Lecture 1.2

• Today – Gases in the atmosphere– Density and pressure– Permanent and variable gases

• Water vapor• Carbon dioxide• Ozone

TA’s office hoursSara Jones: Mon. 12-2 Fri. 12-2Justin Glisan: Mon. 10-11

Tue. 9-10 Wed. 12-2Amanda Cox: Tue. 11-1 Thur. 11-1Lindsey Fennewald: Thur. 9-12

McReynolds Hall, Room 374

Density

• The density of a substance is defined as the amount of mass of a substance in a given volume.

• It can also be defined by a number density that tells us the number of “things” in a given volume.– Number of students in this room– Number of water drops in a cubic

centimeter of cloud

Density

• Is measured in kg m-3 – (or sometimes g cm-3)– Number density is in (number) m-3

• The air in this room (at the surface of the Earth) has a density of ~1.2 kg m-3

• A fluid with a lower density will float on a fluid with a higher density– Decrease the density and it could rise

Pressure

• The air pressure is the force per unit area that the atmosphere exerts on any surface it touches.

• The molecules of the air are in constant rapid motion.

• When a molecule collides with a surface, such as your skin, the molecule exerts a force on that surface.

Pressure and density:

The higher the density the more molecules. More molecules striking a surface means higher pressure

Pressure Units• SI unit Pa (Pascal)

– Or N m-2

– Sea level atmospheric pressure is ~101000 Pa

• Meteorologists also use millibars – mb– Sea level atmospheric pressure is ~1000mb

• They even sometimes use millimeters (inches) of mercury – mm Hg, inches Hg– Sea level atmospheric pressure is ~760 mmHg or 30”

Hg

Pressure Scale & UnitsPressure Scale & Units

Figure 9.4Figure 9.4

Many scales are used Many scales are used to record atmospheric to record atmospheric pressure, including pressure, including inches of mercury (Hg) inches of mercury (Hg) and millibars (mb).and millibars (mb).

The National Weather The National Weather Service uses mb, but Service uses mb, but will convert to metric will convert to metric units of hectopascals units of hectopascals (hPa).(hPa).

The conversion is The conversion is simply 1 hPa = 1 mb.simply 1 hPa = 1 mb.

Measuring Pressure

• To measure atmospheric pressure we use a barometer

Pressure MeasurementPressure Measurement

Changes in atmospheric pressure Changes in atmospheric pressure are detected by a change in are detected by a change in elevation of a barometric fluid or elevation of a barometric fluid or change in diameter of an aneroid change in diameter of an aneroid cell, which indicates changing cell, which indicates changing weather.weather.Average sea level pressure is 29.92 Average sea level pressure is 29.92 in Hg, or 1013.25 mb.in Hg, or 1013.25 mb.Figure 9.5Figure 9.5

Figure 9.6Figure 9.6

Pressure TrendsPressure Trends

Figure 9.7Figure 9.7

Barographs provide a plot of pressure with time, and are useful in Barographs provide a plot of pressure with time, and are useful in weather analysis and forecasting.weather analysis and forecasting.

Altimeters convert pressure into elevation, and are useful in steep Altimeters convert pressure into elevation, and are useful in steep terrain navigation or flying.terrain navigation or flying.

Both use aneroid cells.Both use aneroid cells.

Earth's AtmosphereEarth's Atmosphere

99% of atmospheric gases, including water vapor, extend only 30 99% of atmospheric gases, including water vapor, extend only 30 kilometer (km) above earth's surface.kilometer (km) above earth's surface.

Most of our weather, however, occurs within the first 10 to 15 km.Most of our weather, however, occurs within the first 10 to 15 km.

Figure 1.2Figure 1.2

There is a lot of Nitrogen!

© 1998 Prentice-Hall -- From The Atmosphere, 7th Ed., byF.K. Lutgens and E.J. Tarbuck, p. 6.

Permanent Gases

• Permanent gases have fixed proportions in the atmosphere, both in time and space

• For Dry Air– 78% Nitrogen (N2)

– 21% Oxygen (O2)

– 0.93% Argon (Ar)– The rest is other stuff

• Trace gases and variable gases (eg. CO2)

Variable gases

• Variable gases can have different concentrations in the atmosphere, both in time and space

• The most important variable gas is water vapor

• Other variable gases include carbon dioxide (CO2), methane and ozone

Water vapor

• Is variable– We measure this variability as the humidity

(see later)– From evaporation– Proximity to bodies of water – Air temperature– When it condenses get clouds and

precipitation

Water vapor

• Is important because– It is the only common substance that can

change between gas, liquid and solid at temperatures and pressures that are normal on Earth

– It can ‘hold’ a lot of energy and transport that energy around the planet

– We need water– It absorbs a lot of radiation

Carbon dioxide

• Used by plants during photosynthesis– Plants take in and store carbon as they grow

• Exhaled by animals

• Released by the burning of oil, gas, wood, coal

• Concentrations have been rising around the world for 200 years

Variable & Increasing GasesVariable & Increasing Gases

Figure 1.4Figure 1.4Figure 1.5Figure 1.5

Nitrogen and oxygen concentrations experience little change, Nitrogen and oxygen concentrations experience little change, but carbon dioxide, methane, nitrous oxides, and but carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons are greenhouse gases experiencing chlorofluorocarbons are greenhouse gases experiencing discernable increases in concentration.discernable increases in concentration.

Why is the change in CO2 important?

• Carbon dioxide absorbs longwave (infra-red) radiation

• This creates an imbalance between energy received by the Earth and energy leaving the Earth

• If you want to know why we should care wait for next chapter or look at the atmosphere of Venus (in the book)

Ozone

• At the surface– Is caused by chemical reactions between a

variety of pollutant gases (such as nitrogen oxides)

– Mostly caused by vehicle emissions– Is an irritant

Ozone

• In the stratosphere– Is a beneficial gas that absorbs ultra-violet

radiation– Protects us from this harmful radiation– Is broken down by chemical reactions with

chlorine containing gases (chlorofluorocarbons – CFCs): Man-made compounds used in aerosol sprays, refrigerators and air-conditioners

Summary

• Differences between permanent and variable gases

• Three of the important variable gases– Why are they variable– Why are they important