Topic 1b: Moist Thermodynamics

Question 1

What is the composition of the atmosphere?

Answer 1

Nitrogen (78%)

Oxygen (21%)

Argon (1%)

Water vapour (0-5%)

Question 2

What does Md stand for?

Answer 2

molecular weight of dry air. Md = 28.97

Question 3

How do you find the mass of moist air?

Answer 3

Madd = Cd Md + Cv Mv

= Cd x 28.97 + Cv x18.02

note: Cd + Cv = 1

Question 4

Moist air is ___ dense than dry air. why?

Answer 4

less. because the water vapour molecules replace heavier molecules.

Question 5

what is Daltons law of partial pressures:

Answer 5

Total pressure exerted by a mixture of gases that dont react chemically is equal to the sum of the partial pressures of each gas.

Question 6

What is the partial pressure law for moist air?

Answer 6

P = p’d + e

Where p’d is the partial pressure of dry air

e = the partial pressure of water vapour

Question 7

What expression can be used to determine partial pressure of dry air?

Answer 7

p’d = rho’d Rd T

Question 8

What expression can be used to determine partial pressure of water vapour/

Answer 8

e = rho’v Rv T

where Rv = 461. 5 JK^-1Kg^-1

Question 9

What is the value of ‘3’?

Answer 9

0.622

Rd/Rv

Question 10

What does this equation tell you:

Answer 10

density of moist air. an increase in e (water vapour) decreases the density

Question 11

Define virtual temperature? formula?

Answer 11

Virtual temperature is the temperature that dry air would need to attain to have the same density as the moist air at the same temperature.

note virtual temperature is always greater than actual temperature

Question 12

What are thee measures of water vapour content in air?

1.

2.

3.

Answer 12

1. e = partial pressure in Pa or hPa
2. mixing ratio = Mv/Md
3. q = specific humidity = w / (1+w) given in g/kg

Question 13

When there is no condensation or evaporation mixing ratio is…

Answer 13

conserved

Question 14

Define vitual potential temperature

Answer 14

virtual theta is conserved in the absence of condensation. Equation is a better measure of stability in a moist atmosphere.

Question 15

Define saturation

Answer 15

The point at which air holds as much water vapour as it can. this amount decreases when temperature decreases.

Question 16

What is es

Answer 16

saturation vapour pressure. The pressure exerted by vapour in air when the air is saturated.

Question 17

If e > es there is

Answer 17

net condensation

Question 18

if e < es there will be

Answer 18

net evaporation if there is a source of water

Question 19

What is the Bergeron effect?

Answer 19

the growth of ice crystals in an environment which contains supercooled water

Question 20

Define relative humidity. formula?

Answer 20

ratio of the actual mixing ratio to the saturation mixing ratio.

RH = 100 x w/ws

Question 21

What is the RH when w=ws?

Answer 21

100%

Question 22

When w = 0 what is RH?

Answer 22

0%

Question 23

At a constant pressure what happens when air is cooled?

Answer 23

es reduces and ws reduces unstill the air is saturated. at this point it reaches the dew point temperature Td

Question 24

Define dew point temperature

Answer 24

temperature to which air must be cooled (at a constant pressure) for it to become saturated.

Question 25

What is the physical meaning of latent heat

Answer 25

The energy input required to change liquid water to water vapour without a change in temeprature. Condensation releases latent heat and induces warming. evaporation requires latent heat and induces cooling

Question 26

What is the lifting condenstation level?

Answer 26

The height at which an air parcel is saturated and begins to condense (w = ws)

Question 27

What happens when a cloud is forced over a mountain?

Answer 27

increase in potential temperature on lee (away from wind) side.

increase in tempeature at same level

decrease in water vapour content

warm & dry winds