Lecture #5 Water and ocean structure 22 August 2007
Lecture schedule LectureDateTime 1. Introduction to scientific writing 8/20 9:00-10:20 2. How to write a scientific paper in English 8/20 15:00-16:20 3. Earth structure and plate tectonics 8/21 9:00-11:00 4. Circulation of the atmosphere 8/21 13:00-15:00 5. Water and ocean structure 8/22 10:00-11:00 6. Continental margins and ocean basins 8/22 13:00-15:00 7. Sediment 8/23 10:00-12:00 8. Marine resources 8/23 13:00-14:00 9. Circulation of the ocean 8/24 11:00-12:30
The water molecule is held together by chemical bonds • Atom • the smallest particle of an element that exhibits the characteristics of that element electrons (-) helium atom neutrons (no charge) protons (+)
Element • a substance composed of identical atoms • cannot be broken into simpler substances by chemical means http://en.wikipedia.org/wiki/Image:Periodic_table.svg
The water molecule is held together by chemical bonds • Atom • the smallest particle of an element that exhibits the characteristics of that element • Element • a substance composed of identical atoms • cannot be broken into simpler substances by chemical means • Molecule • > 2 atoms held together by chemical bonds • examples = oxygen (O2) • water (H2O)
Chemical bonds • form when atoms share electrons 2 hydrogen atoms 1 oxygen atom 1 water molecule H2O + covalent bonds
Examples of covalent bonds carbon dioxide CO2 methane gas CH4 atmospheric oxygen O2 http://www.its.caltech.edu/~atobias/o2molecule.gif http://www.nyu.edu/pages/mathmol/textbook/oxygen.gif
Because of the way the water molecule’s oxygen atoms are distributed, the shape of the molecule is “bent” Electrically asymmetric _ +
attracts - + • behaves like a magnet • called a “polar molecule” attracts - + Hydrogen bond • about 5-10% as strong as a covalent bond • allow water molecules to stick to each other cohesion
Cohesion gives water a high surface tension Hydrogen bonds make this possible water strider http://content.answers.com/main/content/wp/en-commons/thumb/6/6b/250px-Wasserläufer_bei_der_Paarung_crop.jpg
What would happen if there were no hydrogen bonds? At normal temperatures and pressure, H20 would form a gas rather than a liquid hydrogen sulfide (H2S) http://www.uwosh.edu/faculty_staff/xie/molecules/h2s-2.gif
Question Time! • What holds molecules together?
Question Time! • What holds molecules together? • 2.Why is water a polar molecule?
heat = temperature Water has unusual thermal characteristics • heat= energy produced by the random vibration of atoms/molecules • measured by • how many molecules are vibrating • how rapidly they are vibrating heat • temperature • measured in degrees
Temperature • a number related to the average kinetic energy of the molecules of a substance • Heat • a measurement of the total energy in a substance • = kinetic energy + potential energy
Bathtub of hot water Candle Which one has • higher temperature? • more heat?
Bathtub of hot water Candle Which one has • higher temperature? Molecules vibrate very rapidly, but very few • more heat?
Bathtub of hot water Candle Which one has • higher temperature? Molecules vibrate very rapidly, but very few • more heat? Molecules vibrate more slowly, but there are many of them
Heat capacity • the heat (measured in calories) required to raise 1 gram of a substance 1o Celsius • different substances have different heat capacities very high • due to large # of hydrogen bonds * water can absorb (or release) large amounts of heat while changing very little in temperature
heat capacity water high sand low
heat capacity water high sand low
Water’s temperature affects its density. Where is the warmest water in a tub ? • at the surface • cold water tends to sink But, where does ice form in a lake? at the surface http://www-personal.umich.edu/~jensenl/visuals/album/2006/ice/IMG_8011.JPG
Density curve shows the relationship between temperature (or salinity) and density 4oC Ice is less dense than liquid water – and thus floats – because the molecules are packed less efficiently. freezing point
water ice angle = 109o angle = 104o
The three states of water to melt to evaporate gas (water vapor) solid (ice) liquid to freeze to condense
gas solid liquid Temperature <0oC 0-100oC >100oC Molecular movement fast fastest slow Density low highest lowest
When heat goes into a substance, one of two things can happen: • rise in temperature • increased kinetic energy + energy or substance • change in state • increased potential energy
States of water to evaporate to melt gas (water vapor) solid (ice) liquid to freeze to condense • very high • all hydrogen bonds must be broken The term latent applies to heat input that does not cause a temperature change but does produce a change of state.
Question Time! How is heat different from temperature?
Surface water moderates global temperature The tendency of a substance to resist a change in temperature with the gain or loss of heat energy is called thermal inertia. Low thermal inertia High thermal inertia Range: -2o – 32o -90o– 50o
Heat No change No change Temperature
Norfolk Wind direction San Francisco
Annual freezing and thawing of ice moderate Earth’s temperature adds 80 calories of heat energy/g liquid water solid (ice) liquid removes 80 calories of heat energy/g liquid water • >18,000 km3 of polar ice thaws and refreezes in the Southern Hemisphere each year • in summer, ice melts, but ocean temperature does not change • in winter, water freezes, but ocean temperature does not change Seasonal extremes are moderated.
Movement of water vapor from tropics to poles also moderates Earth’s temperature • poleward transfer of heat • 2/3 by air • 1/3 by ocean currents
Global warming may be influencing ocean-surface temperature and salinity S N • tropical ocean < 1000 meters is becoming warmer and saltier • water in the far north and south has become fresher
Ocean-surface conditions depend on latitude, temperature,and salinity Evaporation > precipitation Precipitation > evaporation Evaporation > precipitation
Sea surface temperature Sea surface salinity
The ocean is stratified by density. Freshwater density = 1.000 g/cm3 Seawater density = 1.020-1.030 g/cm3 • Seawater density is a function of: • temperature • salinity • pressure most important cold, salty water warm, less salty water is more dense than
The ocean is stratified into three density zones by temperature and salinity • Surface zone • (mixed layer) • 0-150/1000 m Pycnocline Deep zone
Pycnocline: zone in which density increases with increasing depth. http://epswww.unm.edu/facstaff/zsharp/103/lecture%2015-16,%20oceans%20I_files/image010.gif The pycnocline is a combination of the thermocline andhalocline.
Thermocline in the Central Pacific http://svs.gsfc.nasa.gov/vis/a000000/a000200/a000280/thermo_actual_9706.jpg
Thermocline in Lake Biwa, Japan http://www.lbri.go.jp/biwakoview/english/nb/image/02.gif
A water mass is a body of water with characteristic temperature and salinity, and therefore density. These layers are distinct water masses Density stratification usually prevents vertical water movement.
Refraction can bend the paths of light and sound through water Why does it bend?
Refraction can bend the paths of light and sound through water Why does it bend? Due to a difference in density