+ - Water Planet, Water Crisis 2010 Class Notes Topic 2. Water in the earth system Part A: Properties of H 2 O: Why it's so important to us.

Transcription

1 Water Planet, Water Crisis 2010 Class Notes Topic 2. Water in the earth system Part A: Properties of H 2 O: Why it's so important to us. Physical and Chemical properties of H 2 O: Arise from the structure of the molecule: 1) 2 H's bond to an O with H's somewhat crowded together toward one side (104 angle to be exact but donʼt memorize that): + - Why aren't the H's directly opposite each other as one might expect? Electron clouds/orbits around the oxygen nucleus tend to go in certain directions determine where the Hʼs tend to bond. 2) O is an electron thief- tends to pull electrons from H in an H 2 O molecule. The electron "belonging" to each H is most often found near the O, though sometimes it orbits the H. 3) So then: a. Each H consists of a proton with very little electron presence around it; so it has a positive (+) charge b. The O has an excess presence of electrons relative to its 8 protons- negative (-) charge 4) Combine ideas 3a and 3b above, to get this key point: Each H 2 O has a positive end and negative end; we say it is a "polar molecule 5) This means that the + and - ends of H 2 O's have a tendency to attract - or + charged atoms/molecules: a. Other water molecules- Imagine 100 H 2 O's in a tiny bag. Will they be randomly oriented? No, they will arrange themselves with each + end matching up with the - end of another H 2 O, because opposite charges attract each other. This attraction that holds the H2O's together is a hydrogen bond (see more below). b. Ions (most inorganic atoms are charged; deficit or excess of electrons relative to protons) i. Example: Na + ion: Surrounded by the - ends of water molecules. All the charges are nicely satisfied when they are paired up like that. So this is a stable, favorable, low-energy state. ii. This is why water is good at dissolving many substances. Salts like NaCl, proteins, sugars, DNA, etc. all have electric charges that are welcomed into the structure of water because of this pairing/attraction of charges. iii. Note that oil does not dissolve into water well. This is because the oil molecules are not polar: They do not have + and charges overall, or on different parts of each molecule. Water molecules are drawn to charged things and, relatively speaking, are not as happy/stable when next to oil so they tend to not dissolve oil. c. Water is also attracted to surfaces of many solids (most have + or - charges)

2 Hydrogen bonds (H-bonds) are responsible for most of the key properties of water we have discussed in class: 1) H-bonds are somewhat weak; about 1/20 of the strength of O-H bonds within water molecules (if you want numbers, H-bonds are about 5 kcal/mol, whereas O-H bonds in water are about 110kcal/mol). 2) Strong enough to hold H2O s together, to make it a liquid at room T (unlike N 2, O 2, CO 2 ). 3) But weak enough so water is not too far from being a gas; as a result a large amount of water vapor can exist in air at room T. 4) Also weak enough to allow fluid motion at room T- not crystalline/solid (ice). 5) High heat of vaporization, because H-bonds need to be broken to allow H2O s to separate and form a gas. (if you want numbers: 10 kcal/mol, this is high relative to organic liquids like methanol, and liquid N 2 or O 2 ). The high heat of vaporization is very important on planet earth. 6) High heat capacity (if you want numbers: 1 cal/g/deg = kcal/mol/deg). Higher than most other substances like rock, wood, etc. 7) High heat of fusion (melting) (about 1.4 kcal/mol). Takes a lot of energy to rearrange the H- bonds to form/destroy the crystalline ice structure. Note: I do not expect you to memorize numbers above. They are there in case they help you get a sense of the relative energies. H2O is a greenhouse gas: Absorbs heat energy that radiates and would otherwise escape into space. Why aren't N 2 and O 2 greenhouse gases? They are simpler- only 2 atoms. These molecules cannot vibrate in the slow vibration motions that allow H 2 O and CO 2 to absorb heat energy (infrared radiation; waves). Why earth is watery and other planets are not. 1) H is #1 in abundance in solar system. Oxygen #3. 2) But then why isn t the earth dominated by H and He like the gas giant planets? - We are close enough to sun so volatiles (H, He, most water) were blown away - early hot phase, active sun with strong solar wind. Also, earth is small enough so some of those gases escape its gravity. 3) But some H remained inside the earth or was added back after the early hot phase. And because the earth is dominated by O, it is natural for H to be bonded to O, like most similar elements. 4) Earth's current T happens to be good for liquid water. Part B: Water Vapor in the Atmosphere Observations of precipitation patterns on Earth: 1) Latitude belts: Deserts: 30 N: and S latitude Rain Forests: Equatorial (we find some at temperate latitudes also) 2) Mountain belts: Wet zones: Sierra Nevada (California), Mangalore (S.W. India, coastal), Andes, Eastern Hawaii Clearly, there are some important, systematic patterns to where and when precipitation occurs. Letʼs find out why:

3 What is the general cause of precipitation? Amount of water vapor that can be held in air without growing water droplets or ice crystals depends very much on T. This makes sense: Consider a droplet of water in air: 1) Constantly loses some H 2 O's to the air randomly. Constantly gains some from the air. 2) If the amount of vapor in the air is very high, gain> loss, the droplet grows and eventually rains down. 3) If the amount of vapor is low, loss > gain, droplet shrinks and eventually is gone. 4) When there is a balance (gain = loss, we call this equilibrium), the amount of vapor is the maximum amount that can exist without droplets growing and rain occurring. Other names: Saturation, 100% humidity, dew point. 4) At higher T, there is greater vibration of the molecules, and thus greater loss from the droplet, so we need more vapor to reach that balance this. So.. Maximum amount of water vapor that can be held in air is greater at higher T. So then if moist air cools enough, it reaches a T at which droplets or snow/ice crystals form and grow. On earth, precipitation is caused by cooling of air. But...What causes the cooling? 1) Collision of a moist air mass with colder air. As you know from weather forecasts, storms exist at these collision points: Cold fronts, warm fronts. 2) Rising air. Important general point: Moist air rises, just like hot air rises. Hot, moist air rises the fastest. When an air mass rises for any reason it cools. Hereʼs why: Pressure is lower at higher altitude. A rising air mass expands as the pressure decreases. Expansion of a gas always leads to lower temperature. How much does the air cool? 10 C /km maximum (dry air); 5 C /km is rain/snow occurs Rising air generally tops out at 8 to 12 km max (top of the troposphere) At 10 km we expect the air T to be about 65 C cooler than ground If ground T is 20 C, then at 10km altitude, it is -45 C (= -49 F) How much vapor does it drop? If it goes all the way up to 10 km, almost all the vapor is lost. See graph of saturation water vapor versus T. Example: Rising air near the equator- hottest zone, lots of precipitation. Climate there tends to be very wet because of the regional rising air. Visible clouds on satellite images. Example 2: Thunder heads in the summer. Warm moist air rises strongly in column-shaped masses. Precipitation occurs as the air rises, creating the bulging cloud mass and rain or even hail.

4 So where on earth do we expect rising air in latitudinal belts? 1) Hot moist air rises near the equator. Hot because of direct sunlight. Moist because evaporation is MUCH faster at higher T 2) Convection cells: On earth things get complicated because of the Coriolis effect and we have 3 convection cells in each hemisphere (Hadley cells). Cell #1: Air rises near equator; the sinking part of that convection cell is at 30 N or S. Cell #2: The sinking air at 30 latitude is the downgoing part of the next cells that span from 30 to 60 latitude. This cell is odd; air sinks at 30 where the earth surface is warmer and rises at 60 where it is cooler. Cell #3: Rising air at 60, sinks at N or S pole. Satellite images of the earth show bands of clouds (wet weather) caused by rising air near the equator and at roughly 60 N or S. Mountains: The other cause of rising air. (Jargon: The orographic effect) Simple idea: Air moving over a mountain range MUST rise and cool. Picture air moving from west to east over the Sierra Nevada range in CA. 3 km rise causes cooling from, e.g., 20 degrees to zero More than half of moisture precipitates (e.g., 2% H 2 O vapor goes down to <1%;) This drops a large amount of rain or snow Examples: California vs. Nevada; Chile; Mangalore (4 m rain,june to Sept.); Regions of sinking air: Deserts. Air the upper atmosphere is cold and dry. Example: If air starts at 10km up, T might be -45 deg C- this means < 0.1% vapor! Air cannot hold more than that at such a cold T. Bring that air down, and it warms up to > +30 C and it is still very dry. Deserts exist along the 30 latitude lines (e.g., the Sahara). Rain shadows on the downwind side of mountain ranges. As air flows back down to lower altitude, it becomes warmer, and actually ends up warmer than it started (latent heat of water vapor goes into the air as precipitation occurs on the upwind side). And because it lost so much moisture, it is dry. Many mountain ranges have rain shadows on their downwind sides: Nevada is downwind from the Sierra Nevada Range; the Atacama desert is downwind from the Andes; south central India is downwind from the Western Ghats. Monsoon rains and annual cycles. Another complication on top of the Hadley cell pattern. Monsoon: Cycles caused by summer vs. winter T differences between continents and Oceans Summer: Continents heat up more than ocean Hot air rising over continent draws moist air from the oceans inland This brings monsoon rains as air rises to get over mountains or hills on the continent

5 Winter: Continents cool more than ocean Sinking dry air above continents, flows across continents; dry season The Hydrologic Cycle: Global View Youʼve all heard about the hydrologic cycle before: Evaporation, mostly from oceans Precipitation, on both land and ocean Precipitation that falls on land may get stored in lakes, ice, or groundwater for a while (perhaps thousands of years) Eventually the water makes it way back to the ocean to repeat the cycle. But also The atmosphere moves huge amounts of water vapor from low latitude to high latitude. Eventually, the air cools after it moves away from the warm low latitudes Water vapor precipitates- rain or snow. When the precipitation occurs, the latent heat of vapor is released to the air THIS MOVEMENT OF VAPOR FROM LOW TO HIGH LATITUDE IS A HUGE TRANSFER OF HEAT ENERGY FROM LOW TO HIGH LAT. Low latitude areas lose huge amounts of heat via evaporation. When precipitation occurs, the latent heat is given back to the air. High latitudes gain much heat via this mechanism. Without this effect, the earth would have much greater T extremes- Hotter near the equator and colder at higher lat. This is another way in which H 2 O moderates climate on earth.