Lecture 1.2 Today –Gases in the atmosphere –Density and pressure –Permanent and variable gases Water vapor Carbon dioxide Ozone

TA’s office hours Sara Jones: Mon Fri Justin Glisan: Mon Tue Wed Amanda Cox: Tue Thur Lindsey Fennewald: Thur 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 ~ 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 & Units Figure 9.4 Many scales are used to record atmospheric pressure, including inches of mercury (Hg) and millibars (mb). The National Weather Service uses mb, but will convert to metric units of hectopascals (hPa). The conversion is simply 1 hPa = 1 mb.

Measuring Pressure To measure atmospheric pressure we use a barometer

Pressure Measurement Changes in atmospheric pressure are detected by a change in elevation of a barometric fluid or change in diameter of an aneroid cell, which indicates changing weather. Average sea level pressure is in Hg, or mb. Figure 9.5 Figure 9.6

Pressure Trends Figure 9.7 Barographs provide a plot of pressure with time, and are useful in weather analysis and forecasting. Altimeters convert pressure into elevation, and are useful in steep terrain navigation or flying. Both use aneroid cells.

Earth's Atmosphere 99% of atmospheric gases, including water vapor, extend only 30 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 1.2

There is a lot of Nitrogen! © 1998 Prentice-Hall -- From The Atmosphere, 7th Ed., by F.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 (N 2 ) –21% Oxygen (O 2 ) –0.93% Argon (Ar) –The rest is other stuff Trace gases and variable gases (eg. CO 2 )

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 (CO 2 ), 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 Gases Figure 1.4 Figure 1.5 Nitrogen and oxygen concentrations experience little change, but carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons are greenhouse gases experiencing discernable increases in concentration.

Why is the change in CO 2 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