Topic 1b: Moist Thermodynamics Flashcards
What is the composition of the atmosphere?
Nitrogen (78%)
Oxygen (21%)
Argon (1%)
Water vapour (0-5%)
What does Md stand for?
molecular weight of dry air. Md = 28.97
How do you find the mass of moist air?
Madd = Cd Md + Cv Mv
= Cd x 28.97 + Cv x18.02
note: Cd + Cv = 1
Moist air is ___ dense than dry air. why?
less. because the water vapour molecules replace heavier molecules.
what is Daltons law of partial pressures:
Total pressure exerted by a mixture of gases that dont react chemically is equal to the sum of the partial pressures of each gas.
What is the partial pressure law for moist air?
P = p’d + e
Where p’d is the partial pressure of dry air
e = the partial pressure of water vapour
What expression can be used to determine partial pressure of dry air?
p’d = rho’d Rd T
What expression can be used to determine partial pressure of water vapour/
e = rho’v Rv T
where Rv = 461. 5 JK^-1Kg^-1
What is the value of ‘3’?
0.622
Rd/Rv
What does this equation tell you:
density of moist air. an increase in e (water vapour) decreases the density
Define virtual temperature? formula?
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
What are thee measures of water vapour content in air?
1.
2.
3.
- e = partial pressure in Pa or hPa
- mixing ratio = Mv/Md
- q = specific humidity = w / (1+w) given in g/kg
When there is no condensation or evaporation mixing ratio is…
conserved
Define vitual potential temperature
virtual theta is conserved in the absence of condensation. Equation is a better measure of stability in a moist atmosphere.
Define saturation
The point at which air holds as much water vapour as it can. this amount decreases when temperature decreases.
What is es
saturation vapour pressure. The pressure exerted by vapour in air when the air is saturated.
If e > es there is
net condensation
if e < es there will be
net evaporation if there is a source of water
What is the Bergeron effect?
the growth of ice crystals in an environment which contains supercooled water
Define relative humidity. formula?
ratio of the actual mixing ratio to the saturation mixing ratio.
RH = 100 x w/ws
What is the RH when w=ws?
100%
When w = 0 what is RH?
0%
At a constant pressure what happens when air is cooled?
es reduces and ws reduces unstill the air is saturated. at this point it reaches the dew point temperature Td
Define dew point temperature
temperature to which air must be cooled (at a constant pressure) for it to become saturated.
What is the physical meaning of latent heat
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
What is the lifting condenstation level?
The height at which an air parcel is saturated and begins to condense (w = ws)
What happens when a cloud is forced over a mountain?
increase in potential temperature on lee (away from wind) side.
increase in tempeature at same level
decrease in water vapour content
warm & dry winds
