Environmental physics Flashcards
1
Q
What is the total work done by the system over the course of the heat engine cycle?
A
Area contained in a PV diagram
2
Q
Since heat engine cycles are cyclic processes, what can we say about the integral of the internal energy over the entire cycle?
A
It equals zero
3
Q
What is the Carnot cycle (not the steps)?
A
An idealisation of a heat engine where heat is transferred from a hot reservoir to a cold reservoir
4
Q
How is the heat transferred in a Carnot cycle?
A
Via a gas-filled piston which can be isolated from either or both reservoirs
5
Q
In the Carnot cycle, how is work extracted from the system using the piston?
A
The gas in the piston expands or contracts depending on which reservoir it is connected to and the raising and lowering of the piston allows work to be extracted
6
Q
What are the 4 stages of the Carnot cycle?
A
Isothermal expansion, adiabatic (or isentropic) expansion, isothermal compression and adiabatic (or isentropic) compression
7
Q
What is the efficiency of any heat engine?
A
The ratio of the work done to the heat input into the system
8
Q
What is the efficiency of the Carnot cycle, given that it is a reversible and cyclic process?
A
Heat in minus heat out all divided by heat in
9
Q
What are heat pumps?
A
Reverse Carnot engines
10
Q
What is the quantification of how good a heat pump is and what is its equation?
A
Coefficient of performance and heat out over work done
11
Q
What is Carnot’s theorem?
A
All Carnot engines operating between reservoirs at given temperatures are equally efficient and no reversible engine is m ore efficient than the Carnot engine.
12
Q
How do you prove Carnot’s theorem?
A
Imagine a Carnot engine connected to a heat pump in a system with the efficiency of the engine more than that of the heat pump, then there would be a breach of the second law (heat from cold reservoir into the hot reservoir)
13
Q
What is a consequence of Carnot’s theorem?
A
The Carnot cycle is the most efficient possible heat engine operating between reservoirs at specified temperatures
14
Q
From Carnot’s theorem, what is Q (cold) over Q (hot) equal to?
A
T (cold) over T (hot)
15
Q
What is the saturation temperature in regards to steam power?
A
The particular temperature at which water cannot contain any more heat and stay a liquid (under constant pressure)
16
Q
After the saturation temperature, the energy from any heat added to the water beyond this point will go towards what?
A
Converting some of the liquid into vapour, which then increases the volume (not towards increasing the temperature of the water)
17
Q
When there is no more liquid left to be vaporised (still at the saturation temp), what happens when you continue adding heat?
A
Temperature will rise again
18
Q
What does the temperature versus volume graph look like for water under constant pressure that heat is being added to and it turns to vapour?
A
It rises to the saturation temperature and then plateaus for a bit (liquid and vapour phases coexisting) and then the temperature rises again
19
Q
Does the volume of pressurised water vapour increase or decrease with increasing pressure?
A
Decreases
20
Q
Does the saturation temperature of the water/ vapour phase increase or decrease with increasing pressure?
A
Increase
21
Q
What is the critical point on a phase diagram of temperature versus volume for water to vapour transitions?
A
It is at 674 K at a pressure of around 22 MPa and beyond this point, no phase change can be observed
22
Q
On either side of the critical point, what are the lines called and what do they represent?
A
Saturated liquid line, where the transition from a pure liquid to a mixed state is and saturated vapour line, where the transition from mixed to pure vapour state is
23
Q
What is the liquid called on the liquid side of the saturated liquid line on the temperature volume graph?
A
Compressed liquid
24
Q
What is the vapour called on the vapour side of the saturated vapour line on the temperature volume graph?
A
Superheated vapour
25
What is the Rankine engine (not the stages)?
A simplified model of a steam turbine systems such as those used in coal and natural gas power plants
26
What are the 4 stages of the Rankine cycle?
Isentropic compression in a pump, isobaric heating in a boiler, isentropic expansion through a turbine and isobaric condensation in a condenser
27
What is the isentropic compression in a pump stage in the Rankine cycle?
Liquid water brought to high pressure using a pump without adding any heat
28
What is the isobaric heating in a boiler stage in the Rankine cycle?
Fixed pressure, water heated in a boiler until entirely evaporated
29
What is the isentropic expansion through a turbine stage in the Rankine cycle?
Vapour allowed to expand while passing through a turbine from which mechanical work is extracted
30
What is the isobaric condensation in a condenser stage in the Rankine cycle?
Vapour enters a condenser where it is cooled and returned to a liquid state at constant pressure, before starting the cycle again
31
In the Rankine cycle, the pump, boiler, turbine and condenser are considered open thermal systems, why?
Material both enter and exits, carrying energy with it
32
The change in the internal energy in any of the open thermal systems of the Rankine system is of what form?
Change in internal energy of the control volume due to internal processes plus the internal energy change from the material entering the system minus the internal energy change due to material exiting the system
33
What are the two forms of work done on the contents of the control volume?
Flow work, where work is done by the material coming in and out applying pressure to the contents, and shaft work, where work is being done or by the contents on a n external mechanical component
34
What is the flow work?
Work done in displacing the contents in the control volume
35
what is the efficiency of the Rankine cycle?
Specific enthalpy = SE . 1 - (SE of steam passing from turbine to condenser minus SE of water passing from condenser to pump) divided by (SE of steam passing from boiled to turbine minus SE of water passing from pump to boiler)
36
What is the efficiency of a coal fired power plant and is this good?
23% (overestimate) and no this is very low efficiency
37
What is the Keeling curve?
The atmospheric carbon dioxide concentration against time
38
What does the Keeling curve look like?
It increases with time exponentially on average but it is wiggly locally with periodic oscillations
39
What are the periodic oscillation of the Keeling curve due to?
The seasonal death and regrowth of plant matter in the large boreal forests in North America and Russia
40
What was the carbon dioxide concentration before the industrial revolution? (found from air bubbles trapped in Antarctic ice cores)
280 parts per million
41
What are the carbon dioxide concentrations now?
Over 400 parts per million
42
How is acid rain caused?
Sulphur oxides comes from the exhaust from coal power (as well as carbon) and react with the water content of clouds to form sulphuric acid
43
What does acid rain do?
It enters soil and interrupts normal biological processed in the microbiome and increases the acidity of the oceans
44
What is insolation and what letter represents it?
Power per unit area received by a flat surface from the sun and Q
45
What does the insolation of a given surface depend on?
The angle the normal of the surface makes with a direct line to the sun (more hits surface if perpendicular with sun rays)
46
What is the solar zenith angle?
The angle between the suns rays and the normal of the curved surface of the Earth at any point
47
Does the solar zenith angle change with time and why?
Yes because the Earth rotates and orbits the sun
48
What is the latitude?
Measures the angle going north from the equator (North-South direction)
49
What is the longitude?
Measure the angle going east from the Greenwich meridian (East-West direction)
50
What is the obliquity of the ecliptic?
The angle between the ecliptic plane and equatorial plane, the 'tilt' of the Earth
51
What is the ecliptic?
It is the name for the orbital plane of the Earth around the sun
52
What is the equatorial plane?
The plane through the Earth's equator
53
What is a heliocentric and geocentric coordinate system?
Heliocentric one is fixed to the sun and a geocentric one is fixed to the Earth
54
As the Earth orbits around the sun, what does the geocentric coordinate system do?
They rotate with respect to the heliocentric system
55
What is the ecliptic longitude (L)?
The celestial longitude measured with respect to the ecliptic plane, I think the angle the geocentric coordinate system makes with respect to the heliocentric system as the Earth orbits the sun
56
What is the right ascension?
The celestial version of longitude to determine the position of the sun as viewed from the Earth
57
What is the declination?
The celestial version of latitude to determine the position of the sun as viewed from Earth
58
Why does the sun move up and down in the sky over the course of a year?
The declination can be put in terms of the obliquity and the ecliptic longitude and this leads to the declination changing as the Earth moves around its orbit
59
What is the hour angle?
The longitude minus the right ascension
60
At the astronomical sunrise and sunset, what angle will rays of light be to the Earth's surface?
Perpendicular so the zenith angle is + or - 90 degrees
61
When does maximum insolation occur?
On the equinoxes when the sun is directly overhead at noon
62
What are seasonal variations due to on the equator?
Variation in the declination
63
On the north pole, when does the sun rise and set?
Rises on the vernal equinox and set on the autumnal equinox
64
The Earth's surface temperature changes with seasons, but where does the temperature remain essentially constant?
Deep in the ground
65
What is the reasoning behind ground source heat pumps?
Since the temp deep underground is nearly constant, we can use the ground as a heat sink to cool a building in the summer and heat it in the winter
66
The heat flux (flow of energy per unit area per unit time) through a vertical section of the ground obeys what law?
Fourier's law. Heat flux equal minus the thermal conductivity multiplied by the temperature gradient
67
What is the heat equation? (used for ground source heat pumps section)
Partial differentiation of temperature with respect to time equals the thermal diffusivity times the second order partial derivative of temperature with respect to time
68
From the solution the ground temperature equation, why is there a certain depth where the temperature is increased in winter and decreased in summer?
The sign of the cosine in the equation flips in this equation
69
Are ground source heat pumps an efficient way to heat a room and what is their coefficient of performance?
Yes very efficient and 50
70
On a PV plane, what does a ground source heat pump look like?
A Rankine cycle
71
Why are refrigerants used in a vapour compression cycle and what do we use for it at the moment?
They are liquids with low boiling points so it can easily go through repeated phase transitions and propane
72
If we use a frame of reference where the moon just stays where it is (distance between moon and earth is constant), this avoids needing a centrifugal force but what does it mean we need to consider related to gravity?
The gravitational pull on the Earth due to the moon
73
For a test mass on Earth's surface, the angular component of acceleration is called what and what is it responsible for?
Tractive acceleration and it is responsible for the tides
74
Where does the tide raising force point to on the Earth when the tide-raising body considered is the moon?
The point under the moon and its antipode
75
Is the tide-raising force from the sun stronger or weaker than the moon and by how much?
Weaker but only 0.46x that of the moon's force strength
76
Why do the pair of tidal bulges change in magnitude depending on the relative configuration of the sun-Earth-moon system?
The tide raising force from the sun and moon add vectorially
77
When are the tidal bulges maximised in relation to the sun-Earth-moon configuration?
when they form a straight line, known as a syzygy, which occurs on full and new moons
78
What are the maximal and minimal tides called?
Maximal are called spring tides and minimal are called neap tides
79
When are the tidal bulges minimised in relation to the sun-Earth-moon configuration?
When the sun, Earth and moon form a right angle
80
What is a sidereal day and sidereal year? sidereal means 'with respect to the distant stars'
Sidereal day refers to the period of rotation of the Earth with respect to a fixed background and the sidereal year refers to the orbital period of the Earth
81
What does diurnal and semi-diurnal mean?
A diurnal process is one that is periodic and repeats once per day, whereas a periodic process that repeats twice per day is semi-diurnal
82
What is a mean solar day?
The time it takes for the sun to cross the sky and return
83
What is a mean lunar day?
The time it takes for the moon to cross the sky and return
84
What is the frequency of lunar tides?
Twice per mean lunar day
85
What is the frequency of solar tides?
Twice per mean solar day
86
If you combine the amplitude of lunar and solar tides to get a combined tide, what does each term represent?
Two different harmonics of the tidal motion
87
What is the period of the moon circling the Earth?
One sidereal month
88
Does the moon's declination change or stay the same over the course of a day?
It varies
89
What is the diurnal inequality and why does it exist?
The two tides at a particular point on Earth have different amplitudes because the angle at that place to the moons orbit will be different at each tide time in one day (moon's orbit is tilted with respect to the Earth's axis)
90
What are the labels for the semi-diurnal tides due to the moon and sun and also the label for the diurnal lunar tide from the diurnal inequality?
M2, S2 and O1
91
As the moon orbits the Earth, it drags the bulge in it but why is there a lag in this?
Inertia, frictional forces and competing rotation means the motion of the bulges cannot keep up with the motion of the moon
92
The lag between the motion of the bulges and the moon leads to a torque on the Earth, what does this do to the Earth?
It tries to modify the rotation of the Earth to match up with the orbit of the moon
93
What is tidal locking and why does it occur?
The same face of the moon always faces the Earth because of the tidal acceleration from the Earth creating a torque and the moon's rate of rotation nearly matches its orbital period so it always faces
94
When does tidal locking occur?
It appears across the solar system when two bodies have significant tidal interactions
95
Why is the moon slowly moving away from Earth?
To conserve angular momentum in the Earth-moon system and the Earth's angular momentum decreases from tidal locking slowing its rotation
96
What is the tidal range of the open ocean? (amplitude will be half this)
60cm
97
Why are the tidal ranges of the seaside a lot larger than the tidal range of the open ocean?
As the tides roll in, they are slowed and their kinetic energy gets converted into potential energy (height)
98
What is tidal resonance?
The fundamental modes of the surface waves in a channel are excited by the tides in the open sea
99
When does tidal resonance occur?
In narrow channels
100
What is the simplest way to use tidal power to generate power? (using the tidal range from tidal resonance)
Build a barrier over a channel to capture the water at its high point and let the water escape through a sluice gate (via a turbine) at low tides. Converts GPE into usable energy.
101
What is another way to use the tides to generate power?
Extract some of the kinetic energy of the tidal currents using a turbine
102
How was the Earth's primary atmosphere formed?
A protoplanetary disk was formed around the sun and the planets formed by collisional accretion of dust and gas was swept up by planet cores to make their primordial atmospheres
103
How was the Earth's secondary atmosphere formed?
The Earth was hit by a massive impact by a smaller protoplanet, making a freshly molten Earth releasing volatile gases that form the second atmosphere (ie nitrogen, water, methane, CO2). The water from this condensed to form early seas
104
What are the 3 time periods in Earth's history called?
Archean Earth, Proterozoic Earth and Modern Earth
105
Where did the Earth's atmosphere come from?
Asteroid bombardment, outgassing due to volcanic activity and likely meteorites with chromium nitrate in
106
When did oxygen show up in the Earth's atmosphere?
In Earth's Proterozoic stage 2.5 billion years ago in the 'Great Oxigination Event'
107
How did the Argon in our atmosphere originate and what did it not come from?
In potassium decay either through electron capture or positron emission and it did not come from the same origin as that found in the solar nebula
108
Where did the carbon dioxide in our atmosphere come from?
At divergent tectonic boundaries where magma is exposed, outgassing due to volcanic activity and also human activity
109
Where did the Earth's water come from?
Outgassing from volcanoes (small amount), comets/asteroids (not a primary source though) and purple sulphur bacterial uses sulphur in photosynthesis and produces water
110
What are the names of the five major regions that divide the Earth's atmosphere? (closest to Earth to furthest)
Troposphere, stratosphere, mesosphere, thermosphere, exosphere
111
Why is the Earth's atmosphere divided up the way it is (ie what is it dependent on)?
How the temperature changes with altitude (pressure)
112
Do the 'pauses' in atmosphere (eg tropopause, stratopause, mesopause) occur above or below the region its named after?
Above
113
How does temperature relate to altitude in the troposphere?
Temperature decreases with altitude
114
What percentage of the atmospheric mass is in the troposphere?
75%
115
How far up does the troposphere go?
7km at the poles to 17km at the equator
116
How does temperature relate to altitude in the stratosphere (this also has a name)?
The stratospheric inversion means that the temperature starts to increase with altitude
117
What causes the inversion in the stratosphere?
The ozone layer
118
After the tropopause, how far up does the stratosphere go up to roughly?
50km above the Earth's surface
119
Is the stratosphere stable against convection?
Yes
120
As altitude increases, does pressure increase or decrease?
Decrease
121
How does temperature relate to altitude in the mesosphere and what does this cause?
Temperature quickly decreases with altitude, causing strong winds to develop
122
Which region of our atmosphere do most meteors disintegrate?
In the mesosphere
123
At what point in the atmosphere is the temperature the lowest and why?
The mesopause because of carbon dioxide radiative cooling
124
How far up does the mesosphere go roughly?
85km
125
How does temperature relate to altitude in the thermosphere?
Temperature starts to increase with altitude
126
What region in the Earth's atmosphere does the aurora appear and what are they from?
Thermosphere and interactions with nitrogen and oxygen
127
What is the ionosphere composed of?
Ionised gas with different regions of electron density defined by different radiation penetration
128
Where is the ionosphere located in terms of the other regions in our atmosphere?
Mostly in the thermosphere but also overlaps with the mesosphere and exosphere
129
What is the heterosphere?
The part in the atmosphere which is layered by molecular weight and electric charge (gases separate out)
130
What is the homosphere?
The part in the atmosphere where the composition of the atmosphere is uniform. Turbulent mixing causes this composition to be homogeneous.
131
Where is the heterosphere and homosphere of the Earth?
The homosphere is from the Earth's surface up to part way through the thermosphere and then the heterosphere is directly on top of this so a lot of the thermosphere and all of the exosphere
132
What is the exobase?
The lower boundary of the exosphere and it marks the altitude from which the atmosphere is so rarified (very low pressure) that thermal considerations no longer play a dominant role in the particle motion and the constituents of the atmosphere behave as free particles
133
How do gas particles act like in the exosphere?
They no longer act like a gas
134
How is the concentrations of species A written in chemical reactions?
[A]
135
What is the law of mass action?
The reaction rate in a chemical reaction at a given temperature is proportional to the product of the active masses of the reactants
136
What is photodissociation also called?
Photolysis or photodecomposition
137
What is photodissociation?
A chemical reaction which is the interaction of one or more photons with one target molecule and breaks down the molecules
138
What does the ozone consist of?
oxygen 3
139
How do we know the ozone layer exists?
If you look at the sun's spectrum through the Earth's atmosphere, there's a gap in the UV portion of the spectrum
140
What is the name of the explanation of the ozone layer and how many stages are there to it?
The Chapman mechanism and 4
141
What is step 1 of the Chapman mechanism?
High energy photons break dioxygen (molecular oxygen) into two atomic oxygen radicals (atomic oxygen) (this is photodissociation)
142
What is step 2 of the Chapman mechanism?
Atomic oxygen binds with a dioxygen (molecular oxygen) to form ozone and also includes some other stable air molecule (like dinitrogen) to take some excess KE but doesn't change
143
Which part of the Chapman mechanism contributes to the heating of the stratosphere where the ozone is?
The second stage
144
What is step 3 of the Chapman mechanism?
A low energy photon breaks up the ozone into atomic and molecular oxygen (photodissociation again I think)
145
What is step 4 of the Chapman mechanism?
Atomic oxygen and ozone react to make molecular oxygen (2 of them) and this is metastable
146
What do we start and end with in the Chapman mechanism?
Molecular oxygen
147
What does the ozone layer block and what do these things do if it could get through?
UVC and most UVB radiation, and these cause damage to DNA
148
Does UVA make it past the ozone layer and what does this do to us?
Yes and it damages collagen
149
(Chapman Mechanism) What are the symbols of the reaction rate of C1, reaction rate coefficient for C2, reaction rate for C3 and reaction rate coefficient for C4?
J2, K12, J3 and K13
150
Why are the reaction rates/ reaction rate coefficient named with numbers the way they are for the Chapman mechanism?
The number represents the type of oxygen of the reactant side (eg 1 for atomic oxygen) and there are multiple numbers if there is multiple oxygen types on the reactant side
151
Do the photodissociation rates in the Chapman mechanism (J2 and J3 in steps 1 and 3) increase or decrease as you decrease altitude?
Decrease
152
The catalyst to the second step of the Chapman mechanism depends of the density of air, how does this change with altitude?
It increases as you decrease altitude
153
The ozone layer forms when the decrease in photodissociation meets what?
Increase in density when you head to the ground
154
Who is responsible for a lot of the destruction of the ozone?
Thomas Midgley Jr
155
What did Midgley do that destroyed the ozone?
Developed Freon, a type of CFC, as a refrigerant and this destroys the ozone
156
How do CFCs destroy the ozone?
They react with high energy photons to make a free chlorine, which acts as a catalyst for ozone destruction and doesn't get absorbed in the reaction so continues to damage
157
How long does free chlorine hang around in the upper atmosphere before dropping down and dissolving into water?
2 years
158
What year is the ozone expected to be naturally replenished?
2065
159
What is the ideal gas law?
PV=nRT=N(k_b)(N_A)T
160
What is the pressure exerted by a gas on a box that contains it? (in terms of density and velocity)
1/3 times the density times the expectation of the squared velocity
161
What are all the ways an atmosphere can escape from a planet?
Jeans escape (thermal), hydrodynamic escape, solar wind pressure, photochemical, magnetospheric wind, charge exchange
162
What is escape velocity?
The speed something needs to reach to get to infinity and leave the gravitational pull entirely
163
What speeds do the particles need to be to avoid Jeans escape and make the atmosphere stable? (average speeds determined by temperature)
A lot smaller than the escape velocity
164
What factors are depended on to know whether a planet will lose its atmosphere?
The mass and radius of the planet, the mass of the particles and the temperature
165
The pressure for a column of air is just due to what?
The weight of the air
166
What is hydrostatic equilibrium?
The air is not moving because the force pulling down from gravity is balanced by the pressure-gradient force pushing up
167
What is the atmospheric scale height?
The length scale of the atmosphere where pressure and density changes by e
168
What is the lapse rate?
The rate of decrease in temperature with altitude
169
What is the equation for lapse rate?
Minus the derivative of temperature wrt altitude
170
What is the dry lapse rate?
The air is not saturated with water vapour
171
What is the moist lapse rate?
The air is saturated
172
What is the environmental lapse rate?
The local specific lapse rate
173
What equation holds for hydrostatic equilibrium when we assume the only force acting is gravity?
the derivative of pressure wrt to altitude is minus the density times gravity
174
What is an adiabatic process?
A process where no heat or mass is transferred to the surroundings
175
What is the equation for the dry adiabatic lapse rate?
gravity divided by the isobaric heat capacity (constant pressure)
176
What is the dry adiabatic lapse rate for Earth?
9.8 K/Km
177
The adiabatic lapse rate is the largest or smallest positive lapse rate a column of air can have before becoming unstable?
Largest
178
When the lapse rate is less than the adiabatic lapse rate the atmosphere is stable/unstable and convection will/ won't occur?
Stable and won't
179
What is convection?
The transfer of heat due to the motion of its medium
180
What is the principal mode of heat transfer in the atmosphere and why?
Convection because it is difference in temperature that cause the air to move
181
If the lapse rate is stable, what will a displaced parcel of air do?
It will move back to where you took it from
182
If the lapse rate is neutral (equally to the dry adiabatic lapse rate), what will a displaced parcel of air do?
It will stay where you put it
183
If the lapse rate is unstable (more than the dry adiabatic lapse rate), what will a displaced parcel of air do?
It will continue to rise on its own
184
Instead of saying 'hot air rises', we can say if a column of air is .... to convection then hot air will rise?
unstable
185
What is the potential temperature?
It is the temperature a parcel of air would have at a specific pressure if moved adiabatically to that pressure level
186
What is the environmental lapse rate inversion and what happens to gas particles there?
The point between the stable and unstable lapse rates and gas particles tend to get trapped at inversion, where warmer air is above cold air
187
What is the latent heat of vaporisation?
The amount of heat required to convert liquid into vapour
188
On the graph of temp against heat where something goes through all 3 phases, what shoes the energy needed for the separate phase changes?
The length of the plateau region where heat is added but temperature is not raised
189
What is the saturation temperature of water?
The temperature at a fixed pressure at which the liquid and vapour phases of water coexist
190
What is the saturation pressure?
At a fixed temp the pressure at which two phases can coexist
191
What is the dew point?
The temperature at which a sample of air would reach 100% relative humidity and the temperature to which air must be cooled to become saturated with water vapor
192
The dew point is the point where the rate of evaporation matches what?
The rate of condensation
193
What are the condensation and evaporation rates functions of?
Condensation - vapour density. Evaporation - temp
194
Why does the relative humidity rise as the temp falls? (at ground level)
Less vapour is needed to saturate the air when its colder
195
If the temperature drops below the dew point, what happens to a vapour?
It condenses into a liquid
196
What is the lifting condensation level?
The height at which a parcel of moist air would need to be lifted, cooling adiabatically, to reach its dew point
197
What does the lifting condensation level allow us to work out?
The altitude at which clouds will start to form
198
When does orographic uplift ocurr?
Moist air is forced to ascend by a topographic feature (like a mountain) and becomes saturated
199
Orographic uplift works better when the air being lifted is close to saturation already, when could this situation occur?
When the air is transported by winds over the surface of water
200
What forms on the windward side of a mountain after orographic uplift and what occurs on the leeward side?
Clouds on windward side and a rain shadow on the leeward side
201
What happens to the orographic clouds after uplift over a mountain and what does this do?
They condense and precipitate (rain)to release latent heat and warm the air and the wind drives this air to the leeward side, warming it further via compression
202
What is a Fohn wind?
The warm, low humidity air that occurs on the leeward side of a mountain after orographic lift
203
Why do Fohn winds increase the risk of wildfires?
They have a large capacity to carry moisture because it has a higher saturation vapour pressure (from high temp) so it carries moisture away from the already hot area
204
What are the 3 forces at play in a rotating world?
Centrifugal force, Euler force and Coriolis force
205
What sort of deformation of the planet does the centrifugal force do?
It makes the diameter of the planet larger across the equator compared from pole to pole (it bulges around rotational axis)
206
What is the centrifugal force?
The outward force apparent in a rotating reference frame
207
Why can we equate the total potential energy at an equator point and a pole?
They form an equipotential surface, so even though they have different distances to the centre of the Earth they have the same potential
208
When the centrifugal acceleration is large on the planet, what happens?
It beats gravity pushing away from the equator
209
Do you weigh more or less at the equator and why?
Less because gravitational acceleration is slightly different (smaller at equator) due to the bulging from the centrifugal force
210
What is the geoid?
Th equipotential surface of gravity combined with centrifugal force if the whole Earth were covered in ocean (no winds or tides)
211
What is the Euler force due to and why can we ignore it for the Earth?
It is due to non-uniform rotation but Earths rotation is uniform so we can ignore it
212
Why must points near the equator be travelling faster than points near the poles?
The time to rotate is the same but the distance covered by the points near the equator is larger
213
Because planets rotate, how do winds travel and why?
The follow a curved path because winds cannot blow straight from a high pressure to a low pressure region because of the Coriolis force
214
What is the colatitude?
Complementary angle of a given angle ie angle from pole to that latitude (90 - latitude)
215
Is the Coriolis force positive or negative in the North?
Positive
216
Is the Coriolis force positive or negative in the South?
Negative
217
Is the Coriolis force positive or negative at the equator?
Zero
218
What is the Rossby number?
A dimensionless quantity that determines the importance of the Coriolis force
219
If the Rossby number is a lot more than 1, can the Coriolis effects be ignored or taken into account?
Ignored as they are negligible
220
The Coriolis force on the Earth's surface means a body moving in one direction feels a force pointed in another direction, how can we describe the pattern of directions?
For each of the 4 directions, the direction of the force is the next force clockwise to it for northern hemisphere and anticlockwise for southern hemisphere
221
Is the heating from the sun evenly spread across the Earth and why?
No because energy is spread over larger areas at higher latitudes so the equator is heated more efficiently than the poles
222
Does the Earth radiate out the energy from the sun evenly and what does this mean?
Yes it does, so since the sun heats unevenly but the earth radiates evenly some areas experience net warming whilst others experience net cooling
223
Since there is uneven warming with latitude, the system (Earth) is not in thermodynamic equilibrium, what does this mean happens?
Heat tends to transfer away from the equator to balance the system
224
Why is the general circulation model not correct? (2 big Hadley cells)
It does not account for the Coriolis force and the air cools and subsides before it can make it all the way to the poles
225
Where do northerly winds flow from and to? (opposite for southerly)
Flow from north to south
226
Where do easterly winds flow from and to? (opposite for westerly)
Flow from east to west
227
Hadley cells form a discrete cell of what size? (in terms of latitudes)
Around 30 degrees north and south of the equator (2 separate cells)
228
What are Hadley cells in terms of arrows?
They start at the equator and go up (away from the surface) then either clockwise for Northern hemisphere or anticlockwise for Southern hemisphere
229
What are Polar cells?
They run from 60 degrees latitude where they lift warmer air to the poles (90 degrees latitude) where it is cools and descends
230
Where are Ferrel cells?
In between the Hadley and Polar cells so between 30 to 60 degrees latitude
231
How do Ferrel cells circulate air and why?
In the opposite direction to the Hadley and Polar cells and this is due to the drag from their neighbouring cells
232
What is the actual circulation model for the Earth in terms of cell type from South to North and if they are clockwise (C) or anti-clockwise (A)?
Polar (A), Ferrel (C), Hadley (A), Hadley (C), Ferrel (A), Polar (C)
233
Which type or types of circulation cell are heat pumps?
Ferrel cell
234
Which type or types of circulation cell are heat engines?
Hadley and Polar cell
235
What is global warming?
It refers to the long term increase in temperature of the climate system that has been observed since the industrial revolution
236
What is climate change?
It refers to the change in global weather systems as a result of the temperature change from global warming
237
What are easterlies or easterly trade winds?
Beneath the Hadley cells, in the northern hemisphere, a southbound flow air gets deflected west by the Coriolis force (Northeasterly Trades) and in the southern hemisphere, a northbound flow of air gets deflected west by the Coriolis force (Southeasterly trades)
238
What are westerlies?
Beneath the Ferrel cells, in the northern hemisphere, a northbound flow air gets deflected west by the Coriolis force and in the southern hemisphere, a southbound flow of air gets deflected west by the Coriolis force, creating westerly winds
239
What are Polar easterlies?
Same as easterly trade winds but beneath the polar cells instead
240
What are the horse latitudes?
The lines where the Ferrel cells meet the Hadley cells at 30 degrees latitude north and south of the equator
241
What occurs at the horse latitudes?
These are areas of high pressure with no prevailing winds
242
Where and what are the doldrums?
They occur between the Hadley cells and are areas of low pressure. No Coriolis force on the equator so trade winds drop to zero
243
Why is there high pressure at the horse latitudes and low pressure at the doldrums?
At the horse latitudes, the air is drawn downwards to the surface and at the doldrums all the air is drawn upwards away from the surface
244
What deforms the idealised view of the global wind structure?
It is warped by continents and ocean heat transport
245
What is the intertropical convergence zone?
The same as the doldrums
246
What are the subtropical high areas?
Along the horse latitudes
247
What is the Polar vortex?
Large region of cold, rotating air that encircles both of Earth's polar regions
248
Is there high variability in the structure of the polar vortex?
Yes
249
What causes the currents in the ocean?
The Coriolis force combined with drag from the wind
250
What is a gyre?
A large system of rotating ocean currents
251
What are the first two stages of the North Atlantic currents/gyre?
It starts near the equator where trade winds drag the water west towards the gulf of Mexico then deflected northwards from the Coriolis force
252
What are the last two stages of the North Atlantic currents/gyre?
The water drives along the coast of North America and then dragged eastwards by westerly winds before hitting the European landmass and heading south from the Coriolis force to start the cycle again
253
Name the different ocean gyres
North Atlantic, South Atlantic, North Pacific, South Pacific, Indian Ocean and Antarctic Circumpolar current
254
Where is the Great Pacific Garbage patch?
North Pacific Gyre
255
Where are the tropics?
They are 23 degrees latitude north and south of the equator (slightly smaller than Hadley cells)
256
What is the Tropic of Cancer also called?
The Northern Tropic
257
What is the Tropic of Capricorn also called?
The Southern Tropic
258
What do the Tropics represent?
They mark the extent in latitude at which the sun can appear directly overhead
259
What happens at the intertropical convergence zone/ doldrums?
The upward air forces its vapour through its dew point, causing heavy precipitation in the tropics and a band of persistent thunderstorms
260
Where are the tropical rainforests likely to be situated and why?
Along the intertropical convergence zone because of the wet and warm conditions
261
Where is the location of the subtropics?
They extend from the tropics at 23 degrees to 35 degrees latitude both north and south
262
What are also in the subtropics and what is both of its names?
The horse latitudes or subtropical ridges
263
What happens in terms of the humidity at the subtropical ridges?
The air has low humidity even before compression because most moisture was expelled during the ascent of the air
264
What region is defined by warm dry air whose high pressure increases the dew point and surpasses precipitation? (lack of wind also makes rain rare)
Subtropical ridges or horse latitudes
265
What are biomes?
Large-scale environments that are distinguished by characteristic temperature ranges and amounts of precipitation
266
Is Venus' orbital period longer or shorter than its rotational period and what does this lead to?
Shorter, so the slow rotation means the Coriolis force is negligible
267
Why is Venus called a tropical atmosphere?
They only have 2 large Hadley cells
268
What does Mars' air circulation system look like at the equinoxes (spring- Vernal and autumn)?
There is a large Hadley cell centred over the equator with smaller counter rotating polar cells
269
What does Mars' air circulation system look like at the solstices (summer and winter)?
A single planet wide Hadley cell, which causes global dust circulation
270
Which planets are rapid rotators?
Jupiter and Saturn
271
Wind velocities are measured relative to their surface for terrestrial worlds, but giant planets lack this surface, so how is the rotation measured instead?
Relative to their magnetic field
272
Why does the poles on Jupiter receive less solar radiation than the equator?
It has a low axial tilt
273
How can we make a 1st order approximation of the planets composition?
Using the radius and mass to determine bulk density
274
Does the temperature of stars have a large effect of the circulation on their planets?
Yes
275
What is the continuous spectrum of a blackbody dependent on?
Its temperature
276
As the temperature of a blackbody increases, what happens to its spectrum?
The total amount of light emitter per second increases and thus the peak of the blackbody spectrum shifts to shorter wavelengths (bluer)
277
What colour stars relate to the hotter/colder stars?
Hotter stars are more blue whilst colder stars are more red
278
Can stars be approximated as blackbodies?
Yes
279
What does the intensity in Planck's equation for blackbodies depend on?
Wavelength and temperature
280
For short wavelengths, the Planck equation for blackbodies can be approximated with what equation instead?
Wien approximation
281
For long wavelengths, the Planck equation for blackbodies can be approximated with what equation instead?
Rayleigh-Jeans approximation
282
What is Wien's law?
The peak wavelength of a blackbodies spectrum multiplied by its temperature is equal to a constant
283
What does it mean that blackbodies are Lambertian?
They have the same radiance/luminance when viewed from any angle and follows Lambert's cosine law
284
What is Lambert's cosine law?
The intensity of a surface is directly proportional to the cosine of the angle between the normal to the surface and the direction of incident light
285
The total power output or luminosity for a source is given by the flux multiplied by what?
The surface area
286
How can flux be defined?
Rate of energy transfer that crosses a unit area or power output per area or luminosity per unit area
287
The flux of an astronomical source as measured from Earth depends on what?
The luminosity of the object and its distance from the Earth
288
What does it mean that an object is in thermodynamic equilibrium?
It emits the same amount of energy as it absorbs
289
What is the albedo?
The amount of light reflected by the planet from 0 (blackbody that absorbs all incident radiation) to 1 (body that reflects all incident radiation)
290
What is the bond albedo?
The total amount of EM radiation reflected back to space over all wavelengths and phase angles between 0 to 1
291
What is the most simple model we can used to model the atmosphere?
A blanket that absorbs all infrared radiation emitted by the Earth and then re-emits the radiation so the Earth is secondarily heated by the atmosphere
292
What is wrong about the model that approximates the atmosphere as a blanket?
It assumes all infrared radiation is absorbed which isn't true
293
What update can we make to modelling the atmosphere as a blanket to improve it?
Model it with transmission, where the atmosphere is transparent to a fraction of the incoming solar radiation down to the ground and a fraction of the ground radiation out into space
294
The transmission of the Earths atmosphere can be estimated with about what value?
90%
295
The transmittance of thermal radiation is around what value? (going out of the atmosphere from Earth)
20%
296
What is an absorption process?
A process when electromagnetic radiation are transformed into internal energy (heat) through interactions with electrons in atoms and molecules
297
Why must any heating of the atmosphere be due to absorption?
The absorption process in which the internal energy of the atmosphere is increased
298
When is the only time a vibrational mode of a molecule can only be induced by infrared absorption?
The molecule has an electric dipole moment
299
What is an electric dipole?
Structural separation of positive and negative charges
300
How does IR absorption induce a vibration on a dipole?
The wavelength of IR is larger than that of a molecule so the molecule sees a uniform electric field from the IR and this field pushes all the positive and negative charges in opposite directions and causes a fluctuation in the dipole moment
301
Molecules with a pronounced dipole moment will absorb in what section of the EM spectrum?
Infrared
302
What do we call good infrared absorbers present in the Earth's atmosphere?
Greenhouse gases
303
What is a blackbody?
A body that absorbs all wavelengths of light and no light is reflected
304
What are the most common greenhouse gases?
Water, carbon dioxide and methane
305
Who wrote the paper for the first experimental evidence showing the heat-trapping capability of carbon dioxide?
Eunice Foote
306
What is Lambert's law or Beer-Lambert's law?
The spectral radiance as a function of the distance travelled moving down the beam of radiation passing through a column of air (radiance decreases as it passes through gas, which absorbs some of it)
307
How does Schwarzchild's equation extend from Lambert's (Beer-Lambert's) law?
It takes into account that gases do not just absorb EM radiation but also, once heated, will also emit radiation
308
What is optical depth?
A dimensionless measure of how far the light has penetrated down into the atmosphere from the top ie how much stuff the light will pass through
309
What is local thermal equilibrium (LTE)?
Each small part of the system if in thermal equilibrium even though the whole is not
310
What is the absorptivity? (denoted by an alpha)
The fraction of the radiance incident on a material's surface that is absorbed
311
What is the spectral absorptivity?
The fraction of the spectral (per frequency or wavelength) radiance incident on a material's surface that is absorbed
312
What is the emissivity?
The fraction that real materials emit energy compared to the level of energy emitted from a blackbody
313
What is Kirchhoff's law of thermal radiation?
Spectral absorptivity equals spectral emissivity for a body in thermal equilibrium with its environment (including local thermal equilibrium)
314
What assumptions can we make for the grey atmosphere model?
Atmosphere is in local thermal radiative and local thermodynamic equilibrium. Flat Earth and extinction coefficient is the same for all frequencies
315
For the grey atmosphere model, the assumptions that the atmosphere is in (1) local thermal radiative equilibrium and (2) local thermodynamic equilibrium means we can do what?
(1) We can replace the source function with the black body radiance and (2) we can ignore heat transfer
316
What is the Eddington approximation and when is this exact?
The pressure is a third of the energy density and when the radiance is the same in all directions
317
What is the skin temperature?
The temperature at the very top of the atmosphere with no downward flux density (as the atmosphere is transparent to solar radiation)
318
What is the effective temperature of an object?
The temperature that a blackbody would have radiating the same flux density as the object
319
At the top of the atmosphere, what is the flux density direction?
Only up
320
What is the emission height?
The altitude at which the temperature of the atmosphere is equal to the effective temperature
321
What optical depth is the emission height in our atmosphere?
2/3
322
Is there a discontinuity in temperature between the ground and the air immediately above the ground and what does this mean physically?
Yes and that means we predict the air is slightly cooler than the ground
323
The total optical depth of the atmosphere down to ground level increases or decreases with more greenhouse gases and what does this have an effect on?
Increase and this then increases the ground temperature
324
Why is it not true to reality to model the Earth's temperature assuming equilibrium conditions?
The Earth's climate is a dynamic system and changes in global temperature are exacerbated by human action
325
The net radiative flux contributing to the radiative heating of the Earth is given by the difference between what?
The flux incident from the sun and the flux emitted by the Earth- both are dynamic variables that depend on the level of reflection and absorption
326
What is a feedback?
A mechanism by which a change in the Earth's temperature changes the net radiative flux, hence changing the temperature more
327
What is the Ice-Albedo feedback?
A climate process where a change in the area of different ice types alters the albedo and surface temperature of a planet with the change in reflectivity
328
What does the Albedo depend on?
The amount of snow cover over which has a dependence on temperature
329
What does the first part of the Albedo against temperature graph look like and why? (coldest section)
Highest Albedo as it is a mix of ice/snow, and this is a horizontal line until a point where it decreases suddenly
330
What does the second part of the Albedo against temperature graph look like? (intermediate temperature)
Lowest Albedo as it is water and this is a horizontal line until a point where it suddenly increases
331
What does the third part of the Albedo against temperature graph look like? (hottest section)
Intermediate Albedo between that of ice/snow and water and is the clouds section
332
For the flux against temperature graph for the ice-albedo feedback, where do the graphs of flux in and flux out cross and are they stable or unstable points?
Total ice coverage, ice world (stable), partial ice coverage (unstable) and zero ice coverage, water world (stable)
333
For the flux against temperature graph for the ice-albedo feedback, when the flux in is larger than the flux out, is the derivative of temperature wrt time positive or negative?
Positive
334
Where is the Earth on the flux against temperature graph?
A roughly stable point where the system is not trying to increase or decrease
335
If you move the Earth off its roughly stable point on the flux against temperature graph to a different temperature, what will happen?
It will increase or decrease in temperature until it reaches the next stable temperature
336
What is radiative forcing?
The change in energy flux in the atmosphere caused by natural or human factors measured in watts per metres squared
337
What does positive radiative forcing mean?
Earth receives more incoming energy from sunlight than it radiates to space and causes warming
338
The radiative feedback is denoted with an alpha and is minus the partial derivative of net flux with respect to what?
temperature
339
What would a negative feedback parameter mean?
As a body gets hotter it gets less efficient at radiating that heat away
340
What is climate sensitivity?
The increase in the amount of f (radiative forcing) divided by alpha (radiative feedback)
341
What is the feedback response time?
The increase occurs over time scale c (heat capacity) over alpha (radiative feedback)
342
What does the radiative feedback parameter show?
The efficiency with which you are radiating heat away as you get hotter
343
Radiative forcing is approximately proportional to the logarithm of the factor of what?
The factor by which the density of absorbing gases has increased
344
Do greenhouse gases produce positive or negative radiative forcing?
Positive
345
Does the ozone produced positive or negative radiative forcing?
Either depending on where in the atmosphere it is
346
Do clouds and other aerosols produce positive or negative radiative forcing?
Negative
347
What are inviscid fluids and will this be the only type we consider?
Non-viscous fluids and yes
348
What is a fluid?
Any substance which deforms continuously under a sheer stress or any substance which flows (informal)
349
What are examples of fluids?
Liquids, gases and plasmas
350
How does field theory model fluids?
Treating the whole fluid as a single deforming object with properties which vary continuously in space
351
What 2 properties are we focusing on with fluids?
Density and velocity
352
How should we think about local fluid density?
We can draw a control volume and divided by the mass in it and in the limit where the volume shrinks to a point we have the local density
353
How should we think about fluid velocity?
We can draw a control volume and then deform its borders as the fluid flows such that no material enters or leaves, so that object has a well-defined velocity. A velocity at a point is the limit where the volume shrinks to a point
354
What are the two types of forces acting on a given fluid element?
Body forces like gravity and surface forces due to neighbouring elements
355
What two groups can surface forces be divided into?
Pressure forces from neighbouring elements banging and pressing up against the sides and viscous forces from the neighbouring elements sticking and scraping against the sides (will be ignoring viscous forces moving forward)
356
What is the direction of the net force due to the pressure on a fluid element and what sign is it?
Inwards so negative
357
What is the Lagrangian specification for a frame of reference and do we want this?
Frame of reference moved and deforms with fluid and no
358
What is the Eulerian specification for a frame of reference and do we want this?
Frame of reference fixed as fluid moves and deforms and yes
359
What is the material derivative (or the advective derivative)?
The equation that converts the Langrangian specification to the Eulerian specification
360
What is the continuity equation a statement of in fluid mechanics?
Conservation of mass
361
Incompressible flow is also known as what?
Isochoric flow
362
What equations can be used to define incompressible flow?
The derivative of density wrt time equals zero OR (better) the divergence of the velocity is zero
363
What can we treat as incompressible?
Liquids (like water) and generally gases too (they are only weakly compressible)
364
How is hydrostatic equilibrium defined for a fluid?
The state of a fluid which is not moving
365
What is a streamline?
A curve whose tangent at any point is given by the local fluid velocity vector and they give instantaneous directions for a test particle dropped in the fluid
366
What are streaklines also known as and why?
Dye lines because we can imagine it as if we continuously inject dye at a point in the fluid
367
What is a streakline?
It is the line consisting of all the points where the fluid has passed through some fixed point during its motion.
368
What is a pathline?
It gives the trajectory of a selected fluid element moving away from a particular starting point
369
What is the difference between pathlines and streaklines?
Finding them are equivalent except the initial conditions and the interpretation is different
370
How are the streaklines and pathlines parameterised?
Streaklines by tau (they go up in tau) and can be drawn up to any reference time t. Pathlines start at t=0 and goes on forever in t.
371
When can we assume two-dimensional fluid flow and an example?
The velocity component in one direction is negligibly small, like flow over an even surface or flow in thin films. Wind across the Earth's surface can be treated as 2D
372
What is the stream function?
A scalar function that satisfies the solution for a incompressible flow in 2D
373
What is the velocity in terms of the stream function?
The curl of (the stream function times by the unit z vector)
374
How are streamlines and the stream function connected?
The stream function is constant on stream lines
375
The stream function is constant on streamlines and there is no flux through the boundaries of a system, so the stream function is constant on boundaries. What does this mean?
Boundaries are streamlines
376
What is the vorticity and its symbol?
The local 'spinning' of a fluid and it is an omega symbol.
377
What does the vector of vorticity give you?
It gives the axis and magnitude of rotation of a stick dropped in the fluid at a particular point
378
What is the equation for the vorticity?
The curl of the local velcoity
379
What equation is true for irrotational fluid?
The vorticity is zero so the curl of the velocity is zero
380
A fluid is irrotational when the path lines are what to each other?
Parallel
381
What can we say if the vorticity is zero everywhere at any given time?
It will always be zero everywhere
382
What properties does the weak form of the Bernoulli's principle require the fluid to have?
Inviscid, incompressible, steady and irrotational
383
If a fluid is steady, what does this mean?
Partial derivatives wrt time is zero
384
What does the 'head' in Bernoulli's principles consist of?
The velocity head and the hydraulic head, which itself has two parts- the pressure head and the elevation
385
For the weak form of Bernoulli's principle, what can be said about the total head and what does this mean?
The total head is a constant throughout the fluid and this means the local energy density is a constant across teh fluid
386
For the weak form of Bernoulli's principle, what can be said about the total head and what does this mean?
The total head is a constant throughout the fluid and this means the local energy density is a constant across the fluid
387
What properties does the strong form of the Bernoulli's principle require the fluid to have?
Inviscid, incompressible and steady (it can be rotational)
388
For the strong form of Bernoulli's principle, what can we say about the total head?
The total head is constant along streamlines
389
What properties does the compressible form of the Bernoulli's principle require the fluid to have?
Inviscid, steady and irrotational (it can be compressible)
390
For the compressible form of Bernoulli's principle, what can we say about the total head?
The total head is constant and takes a weird form with an integral in it
391
What is the velocity potential?
The scalar field for velocity when the fluid is irrotational, incompressible and inviscid
392
When a velocity potential exists, what do we call the flow?
'Potential flow'
393
Why do we know the velocity potential exists for irrotational fluids?
The curl of a gradient is zero so this satisfies the irrotational condition
394
What is a gravity wave?
The vertical motion of water driven by gravity
395
What are the three sections of a gravity wave that we treat separately?
Free surface, bulk and seabed
396
What assumptions do we make for gravity waves?
The water is irrotational and incompressible and at first we ignore the y direction so only 2D
397
What is the only thing we need to consider in the bulk of gravity waves?
The incompressibility (divergence of velocity is zero)
398
What do we assume about the free surface of gravity waves and how are they characterised?
The mean sea level is z=0 and there is no breaking at the surface so a fluid element on the surface stays on the surface
399
Why can non-linear terms be ignored for gravity waves solutions?
We assume the height of the water above sea level and fluid velocity are only small perturbations
400
What does the solution for the gravity waves (or water waves) look like?
A dispersion relation (omega/ frequency squared)
401
When can we use the shallow water equations?
Any thin layer of incompressible fluid with vertical waves moving under gravity, when the the wavelength is much greater than the depth
402
What are Poincare waves?
Waves that satisfy the solutions for rotating shallow water, whose path lines look like ellipses. They are affected by gravity and the Coriolis force
403
How are coastal Kelvin waves formed?
They are Poincare waves that get stopped by the coast and reflected and trapped
404
What do Kelvin waves look like?
After the fluid element gets reflected by the coast its direction gets reversed and created a new ellipse, taking if further along the coast
405
What direction do coastal Kelvin waves travel in compared to the great ocean gyres?
Opposite directions
406
Where else do you get Kelvin waves and why?
Around the equator because the Coriolis parameter changes sign
407
What is the El Nino in the South Pacific?
Periodic equatorial Kelvin waves form and carry warm water to the coast and make coastal Kelvin waves along Peru
408
What does the warm water with the El Nino also bring?
High air pressure and an increase in humidity, which brings extreme weather phenomena
409
What defines geostrophic wind?
Wind velocity is non-zero but it is not materially accelerating (zero material derivative)
410
When does geostrophic wind occur?
The pressure gradient balances with gravity and the Coriolis force
411
What assumptions do you need to make for geostrophic wind?
The Rossby number is small, mean velocity divided by system size is smaller than the Coriolis parameter, no frictional forces (not near ground level), no variations in Coriolis parameter (small enough system), Coriolis parameter never zero
412
In geostrophic flow, the stream function determines what type of surfaces? (not the same as but is a function of it)
Isobaric surfaces
413
What are isobars on weather maps?
They are curves of constant pressure at a fixed height
414
In geostrophic flow, isobars are what?
Streamlines
415
There is a right hand rule, where the thumb points upwards, the index finger points along the pressure gradient towards increasing pressure. What does the middle finger represent?
The direction of the wind
416
What are low-pressure systems called and what do they do?
They are called cyclones and they spin anti-clockwise in the northern hemisphere (opposite in southern where f is negative)
417
What are high-pressure systems called and what do they do?
Anticyclones and the spin clockwise in the northern hemisphere (opposite in southern where f is negative)
418
What other names for a tropical cyclone depending on where they happen?
A hurricane (North Atlantic), typhoons (South Pacific)
419
What is a tropical cyclone?
A rapidly rotating wind with a very low pressure core at its centre
420
Can we use geostrophic approximations for tropical cyclones and why?
No because the wind speeds are very high so the Rossby number can be very high
421
What do we use to model a hurricane/ tropical cyclone?
The incompressible Euler equations in a rotating frame in cylindrical coordinates
422
What is gradient wind?
Wind that accounts for air flow along a curved trajectory and works well for hurricanes
423
In the eye of a hurricane, how does it rotate?
Rigid body motion
424
For a hurricane, what is the relationship between velocity and radius (from the centre) for small and large r?
For small r, it is directly proportional. For large r, it is inversely proportional
425
What is the thermal wind shear?
The velocity difference between neighbouring isobaric surfaces (velocity gradient) runs at right angles to the temperature gradient
426
What two factors should be known about thermal wind?
Its direction is perpendicular to the temperature gradient and its size depends on the magnitude of the pressure drop
427
When is thermal wind the fastest?
The variation in temperature is greatest and where there are sharp pressure drops between neighbouring isobaric surfaces
428
Where in the atmosphere does the jet stream occur and the thermal wind is fastest?
Around the tropopause, where the Hadley and Ferrel cells meet and where the Ferrel and polar cells meet
429
What are jet streams and what are the different ones called?
Very fast currents. They occur at latitudes of around 60 degrees called the polar jet and around 30 degrees for the subtropical jet
430
What direction do the jet streams flow in both hemispheres?
West to east in both hemispheres and at rights angles to the variation in temperature (increases in the direction of the equator)
431
What shape and size are jet streams?
Shaped like ribbons, around 5km vertically, few hundred kilometres width and a thousands of kilometres long
432
Does the jet stream move over the course of the year and why?
Yes because it follows temperature gradients
433
What are Rossby waves?
Waves on the jet stream like waves on a string
434
What regions do jet streams separate?
Cool polar air and warm tropical air
435
Why can a trough in the Rossby waves be associated with a region of low pressure?
Pressure gradients run at right angles to the velocity
436
What happens if a trough in a Rossby wave is 'pinched off'?
A cyclone will form
437
For meteorology length scales, what method of heat transport should we expect?
Solely by advection and we assume diffusion is negligible
438
What is advection?
The motion of particles along the bulk flow
439
What is diffusion?
The net movement of particles from high concentration to low concentration
440
What are air masses?
Well-defined patches of warm and cold air with clear boundaries, called fronts and they determine ground-level weather
441
Why is the typical configuration of fronts involve a wedge of cold air slipping beneath a region of warm air?
Hot air is lighter than cold air
442
What are the leading edge of an air mass called depending on its temperature and movement?
Warm front, cold front or stationary front if not moving
443
From the ground-level perspective, what determines if a front is a warm or cold front?
If the boundary moves towards or away from us
444
What symbols represent the different types of fronts on weather maps?
Blue triangles for cold fronts, red circles for warm fronts, mix of box for stationary fronts and purple triangles and circles for occluded fronts
445
What properties should we expect to change when crossing a weather front?
Sharp jump in temperature and density but a continuous variation in pressure
446
What could we find from Margule's formula?
The velocity on either side of a weather front
447
What happens at the point of occlusion where to fronts meet?
There would be a cusp, which has a very low pressure (near zero), which can give rise to a cyclonic pressure system
448
What type of front travels faster than the other and why?
Cold front are usually faster than warm fronts because they have a higher inertia
449
How are occluded fronts formed?
A cold front overtakes a warm front and the cold region slips below the warm region and lifts it above the ground
450
At the interface between fronts, hot air gets cooled or cold air gets warmed and what does this change for the air?
The dew point and hence the weather
451
Name some cloud types
Cumulonimbus, nimbostratus, cumulus, stratus, stratocumulus, altocumulus, altostratus, cirrocumulus, cirrostratus, cirrus
452
Does the air temperature affect dew point and why?
As temperature increases and air expands, the air has more space to absorb water. The dew point increases with an increase in air temperature
453
For the duration of a cold front passing, what weather and clouds can you expect and why?
Rain because the air's water content hits its dew point and condense into rain. The clouds are cumulonimbus or nimbostratus
454
For the duration of a warm front passing, what weather and clouds can you expect and why?
Rain because the warm air will be lifted above the cold air mass and as it ascends its moisture content will condense. This can turn into snow if the cold air is cold enough (eg winter). The clouds are cumulonimbus or nimbostratus
455
What weather happens at ground-level around a stationary front?
Rain until the front shifts
456
What weather happens ground-level under occluded fronts?
Sudden stormy weather that is suddenly cool and wind speeds will increase
457
What are warm and cold occlusions?
If the air behind the cold front is warmer than the air ahead of the warm front then it is a warm occlusion and warmer conditions will follow. If the air behind the cold front is colder than the air ahead of the warm front then it is a cold occlusion.
458
What are the big six air masses that affect the UK?
Polar Maritime, returning Polar Maritime, Arctic Maritime, Polar Continental, Tropical Continental and Tropical Maritime
459
What are the conditions in which air masses originate?
Fairly uniform and consistent conditions
460
Are maritime air masses (over oceans) more or less humid than continental air masses (over land)?
More
461
Where does the Polar Maritime air mass originate and how does it travel to us?
Over Greenland and prevailing north-westerly winds bring it here (most common air mass)
462
What are some of the characteristics of the Polar Maritime air mass?
Starts as cold and dry but gets warm and wetted as it passes over the Atlantic. It is heated from below so carries frequent showers
463
Where does the returning Polar Maritime air mass originate and how does it travel to us?
Over Greenland (same as Polar Maritime) but takes a longer route, as it travels south into the Atlantic before returning northwards and coming to us
464
What are some of the characteristics of the returning Polar Maritime air mass?
Unstable at the beginning of its journey but when it reaches us its more stable and leads to dry and cloudy weather
465
Where does the Arctic Maritime air mass originate?
Over the Arctic ocean
466
Where does the Polar Continental air mass originate and when does it usually come to us?
Over vast snow fields in Eastern Europe and Russia and arrives late winter/early spring
467
What are some of the characteristics of the Arctic Maritime air mass?
Cold and dry and when it reaches land over Scotland its heated from below, leading to instability (hail and snow)
468
What are some of the characteristics of the Polar Continental air mass?
Very dry and very cold
469
Where does the Tropical Maritime air mass originate?
In the warm waters of the Atlantic (Sargasso sea)
470
Where does the Tropical Continental air mass originate?
In North Africa or over the Sahara
471
What are some of the characteristics of the Tropical Maritime air mass?
Starts warm and wet and gets cooled from below as it travels, increasing its stability. Brings us warm to mild weather with possible drizzle
472
What are some of the characteristics of the Tropical Continental air mass?
Uniformly warm and stable but frequently picks up dust for Saharan sandstorms and pollution from being over Europe so hot and hazy weather
473
When air is warmed what will it do and how will this affect the pressure? (opposite for when it is cooled)
It will ascend and create a low pressure system
474
Why is the sky blue and why are sunsets red?
Rayleigh scattering is the scattering of light by particles smaller than the wavelength. It prefers to scatter bluer light, so this scatters upwards to the sky and when the sun sets we see more of the red end of the scattering
475
Why is the red effect of sun sets stronger for more polluted or smoky areas?
Larger particulates scatter more strongly
476
Which directions does the sun set and rise?
Rises in the East and sets in the West
477
Why will light seem more red at dawn or dusk if it passes through a high pressure system?
Trapped aerosols
478
If you see a red sky at night, where is the high pressure system?
To the West
479
If you see a red sky in the morning, where is the high pressure system?
To the East
480
What type of winds do we get in the UK (direction) and so is an air mass to your west moving towards or away from you?
Westerlies (west to east) so moving towards you
481
Is a high or low pressure system associated with a descending warm air mass and still, pleasant weather?
High
482
What is the stability?
It is the degree of resistance of a layer of air to vertical motion (so if it is stable, convection won't happen)
483
Is the temperature inversion stable or unstable and why?
Extremely stable, as temperature increases with altitude so the lapse rate is lower than the dry adiabatic lapse rate
