Atmospheric Thermodynamics Final Exam

Mixing Ratio (w):

Mass of water vapor per unit mass of dry air

aturation Mixing Ratio (wsw)

Mass of water vapor per unit mass of dry air at saturation

Specific Humidity (q)

Mass of water vapor per unit mass of moist air

Relative Humidity (r)

The ratio (or percentage) of water vapor mass in a moist air parcel

to the vapor mass the air parcel would have if it was saturated

with respect to liquid water

Dew Point Temperature (Td)

Temperature at which saturation (with respect to liquid water)

is reached when an unsaturated moist air parcel is cooled at

constant pressure

Wet-Bulb Temperature (Tw)

Temperature at which saturation (with respect to liquid water)

is reached when an unsaturated moist air parcel is cooled by

the evaporation of liquid water

Moist Potential Temperature (θm)

Temperature an unsaturated moist air parcel would have if it

were to expand or compress from (p, T) to the 1000 mb level

Temperature at the Lifting Condensation Level (TLCL)

Temperature at which an ascending unsaturated moist

air parcel first achieves saturation due to adiabatic cooling

and condensation begins to occur

Pseudo-Adiabatic Equivalent Potential Temperature (θep)

Temperature an unsaturated moist parcel would have if it:

• Dry adiabatically ascends to saturation (to its LCL)

• Moist adiabatically ascends until all water vapor was

condensed into liquid water and falls out of the parcel

• Dry adiabatically descends to 1000 mb

Wet-Bulb Potential Temperature (θw)

emperature a saturated moist air parcel that contains condensed

water would have if it descends adiabatically to 1000 mb

Virtual Temperature (Tv)

The temperature a moist air parcel would have if the water vapor

molecules were replaced by dry air molecules

Virtual Potential Temperature (θv)

Temperature a moist air parcel would have if it were to expand

or compress from (p, Tv) to the 1000 mb level, and the parcel

contained no water vapor (i.e., vapor was replaced by dry air)

geopotential height (Z)

actual height normalized by the globally

averaged acceleration due to gravity at the Earth’s surface (

Stable

Returns to its original position

after displacement

Neutral

Remains in new position after

being displaced

Unstable

Moves further away from its original

position after being displaced

Archimedes Principle

The buoyant force exerted by a fluid on an object

in the fluid is equal in magnitude to the weight of fluid

displaced by the object.

Convective Condensation Level (CCL)

Pressure (or altitude) to which an unsaturated moist air parcel,

if heated sufficiently from below, will rise dry adiabatically until

it first becomes saturated

Convective Temperature (Tc)

Surface temperature that must be reached to start the formation

of clouds by solar heating.

Equilibrium Level (EL)

Pressure (or altitude) at which the temperature of a rising buoyant

air parcel (i.e., a parcel warmer than its local environment) becomes

equal to the temperature of the environment

Showalter Index (SI) Skew T Procedure:

Skew-T Procedure: 1. Find the LCL for a parcel lifted from 850 mb

2. Find the LFC for the same parcel

3. From the LCL move up a moist-adiabat to 500 mb

4. Subtract the parcel temperature (Tp) at 500 mb from

the environmental temperature (Te) at 500 mb

Showalter Index (SI) Forecast Guidelines

+1 to +3 Showers are probable – Thunderstorms possible

(need strong forced ascent)

0 to -3 Unstable – Thunderstorms probable

-4 to -6 Very Unstable – Severe thunderstorms possible

less than -6 Extremely Unstable – Severe thunderstorms probable

(Tornadoes are possible)

Lifted Index (LI) Skew T Procedure

Identify the lowest 100 mb of the sounding

. Find the mean wsw and mean θ in the lowest 100 mb

. Follow the mean wsw and mean θ up to the LCL

.From the LCL move up a moist-adiabat to 500 mb

. Subtract the parcel temperature (Tp) at 500 mb from

the environmental temperature (Te) at 500 mb

Finding mean conditions for LI

Identify the lowest 100 mb

2. Identify the maximum and minimum θ within the 100 mb

3. Mean θ is located 50 mb above the surface halfway between θmax and θmin

4. Identify the maximum and minimum wsw within the 100 mb

5. Mean wsw is 50 mb above the surface halfway between wsw-max and wsw-min

Note: The mean θ and mean wsw may NOT fall along the observed sounding

Lifted Index (LI) Forecast Guidelines

0 to -2 Thunderstorms possible (need strong forced ascent)

-2 to -5 Unstable – Thunderstorms probable

less than -5 Very Unstable – Severe thunderstorms probable

K Index (K) Definition

Measure of thunderstorm potential based on:

• Environmental lapse rate (T850 – T500)

• Moisture content of the lower atmosphere (Td 850)

• Vertical extent of moist layer (T700 – Td 700

K Index Forecast Guidelines

K < 15 0% chance of thunderstorms

15 – 20 < 20% chance of thunderstorms

21 – 25 20-40% chance of thunderstorms

26 – 30 40-60% chance of thunderstorms

31 – 35 60-80% chance of thunderstorms

36 – 40 80-90% chance of thunderstorms

K > 40 > 90% chance of thunderstorms

Total Totals (TT) Definition

Used to identify areas of potential thunderstorm development:

• Environmental lapse rate between 850 and 500 mb (T850 and T500)

• Low-level moisture (Td850)

Total Totals (TT) Forecast Guidelines

TT < 45 No thunderstorm activity

45 – 50 Weak potential for thunderstorm activity

50 – 55 Moderate potential for thunderstorm activity

TT > 55 Strong potential for thunderstorm activity

Convective Inhibition (CIN) definition

• The energy that must be overcome to make a parcel buoyant

• Energy is overcome by forced ascent

• The negative area below the LFC between the environmental sounding

and the temperature of a lifted parcel

Convective Available Potential Energy (CAPE)

Buoyant energy available in the atmosphere

• Forced ascent is usually required to tap into this energy

• The positive area above the LFC between the environmental sounding

and the temperature of a lifted parcel