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1. Water Vapor in Air
Water appears in all three phases in the earth’s atmosphere - solid, liquid and vapor - and it is one of the most important components, not only because it is essential to life, but because the release of latent heat of vaporization when vapor condenses into the liquid or solid phases, together with the earth’s rotation, drives the large-scale circulation of the atmosphere. Here, we show various measures of the variable atmospheric water vapor content.
- VAPOR PRESSURE e
To a good approximation, water vapor behaves as a perfect gas. Thus, its equation of state is:
e = ρvRvT (1) eαv = RvT (2)
eαv =
RdT
e is vapor pressure Rv = R∗/Mv = 461. 5 Jkg−^1 K−^1 and Mv = 18. 01 gmol−^1 , = Mv/Md =
- The vapor pressure is the partial pressure of the water vapor.
- SATURATION VAPOR PRESSURE es The saturation vapor pressure is the partial pressure of the water vapor in equilibrium with a plane surface of pure water. That means that the rate of condensation is equal to the rate of evaporation. es is a function of temperature alone, it doesn’t depend on the vapor content of the air. ex(T ) is highly nonlinear as it increases rapidly with increasing temperature. This explains why the amount of atmospheric water vapor will likely increase with global warming caused by increasing concentration of greenhouse gases. This pressure is defined by the integral of the Clau- sius Clapeyron equation [you will do this in a HW]. There are also approximate expressions for es, one of the most used is:
es = 611exp
17. 27 T
237 .3 + T
where es is in Pascals and T is in Celsius.
- MIXING RATIO w
Defined as the mass of water vapor mv per unit mass of dry air.
w =
mv md
e/(RvT ) (p − e)/(RdT )
e p − e
where p = pd + e, with p the total pressure and pd the pressure due to the dry air component. It is often expressed as g/kg. Very moist air might have a mixing ratio of 18 g/kg.
- SATURATION MIXING RATIO ws
Mixing ratio if the air were saturated. Defined as:
ws =
es p − es
- SPECIFIC HUMIDITY q
The specific humidity is defined as the mass of water vapor per unit mass of moist air,
q =
mv md + mv
e/(RvT ) (p − e)/(RdT ) + e/RvT
e p − (1 − )e
We can calculate the amount of water vapor contained in a unit area column of air which extends from the earth’s surface to the top of the atmosphere as:
P W =
∫ P 0
0
q
dp g
- SATURATION SPECIFIC HUMIDITY qs
qs =
es p − (1 − )es
Notice that qs and ws depend on pressure and temperature only - not on the water vapor content of the air.
7) RELATIVE HUMIDITY RH
Ratio (expressed as percentage) of the actual mixing ratio to the saturation mixing ratio:
RH = 100
w ws
e es
q qs
Air can have a RH in excess of 100% especially if the air is devoid of aerosols. Such air is supersaturated.
8) VIRTUAL TEMPERATURE
When there is a mixture of water vapor and dry air, it is convenient to retain the gas constant for dry air and use a fictitious temperature called virtual temperature in the ideal gas equation. The density of a mixture of dry air and water vapor is:
ρ =
md + mv V
= ρd + ρv =
p − e RdT
e RvT
p RdT
[
e p
]
p = ρRdTv (12)
where
Tv ≡
T
1 − ep (1 − )
w = ws(Td, p) =
es(Td) p − es(Td)
It follows that the relative humidity at pressure p and temperature T is given by:
RH = 100
ws(Td, p) ws(T, p)
Equation 23 is a transcendental equation that must be solved graphically of through an iterative process. To do this we can use a calculator that does iterations, a computer or manually using a Newton-Raphson procedure. The Newton-Raphson procedure is as follows:
i. Re-write Equation 23 as f (Td) = 0
ii. Using Taylor series, we know that f (Td) = f (T (^) d^0 ) + f ′(T d^0 )[T (^) d^1 − T (^) d^0 ] where f ′^ = df /dTd
iii. Solve for next estimate T (^) d^1 = T (^) d^0 − f^ (T^ d^0 f ′(T (^) d^0 )
iv. Repeat iteratively to converge to the true solution T (^) dn +1= T (^) dn − f (T (^) dn f ′(T (^) dn )
Graphically, the simplest way to find Td is using the Skew T-log p chart. The dew-point tem- perature is always less than or equal to the actual temperature Td ≤ T.
- FROST-POINT TEMPERATURE Tf
Temperature to which moist air must be cooled isobarically, with p and q held constant, for the air to reach saturation with respect to a plane surface of pure ice. Given by:
w = ws(Tf , p) =
es,i(Td) p − es,i(Td)
This is meaningful only when (Td < 0 ◦C). We will see later that es, i(T ) < es(T )
- WET-BULB TEMPERATURE, Tw
Temperature to which air may be cooled by evaporating water into it a constant pressure until saturation is reached. w is not held constant in this process. The first law of Thermodynamics for an isobaric process is:
dq = (1 + 0. 87 w)cpddT (26) if we neglect the term multiplying w as it is sufficiently small, we can work with the dry air constants.
dq = cpddT (27) If moist air consists of one gram of dry air and w grams of moist air, the heat lost by the moist air required to evaporate dw grams of water is:
(1 + w)dq = −Ldw (28) where L is the latent heat of vaporization. So
cpddT = −
Ldw 1 + w
≈ −Ldw (29)
Neglecting the weak dependence of L on T we can integrate 29 from (T, w) to [Tw, ws(Tw, p)] to obtain:
Tw = T −
L
cpd
(ws(Tw, p) − w) (30)
This is also a transcendental equation that must be solved iteratively. The wet bulb temperature cannot be found on a skew T-log p diagram.
- EQUIVALENT TEMPERATURE Te
Temperature a sample of moist air would attain if all the moisture were condensed out at con- stant pressure. We obtain the expression for Te by integrating equation 29 from (T, w) to (Te, 0).
Te = T +
L
cpd
w (31)
Thus, Te ≥ T
- ADIABATIC SATURATION TEMPERATURE Ts
Temperature at which saturation is reached when moist air is cooled adiabatically with w held constant. Easily understood in a skew T-log p diagram. If an parcel ascends along its adiabat which has potential temperature θ, both temperature and pressure decrease. The saturation vapor pressure decreases more rapidly than p, so the saturation mixing ratio decreases. Eventually, the air parcel will reach a point where the decreasing saturation mixing ratio just equals the mixing ratio of the air parcel.
w = ws(Ts, ps) =
es(Ts) ps − es(Ts)
where ps is the adiabatic saturation pressure and Ts = T (ps/p)κ, is the adiabatic saturation temperature. This is, yet another, transcendental equation which must be solved iteratively. We may speak interchangeably of condensation and saturation at a point. The level at which an air mass attains saturation by adiabatic ascent is called the lifting condensation level LCL. We can obtain the pressure of the LCL as
ps = p
Ts T
) 1 /κ (33)
We can find the altitude of the LCL using the hypsometric equation between p and ps
FIG. 1. Saturation vapor pressure es over a plane surface of pure water and the difference between es and the saturation vapor pressure over a plane surface of ice esi
FIG. 2. Column Precipitable Water from NCEP/DOE Reanalysis.
