__Determining Meteorological Parameters from a Sounding__

The temperature and dewpoint curves that are plotted on the Skew-T are used for much more than simply identifying the temperature and dewpoint at a certain level in the atmosphere. There are many parameters that are not directly reported on a sounding but that can be calculated by using the lines discussed in Section 1.

**Mixing Ratio (w) – **the ratio of the
mass of water vapor to the mass of dry air in a sample of moist air.

*Units:
*__g of water vapor__ per kg of dry air

*Procedure:
*Read the value of the saturation mixing ratio line that crosses
the dewpoint curve at that pressure.

**Saturation Mixing
Ratio (w _{s}) – **the mixing ratio that a sample of air would have if it
were saturated.

*Units:
*__g of water vapor__ per kg of dry air

* Procedure:*
Read the value of the saturation mixing ratio line that crosses the
temperature curve at that pressure.

**Relative Humidity
(RH) – **the ratio of the amount of water vapor in a volume of
air to the amount of water vapor that the volume could hold if it were
saturated

*Units:
*expressed in percent

* Procedure:*
Compute from the equation: RH = 100% * (w/w

**Vapor Pressure (e) – **the part of the
total atmospheric pressure that is contributed by water vapor

*Units: *millibars

* Procedure:
*From the dewpoint curve at a given pressure level, follow the
isotherms to the isobar at 622 mb. Read the value of the
saturation mixing ratio line that passes through this point to
get vapor pressure.

**Saturation Vapor
Pressure (e _{s}) -- **the partial pressure that water would add to the total
atmospheric pressure if the air was saturated

*Units: *millibars

* Procedure:
*From the temperature curve at a given pressure level, follow the
isotherms to the isobar at 622 mb. Read the value of the
saturation mixing ratio line that passes through this point to
get saturation vapor pressure.

**Wet Bulb
Temperature (T _{w}) – **the lowest temperature to which a volume of air at
constant pressure can be cooled by evaporating water into it.

*Units:* Kelvin

* Procedure:
*From the dewpoint curve at a given pressure level, draw a line
upward along a saturation mixing ratio line. Then from the
temperature curve at the given pressure level, draw a line
upward along a dry adiabat until it intersects with the first
line drawn. From this intersection, follow a saturation
adiabat down to the given pressure level. The value of the isotherm
running through this point is the wet bulb temperature.

**Potential
Temperature (θ) – **the temperature a parcel of air would have if it were
brought dry-adiabatically down to 1000 mb.

* Units: *Kelvin

* Procedure:
*From the temperature curve at a given pressure level, follow the
dry adiabat that intersects the T curve down to 1000 mb. The
isotherm value at this intersection is the potential temperature
of the parcel at the given pressure.

**Wet
Bulb Potential Temperature (θ _{w})
-- **the
wet bulb temperature a parcel of air would have if it were brought
down to 1000 mb saturation-adiabatically.

* Units:
*Kelvin

* Procedure:
*Find the wet bulb temperature and follow the saturation adiabat
to the 1000 mb isobar. The temperature value at this
intersection is the wet- bulb potential temperature.

**Equivalent
Temperature (T _{E}) -- **the temperature of a parcel if all the moisture was
condensed out by a moist adiabatic process using latent heat to heat
the parcel.

* Units:
*Kelvin

* Procedure:
*From the dewpoint curve at the given pressure level, follow a
line upward along a saturation mixing-ratio line. Then from
the temperature curve at the given pressure level, follow a
line upward along a dry adiabat. The intersection of these two
lines is the LCL. From this intersection, follow a saturation
adiabat upward to a pressure where the saturation and dry
adiabats are parallel; where all the moisture has been condensed out of
the parcel. Then follow a dry adiabat down to the original pressure
level. The temperature at this level is the
equivalent temperature.

**Equivalent
Potential Temperature (θ _{E}) – **the temperature of a parcel if all the moisture was
condensed out by a moist adiabatic process using latent heating to heat
the parcel and then the parcel brought dry-adiabatically down to 1000
mb.

* Units:
*Kelvin

* Procedure:
*Find the equivalent temperature for the given pressure level,
then from this point, follow a dry adiabat down to 1000 mb.

**Virtual
Temperature (T _{v}) – **the temperature at which dry air would have the same
density as moist air at the same pressure.

* Units:
*Kelvin

* Procedure:
*The virtual temperature can be computed by the equation: T

**CAPE (Convective Available Potential Energy) – **the maximum energy available on an ascending air parcel.

*Units: *Joules per kilogram: (J/kg*)
Procedure: *To find CAPE on a Skew-T/Log-P Diagram you first must find the level of free convection (LFC). From the LFC, follow the air parcel up along a moist adiabat until it intersects with the temperature curve. This will be the level of neutral bouyancy (LNB). The area to the left of the temperature curve between the LFC and the LNB is CAPE. CAPE is often called Positive Area which is proportional to the amount of kinetic energy the parcel can gain from
the environment.

**CIN (Convection Inhibition) – ** the energy needed to lift an air parcel vertically and pseudoadiabatically from its originating level to its level of free convection (LFC).

*Units:* Joules per kilogram: (J/kg)

*Procedure*: Take the surface dew point temperature and raise it adiabatically. At the same time, take the surface temperature and raise it pseudoadiabatically. Raise these two parcels until they intersect. From the point of intersection, raise the parcel pseudoadiabatically until it crosses the temperature curve. This point is the level of free convection. The area under the LFC and to the right of the temperature curve is CIN. CIN is often called Negative Area which is proportional to the amount of kinetic energy that must be supplied to move the parcel
to the LFC.

**Lifting Condensation Level (LCL) -- **the height
at which a parcel of air becomes saturated when it is lifted
dry-adiabatically.

* Units:
*millibar

* Procedure:
*From the dewpoint curve at the given pressure level, follow a
line upward along a saturation mixing-ratio line. Then from
the temperature curve at the given pressure level, follow a
line upward along a dry adiabat. The intersection of these two
lines is the LCL.

**Convection
Condensation Level (CCL) -- **the height to which a parcel of air if heated
sufficiently from below will rise adiabatically until it is just
saturated. This is the height of the base of cumuliform clouds produced
by surface heating.

*Units:
*millibar

* Procedure:
*Find the average mixing ratio in the lowest 50 mb and follow this
mixing ratio line up to where it intersects the temperature
sounding. This point is the height of the CCL.

**Convection
Temperature (T _{c}) -- **the surface temperature that must be reached to start
the formation of convection clouds by surface heating.

* Units:
*Kelvin

* Procedure:*
First determine the CCL on the sounding. From the CCL on the
temperature curve, follow a dry adiabat downward to the surface. The
temperature at this intersection is the convection temperature.

**Mixing
Condensation Level (MCL) -- **the lowest height at which saturation occurs after the
complete mixing of the layer.

* Units:
*millibar

* Procedure:*
Follow the mixing ratio line that passes through the surface
dewpoint up to where it intersects the temperature sounding. This is
the MCL.

**Level of Free
Convection (LFC) -- **the height at which a parcel of air that is lifted dry
or moist adiabatically becomes less dense (warmer) than the surrounding
air.

* Units:
*millibar

* Procedure:
*Find the region of positive area on the sounding. The pressure
level at the bottom of the positive area that is closest to
earth's surface is the LFC. Just below this point, the
temperature of the parcel and the temperature of the
environment should be equal.

**Equilibrium Level
(EL) -- **the height where
the temperature of a positively buoyant parcel of air becomes equal to
that of the surrounding atmosphere and above this level the parcel
becomes negatively buoyant.

* Units:
*millibar

* Procedure:
*Locate the positive area on the sounding. The equilibrium level
is the point at the top of the positive area where the
temperature curve and the saturation adiabat that goes through
the LFC meet.

**Note:
Many of the temperature variables need to be converted from Celcius
to Kelvin, (C+273=K).**

*Page References*

Department of Atmospheric Science – University of Wyoming . Use of Skew T-log p diagram . Air Weather Service (MAC), United States Air Force. (1969).

American Meteorological Society. Glossary of Meteorology Online. Allen Press. 2005. http://amsglossary.allenpress.com/glossary