STEC 521: Lesson 2
IDEAL GAS LAW.
Lesson 1
Lesson 3




OVERVIEW:

We will discuss the concepts of atmospheric pressure, temperature, and density; the relationship of water vapor content to air density; virtual temperature; the ideal gas law; and sea-level equivalent pressure.



OUTLINE:

1. Basic concepts.

1.1. MKS system base units:
1.1.1. Length [meters].

1.1.2. Mass [kilograms].

1.1.3. Time [seconds].

1.1.4. Temperature [Kelvins].

1.2. Derived units:
1.2.1. Velocity = Distance/Time [meters/second].

1.2.2. Acceleration = Velocity/Time [meters/second2].

1.2.3. Force = Mass x Acceleration [(kilogram x meter)/second2] or [Newton].

1.2.4. Pressure = Force/Area [Newton/meter2] or [Pascal].

1.2.5. Energy = Mass x Velocity2 [(kilogram x meter2)/second2] or [Joule].

1.2.6. Heat is equivalent to energy.

2. The behavior of the atmosphere approximates that of an "ideal" gas. An ideal gas is one where the only interactions between particles are those relating to direct collisions, i.e., there are no intermolecular forces. The Ideal Gas Law for a moist airmass is:

p = rhoa Rd Tv

where:
p = Pressure [pascals] (or [Newton/m2])
rhoa = Density [kg/m3]
Rd = Gas constant for dry air = 287 Joules/(kg dK)
Tv = Virtual temperature [dK]
See Lutgens and Tarbuck appendix A for units discussion.

3. Pressure: Ideally, a measure of the total weight of the atmosphere above a certain point on Earth's surface. Falls off logarithmically with height. Typical value at sea level is 1.0135 bars.
3.1. Units: Newtons/m2, Pascals, bars, inches of mercury.

3.2. Measurement device: Barometer, various types.

3.3. Sea-level (equivalent) pressure.

3.4. Mean annual surface isobars.

4. Temperature: A measure of the bulk kinetic energy of an airmass. Profile varies considerably depending on the vertical region of the atmosphere. Typical value at sea level is 15 degrees Celsius.
4.1. Units: Degrees (Fahrenheit, Celsius, or Kelvin).

4.2. Measuring device: Thermometer.

4.3. Relationship to specific heat; energy content; specific heat coefficients.

4.4. Relationship to kinetic energy.

4.5. Mean vertical temperature distribution.



The layers of the Earth's atmosphere.
(Graphics credits)


4.6. Mean annual surface isotherms.

5. Moisture content: Normal range (by mass) within the Earth's atmosphere -- a trace to 4 %. Most of this is concentrated in the lowest 1500 feet.
5.1. Indicators: Several quantities, including vapor pressure, mixing ratio, dew point, relative humidity, and others.

5.2. Measurement device: Primary is pychrometer.

5.3. Phases and phase-changes in water content.

Phases of water in the Earth system.


5.4. Phase changes and latent heat.

5.5. Definition and temperature dependence of saturation.

5.6. Total amount of water in the Earth system.


The Earth's water budget.
(Graphics credits)


5.7. Virtual temperature: The hypothetical temperature of a completely dry airmass that results in the same density as that of a slightly cooler, moist airmass.

6. Density: A measure of the total amount of mass in a given volume. Falls off logarithmically with height. Typical value at sea level is 1.2 kg/m3.
6.1. Units: May be either mass/volume (kg/m3) or number/volume (m-3).

6.2. Inversely related to temperature and moisture content.

6.3. Directly related to pressure.


LAB:

Use sling psychrometer and tables (or calculator) to determine:

1. Wet- and dry-bulb temperatures (T and Tw).
2. Dew point (Td).
3. Relative humidity (RH).
4. Vapor pressure and saturation vapor pressure (e and es).
5. Mixing ratio and saturation mixing ratio (w and ws).
6. Virtual temperature (Tv).

HOMEWORK:

1. Read Lutgens and Tarbuck chapter 3 (all), chapter 4 (pp. 86 - 99), and chapter 6 (pp. 156 - 170).

2. Skim Bannon et al., 1997.



An equations summary is here.

Lesson 1
Lesson 3

This page was last updated on 01/13/2002.