Moisture Variables on Skew T Log P Diagram

1
Moisture Variables on Skew T Log P Diagram

• AOS 330 LAB 8

2
List of Variables

• Mixing ratio (w)
• Saturation mixing ratio (ws)
• Specific Humidity (q)
• Vapor pressure (ev)
• Saturation vapor pressure (es)
• Relative humidity (RH)
• Dewpoint (Td)
• Dewpoint Depression
• Virtual Temperature (Tv)
• Wet-Bulb Temperature (Tw)

3
Mixing Ratio (w or rv)

• Mixing ratio (rv) a way to tell how much vapor
  there is relative to a mass of dry air
• It is conserved as long as there is no
  condensation or evaporation.
• Units g kg-1

4
Specific Humidity (q)

• Mass of water vapor per unit mass of moist air
• But mass of water vapor is very small compare to
  the total mass (1-2 of the total mass)
• , (evltlt p)

5
Vapor Pressure (ev)

• ev partial pressure of vapor in (Pa)
• es saturation vapor pressure over plane surface
  of pure water

6
Saturation Vapor Pressure (es)

• Vapor pressure ev - most directly determines
  whether water vapor is saturated or not.
• ev lt es(T) subsaturated, evaporation
• ev es(T) saturated
• ev gt es(T) supersaturated, condensation
• es only depends on temperature.
• es(T) increases with increasing temperature.

7
Relative Humidity

• Subsaturated RH lt 100
• Saturated RH 100
• Supersaturated RH gt 100
• Depends on both vapor pressure ev and the air
  temperature T

8
Dewpoint (Td)

• Consider the case ev lt es(T), we could always
  reduce es(T) to ev by lowering the temperature.
• Dewpoint is the temperature at which moist air
  became saturated over a plane surface of pure
  water by cooling while holding ev constant.
• Only depends on the vapor pressure ev.

9
Saturation Mixing Ratio (ws)

• It is the mixing ratio for which air is saturated
  at specific T and P.

10
Saturation Mixing Ratio (ws)

• Depend on both temperature and pressure
• In units of (g kg-1)
• If we choose P to be 622 hPa, then

11
To find saturation vapor pressure (es)
A temperature of 3 deg C at 622hPa is correspond
to a saturated mixing ratio of 7.8 g kg-1. The
saturation vapor pressure is 7.8 hPa.
12
Dewpoint, Mixing Ratio, and Dewpoint Depression
13
Virtual Temperature (Tv)

• To apply ideal gas law to mixture of air and
  vapor
• Moist air equation of state

14
Wet-Bulb Temperature (Tw)

• It is the temperature to which air is cooled by
  evaporation until saturation occurs.
• Assume that all of the latent heat of
  vaporization is supplied by the air
• Normands Rule
• To find Tw, lift a parcel of air
  adiabatically to its LCL, then follow moist
  adiabat back down to parcels original P

15
One Other Variable T

• Potential Temperature T
• Temperature of the parcel if it were compressed
  or expanded dry adiabatically to 1000 hPa.
• Conserved in dry adiabatic process

16
Critical Levels on Thermodynamic Diagram
17
Lifting Condensation Level (LCL)

• The level at which a parcel lifted dry
  adiabatically will become saturated.
• Find the temperature and dewpoint of the parcel
  (at the same level, typically the surface).
  Follow the mixing ratio up from the dewpoint, and
  follow the dry adiabat from the temperature,
  where they intersect is the LCL.

18
Finding the LCL
19
References

• Petty, G (2008). A First Course in Atmospheric
  Thermodynamics, Sundog Publishing.
• Potter and Coleman, 2003a Handbook of Weather,
  Climate and Water Dynamics, Climate, Physical
  Meteorology, Weather Systems and Measurements,
  Wiley, 2003