Water vapour, evaporation and precipitation

Question 1

The rate oof change of pressure, P, with elevation or altitude z is given by

Answer 1

dP/dz = -ρg

Question 2

what is the equation for density of dry air

Answer 2

ρda = P/RdaT

Question 3

what is Rda

Answer 3

287 J/kg/K

Question 4

What is the average near surface rate of decrease of temperature with altitude

Answer 4

dT/dz = -6.5 C°/km

Question 5

what is the density of air at sea-level pressure, and density of dry air

Answer 5

Pa = 101.3kPa
ρda = 352/T

Question 6

what is the equation for the density of water vapour

Answer 6

ρv = e/RvT

Question 7

What is Rv equal to

Answer 7

461 J/kg/K

Question 8

define atmospheric pressure

Answer 8

the sum of the partial pressures of the gases in the atmosphere
P = Pda + e

Question 9

define vapor pressure

Answer 9

the partial pressure of water vapour, e

Question 10

Is the mass of water greater or less than the mass of dry air

Answer 10

Mass of H2O is less than the mass of dry air

Question 11

moist air can be treated as a mixture of

Answer 11

dry air an water vapour
ρma = ρda + e

Question 12

Equation to find the molecular weight of moist air

Answer 12

Mma = e/ρma MH2O + ρda/ρma Mda

Question 13

is the density of moist air greater than or less than the density of dry air

Answer 13

density of moist air is less than the density of dry air
ρma < ρda

Question 14

what is the equation for the density of moist air

Answer 14

ρma = P/RdaT (1 - 0.378 e/P)

Question 15

What are the 6 ways to quantify water vapor

Answer 15

vapor pressure
absolute humidity
specific humidity
relative humidify
Dew point
precipitable water

Question 16

what is vapour pressure

Answer 16

the partial pressure of water vapour in the air

Question 17

What is e*

Answer 17

saturation vapour pressure. It is the maximum vapor pressure that is thermodynamically stable. Also, the maximum amount of water vapor that the atmosphere can hold at that temperature.

Question 18

e* is a function of

Answer 18

temperature

Question 19

what is the equation for e* for a liquid, T > 0 C

Answer 19

e* = 611 e^(17.27T/(T + 237.3))

Question 20

What is the equation for e* for ice, T < 0C

Answer 20

e* = 611 e^(21.87T/(T + 265.5))

Question 21

If we are at e*, how can we get condensation to occur

Answer 21

lowering of the temp or adding more water vapor

Question 22

describe absolute humidity

Answer 22

water vapor density
ρv
the mass of water vapour per unit volume of air

Question 23

equation for the density of water vapor / absolute humidity

Answer 23

ρv = e/RvT

Question 24

describe the specific humidity

Answer 24

q = ρv/ρda = 0.622 e/P
ratio of water vapor density to the dry air density

Question 25

describe relative humidity

Answer 25

ratio of the water vapor pressure to the saturation vapor pressure e*

Question 26

describe dew point

Answer 26

The temperature to which a mass of air with a given vapor pressure has to be cooled in order to reach saturation point Tdp

Question 27

describe precipitable water

Answer 27

If all the amount of water vapor in a column of the atmosphere of unit area extending from the surface to the top of the atmosphere is condensed into liquid water

Question 28

Describe the 5 steps of the evaporation process

Answer 28

1. Assume that the vapor pressure equals the saturation vapor pressure at the surface temp (e*(Ts) = e(Ts)) 2. the water molecules at the surface are attracted to the ones under it. However, some have enough energy to break the hydrogen bonds and go above the surface. This escape energy increases as Ts increases 3. Some water mcl may re-enter the water. equilibrium of entry and exit is reached 4. water vapor in the air is at steady state, it cant increase anymore. ea (actual saturation) < e* because wind and temp prevents the air to be completely saturated. 5. evaporation rate ∝ (es* - ea) ( So if ea << es* the air has a high capacity to hold more water vapor and evaporation occurs faster.

Question 29

Is the air dry or humid if ea is almost equal to es*

Answer 29

humid

Question 30

Is the air dry or humid if ea is a lots less than es*

Answer 30

dry

Question 31

If es* > ea we have

Answer 31

evaporation

Question 32

is es* < ea we have

Answer 32

condensation

Question 33

is es* = ea we have

Answer 33

nothing

Question 34

What happens when humidity is 100%

Answer 34

Then we know es* = ea and then the evaporating water will sondense in the air to form fog or mist

Question 35

The actual vapor pressure 𝑒𝑎 of water in the air at a given temperature 𝑇𝑎 reflects the amount of water vapor present. The saturation vapor pressure 𝑒∗(𝑇𝑎), on the other hand, represents the _________ vapor pressure the air can hold at that temperature before becoming __________.

Answer 35

maximum saturated

Question 36

𝑒𝑎 can be less than or equal to 𝑒∗(𝑇𝑎) . This means the air can either be _________ (relative humidity <100%) or ______ _________ (𝑅𝐻=100%).

Answer 36

undersaturated fully saturated

Question 37

Evaporation will occur even if the relative humidity equals 100%, as long as 𝑒s* ___ 𝑒a

Answer 37

>

Question 38

Evaporation occurs in exactly the same way whether the underlying surface is ________ or _________. The only difference is that the saturation vapor pressure/evaporation for an ice surface at a given temperature is slightly _______

Answer 38

liquid water ice/snow lower

Question 39

what are the 3 inferences we can make from the evaporation process

Answer 39

Inference #1: es* > ea we have evaporation, opposite is condensation Inference #2: When humidity = 100% we have fog and mist. evaporation happens as long as es* > ea Inference #3: Evaporation occurs in exactly the same way whether the underlying surface is liquid water or ice/snow. The only difference is that the saturation vapor pressure/evaporation for an ice surface at a given temperature is slightly lower

Question 40

The latent heat of ____________ λv, is the quantity of heat energy that is absorbed to break the hydrogen bonds when __________ takes place. This same quantity is released when the bonds are reformed upon _____________

Answer 40

vaporization evaporation condensation

Question 41

__________ is always accompanied by a transfer of heat out of the evaporating surface; i.e., the surface cools.

Answer 41

evaporation

Question 42

_________ in the atmosphere is always accompanied by a transfer of heat into the air; i.e., the air warms.

Answer 42

condensation

Question 43

__________ on the surface is always accompanied by transfer of heat to the surface; i.e., the surface warms.

Answer 43

condensation

Question 44

what are the two cases for latent heat exchange

Answer 44

Case #1: The rate oof latent heat-transfer in evaporation at water surface Case #2: The rate of latent-heat transfer in sublimation at ice surface

Question 45

What is the equation for the first case of latent heat exchange: The rate oof latent heat-transfer in evaporation at water surface

Answer 45

λE = λv ρw E

Question 46

what is the equation for the second case of latent heat exchange: The rate of latent-heat transfer in sublimation at ice surface

Answer 46

λE = ( λf + λv) ρw E

Question 47

What is the difference between the equation to find the rate of latent heat-transfer for evaporation vs sublimation

Answer 47

the evaporation only considers the latent heat of fusion λv The sublimation is the sum of the latent heat of vaporization and latent heat of fusion, λf + λv

Question 48

Is the latent heat of fusion smaller or larger than the latent heat of vaporization

Answer 48

smaller

Question 49

does λv and λf depend on temp

Answer 49

λf doesnt depend on Temp. whereas λv does

Question 50

What are the 4 steps to the precipitation process

Answer 50

1. Cooling to the dew point 2. Condensation 3. Water droplet growth 4. Importation of water vapor

Question 51

Describe the first step of the precipitation process

Answer 51

Cooling: Without condensation: air cools at the dry adiabatic lapse rate Γda = 1∘C/100 m With condensation: The release of latent heat reduces cooling to the saturated adiabatic lapse rate: Γs 0.5∘C/100 m weighted average: of the previous two Γ = 0.65∘C/100 m Rising air expands and density decreases as pressure decreases Uplift rates: 1m/s for frontal zones and 30 m/s for thunderstorm. The thunderstorm drives stronger colling and precipitation

Question 52

describe the second step of the precipitation process

Answer 52

Condensation: When air above the freezing point is cooled to the dew-point, liquid water is thermodynamically stable and randomly colliding water-vapor molecules tend to form tiny water droplets.

Question 53

_____________ releases latent heat, which tends to sustain the _____________ process by warming/heating the surrounding air, reducing its density (a), and increasing its tendency to rise, cool, and produce more ____________.

Answer 53

condensation precipitation condensation

Question 54

For example, some 10^8 (100 million) molecules (H2O) must collect to form a stable water droplet of about _____ diameter.

Answer 54

10^–4 mm (0.1 microm)

Question 55

Describe the third step of the precipitation process

Answer 55

Condensation clouds form when: 1. water vapor is present 2. there is sufficient degree of uplift to bring cooling to the dew point In order for precipitation to fall from clouds, droplets must grow in size such that: 1. falling velocities exceeds the rate of uplift 2. can survive evaporation as they fall The droplets grow in size by: 1. coalescence 2. The e* of ice < e* liquid. This difference allows water to evporate and condense into ice. So they ice grows at the expense of the liwuid water

Question 56

describe the fourth step of the precipitation process

Answer 56

Importation of water vapour: shows that in order to generate significant amounts of precipitation, vapor-bearing winds must continuously provide water vapour to precipitation-producing clouds, which is called importation of water vapour.