection
- Mixes heat meridionally (equator to pole) by advection
- Redistributes water via hydrological cycle
- Absorbs most energetic solar radiation (external UV shield)
6
Q
What is the difference between convection and advection?
A
Convection is movement of air by heating whilst advection is movement of air by pressure differences
7
Q
What are the 5 spheres in the atmosphere from closest to furthest from the Earth’s surface?
A
Troposphere –> stratosphere –> mesosphere –> thermosphere –> exosphere
8
Q
Which sphere in the atmosphere do most weather systems occur in?
A
Troposphere
9
Q
What is the homosphere and heterosphere?
A
Homosphere = troposphere –> mesosphere edge (homopause), homogeneous mixture of gases
Heterosphere = 100km –> 500km, heterogeneous mixture of gases
10
Q
Does pressure increase or decrease with increasing altitude in the atmosphere?
A
Decrease
11
Q
What is the mean free path in a gas?
A
Average distance travelled by a molecule between collisions with other molecules (dependant on T and P)
12
Q
At homopause in the atmosphere, what is the mean free path of gases approximately equivalent to?
A
The average scale of turbulent motion (below = mixing by turbulence, above = mixing by diffusion)
13
Q
Approximately at what altitude is the critical level in the atmosphere and what is mean free path approximately equal to at the critical level?
A
~500km, the distance to escape the Earth’s gravitational field
14
Q
What causes overturning/mixing/an unstratified atmosphere?
A
Hot, less dense layers below cold, more dense layers (not stable)
15
Q
Why does temperature increase in the stratosphere?
A
Due to the presence of ozone which absorbs solar radiation
16
Q
Why is the stratosphere stratified?
A
Temperature increases throughout layer so no turbulence/overturning/mixing
17
Q
What are 4 features of the troposphere?
A
- ‘Overturning sphere’
- Vertical motion
- Contains almost all atmospheric water
- Most weather systems occur in the troposphere
18
Q
Why does overturning occur at the planetary boundary layer?
A
Wind velocity decreases to 0 at surface so friction causes overturning
19
Q
What are 2 reasons why weather forecasting is difficult?
A
- Sun heats atmosphere but mostly indirectly by heating Earth’s surface
- Atmosphere is sensitive to change
…So no simple cause and effect
20
Q
What is used to collect and transmit weather data?
A
Radar/station data/satellite data/civil aircraft/sondes –> national meteorological centre –> regional meteorological centre
21
Q
What are the 2 types of weather stations used to make observations in the UK?
A
- Synoptic stations (send data directly to Met Office) e.g manned Met Offices, part manned stations, automatic stations
- Daily climate stations e.g. voluntary climate observers
22
Q
In which direction do winds move around high and low pressure systems?
A
High pressure systems: Clockwise
Low pressure systems: Anti-clockwise
23
Q
What are cold and warm air fronts and how are they represented in a forecast?
A
Cold air front: Cold air mass, triangles
Warm air front: Warm air mass, semi-circles
24
Q
What are 2 ways weather observations are made consistent?
A
- Every detail of station observation is regulated by Met Observer’s Handbook in UK
- Observations are referenced to mean sea level pressure as pressure differs with altitude
25
What are the 2 types of thermometer used to measure temperature manually and how do they work?
Mercury-in-glass or alcohol-in-glass thermometers, work via expansion + contraction of liquid
26
At what time is air temperature always measured and what other measurements are taken?
0900, maximum and minimum temperatures since previous observation
27
What is used to protect thermometers from external factors and how does it work?
Stevenson screen - naturally ventilated + shielded from solar/terrestrial radiation + rain
28
What are the units of absolute humidity and saturation vapour pressure?
Absolute humidity: pw
Saturation vapour pressure: ew
29
What is saturation vapour pressure?
The maximum amount of water vapour that air, above liquid/solid water, can contain at equilibrium (depends very sensitively on T)
30
What is relative humidity (%RH) and what is the equation for %RH?
Ratio of partial pressure to vapour pressure of water/ice, %RH = 100 (pw/ew)
31
How does a psychrometer measure relative humidity?
Ordinary thermometer ('dry bulb') is matched with thermometer covered in wet muslin cloth ('wet bulb') and diff. in temp. is related to relative humidity e.g. when Twet < Tdry, heat has been used to evaporate water from cloth so T decreases
32
What meteorological instrument is used to measure wind speed and how does it work?
Calibrated cup anemometer, counts no. rotations per minute
33
What scale is used to subjectively measure wind speed when instruments aren't available?
Beaufort scale (0-12, e.g. Force 8 = gale)
34
Why is wind speed measured at a standard height of 10m?
Friction reduces wind speed close to the ground
35
What are 2 ways of measuring wind direction?
1. Wind vane (measures angle from north, clockwise, in °)
2. Run of wind (cumulative wind measured by connecting anemometer to a mile-ometer, standard height = 2m)
36
What causes winds?
Pressure differences (winds travel from high --> low pressure
37
What instrument is used to measure atmospheric pressure and how does it work?
Barometer, reads station pressure and converts to mean sea level pressure using station altitude
38
What units are used for atmospheric pressure and how many Pa is this unit equivalent to?
mbar (requires accuracy to 0.1 mbar), = 100 Pa
39
How does an aneroid barometer work and what is it used for?
On a barograph, pressure is recorded on a rotating chart, as pressure decreases sealed metal chamber expands, used to study pressure tendency (change in pressure over time)
40
How does precipitation radar measure precipitation?
Raindrops reflect radio waves, strength of radar echo is proportional to rain intensity, gives info on mean size of raindrops + velocity of weather system
41
What are the low, medium, and high cloud types?
Low: Cumulis, stratus, fog, stratocumulus, nimbostratus
Medium: Altostratus, altocumulus
High: Cirrus, cirrostratus, cirrocumulus, contrails
All 3: Cumulonimbus
42
Why are less upper air measurements needed than surface measurements?
Weather changes on much larger scales
43
What do weather satellites do?
Record intensity of EM radiation in particular wavelength regions --> data transmitted to ground base --> corrected for known instrument + re-transmitted to users
44
What are the 2 types of weather satellite?
1. Geostationary orbits (fixed position relative to Earth on equatorial plane, 35,000 km above)
2. Polar orbiting (850 km above, return over same location twice a day, coverage better at poles than Equator)
45
What is the width of a satellite view called?
Swath
46
Why are weather satellite images taken in the infrared region where absorption by water vapour is very strong?
Atmosphere is transparent to some wavelengths but opaque to others
47
What is the source of energy that drives climate?
The Sun
48
What 2 equations are used to calculate energy from the Sun?
1. Stefan-Boltzmann Law (W m^-2): F = σT^4
2. Wien's Law: λ = 3000 / T
49
In which type of radiation is peak energy output of the Sun?
Visible radiation
50
What are the 2 heat sources from Earth?
1. Geothermal energy from Earth's interior
2. Gravitational energy (via tides)
51
What do IR and far-IR absorption cause?
IR: Vibrational excitation e.g. CO2
Far IR: Rotational excitation e.g. water vapour
52
Do molecules scatter more efficiently at shorter or longer wavelengths?
Shorter
53
At what wavelength is the atmospheric 'window' where heat from Earth can escape but can be blocked by clouds?
10 µm
54
What drives the meridonial heat flux (ocean and atmosphere carry heat poleward)?
Flux of solar energy is greatest at Equator + there is a larger atmospheric path length at high latitudes (so oblique solar irradiance) so there is a surface temp. gradient
55
Which form of the Ideal Gas Law equation (pv = nRT) is more convenient for an unbounded gas with known composition e.g. the atmosphere?
p = mass / volume
56
What is the atmospheric equation of state?
p = ρRT where R = specific gas constant for dry air (287 J K-1 kg-1)
57
Define isobars
Contour lines of equal pressure, hPa (isobar spacing directly proportional to wind speed e.g. closer spacing = faster wind speed)
58
What kind of trend is the decrease in pressure with increasing height?
Exponential
59
What 3 things do gravitational forces vary with?
1. Distance from centre of Earth
2. Variations in Earth's shape/composition
3. Centrifugal acceleration due to Earth's rotation
60
What is geopotential height measured in and how many metres is this equal to?
Decameters, 1 dam = 10 metres
61
What is thermodynamics?
Change in heat of a system is equal to change in energy of the system
62
What is the first law of thermodynamics?
dq = C(p) dT - (1/ρ)dp = 0
63
What is the equation for calculating the temperature of an air parcel at any pressure (as long as it moves adiabatically)?
T(2) / T(1) = (P(2) / P(1))^(R/C(p))
64
What are adiabatic processes?
Processes that involve no heat exchange with the surroundings
65
What is the equation for potential temperature?
θ = T (p(0) / p)^(R/C(p))
66
What are 3 sources of climatic heating?
1. Conduction of heat from surface
2. Frictional heating at surface
3. Latent heating from condensation of water vapour into clouds
67
What is the value of dry adiabatic lapse rate (DALR)?
9.8 K km-1
68
When (-dT/dz) = DALR, is the air column neutral, stable, or unstable?
Neutral (air requires no extra energy to move along adiabats so any small mechanical perturbation will allow air parcels to swap places spontaneously)
69
When (-dT/dz) < DALR, is the air column neutral, stable, or unstable?
Stable (air above has higher θ than air below so mixing is suppressed)
70
When (-dT/dz) > DALR, is the air column neutral, stable, or unstable?
Unstable (air above has lower θ than air below so lower air has excess heat + air column spontaneously overturns)
71
What are the heat capacities (C(p)) of dry air, water, and land?
Dry air = 1004 J kg-1 K-1
Water = 4200 J kg-1 K-1
Land = 800 J kg-1 K-1
72
What is heat capacity numerically equal to?
The amount of heat required to increase 1kg of a substance to 1K
73
How do evaporation + condensation occur?
Molecules on liquid surface with sufficient energy evaporate, molecules in air collide with surface and condense if they stick (energy released when condensation occurs)
74
When will a warm puddle evporate?
If air is sub-saturated relative to vapour pressure of warm puddle
75
When is air supersaturated and what happens when it is supersaturated?
When partial pressure > vapour pressure that would exist over a surface of water at equilibrium, excess water vapour is condensed to form droplets (cloud/fog)
76
What kind of particles make cloud droplet production more efficient?
Aerosol particles
77
What is the specific heat of vaporisation of water and specific heat of fusion of ice?
Specific heat of vaporisation of water = 2500 kJ kg-1
Specific heat of fusion of ice = 334 kJ kg-1
78
What is the form of energy used for phase change e.g. condensation?
Latent heat
79
Describe convective cloud formation
Air lifted adiabatically causes cooling --> eventually Pw (partial pressure of water) > e (saturation vapour pressure) --> clouds form (releases latent heat)
80
What Law shows motion relative to air (balance of gravitational acceleration + frictional drag)?
Stoke's Law (velocity, v = 1.2x10^8 r^2)
81
What are the different motions relative to ground for the cloud types: stratus, cumulus, lenticular, and cumulonimbus (depends on Stoke's Law and updraft velocity)?
Stratus: ~10 cm s-1
Cumulus: ~1 m s-1
Lenticular: ~1 m s-1
Cumulonimbus: ~10 m s-1
82
What are the equations for the terminal velocity of rain and hail with radius >30m and with >1000m and why are they different?
>30m: v = 8x10^3 r
>1000m: v = 250√r
Turbulence and drag behind drop change terminal velocity
83
Cloud droplets don't freeze at 0°C, so what temperature are they supercooled at?
T(f) < -40°C
84
What does freezing give out to the air in a cloud?
Heat
85
What happens when e(ice) < e(supercooled liquid)?
Initial rapid growth of ice particles by condensation
86
How are snowflakes formed?
Deposition of vapour onto ice (hexagonal symmetry, v. efficient coalescence)
87
How is hail formed?
Impaction of liquid drops on ice T 0°C, liquid spreads over surface clear (high density ice particle)
88
How is graupel (soft hail) formed?
Impaction of liquid drops on ice T << 0°C, liquid doesn't spread much + air trapped in some ice layers (opaque, low density ice particle)
89
How does the temperature decrease of an air parcel lifted adiabatically change while water is condensing?
Temp. decreases more slowly
90
Where in the atmosphere is the difference between DALR and SALR greatest?
At high temperatures (low altitudes in troposphere) where the saturated water vapour mixing ratio changes rapidly
91
What is horizontal position given relative to?
The centre of the Earth, the Equator, and the Greenwich Medium (frame of reference rotates with Earth - looked at from North Pole, rotation is anti-clockwise)
92
What is the difference between centrifugal and Coriolis force?
Centrifugal force acts continuously on any object on Earth whilst Coriolis force acts on moving objects only
93
What does horizontal deflection of the require?
A force acting in the horizontal (greatest at poles and zero at Equator)
94
In which direction are N.hemisphere air parcels deflected from their direction of movement and what does the magnitude of deflection depend on?
Right, depends on velocity of air (Coriolis force deflects an air parcel but doesn't affect its velocity)
