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Chapter 7

GG101 Chapter 7 Notes.docx

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James Hamilton

GG101 Chapter 7 Notes- Water and atmospheric Measure  Pure water is colourless, odouress and tasteless; yet, because it is a solvent (dissolves solids), pure water rarely occurs in nature.  Water weights 1g/cm^3  Constitutes nearly 70% of our body mass; we can only survive 2-3 days without water  Earth’s hydrosphere contains about 1.36cu. kilometers of water o Most water came from icy comets and hydrogen/oxygen laden debris  Outgassing: a continuing process by which water and water vapes emerge from layers deep within and below the crust  In the early atmosphere, massive quantities of outgassed water vapes condense and then fell to Earth in torrential rains. For water to remain in Earth’s surface, land temp’s had to stay below 100 Degrees C  Most water that was originally on Earth is still here today  Water is lost when it dissociates into hydrogen and oxygen and the hydrogen escapes Earth’s gravity to space  Eustacy: describes global sea-level condition; volume in the oceans  Glacio-eustatic factors: changes in the amount of water stored in glaciers and ice o In cooler times, sea level LOWERS due to more water being stored in glaciers and ice sheets o In warmer time, more water is melted therefore higher sea levels  From a geographic point of view, ocean and land surfaces are unevenly distributed  When you look at Earth from certain angles, it appears to have an oceanic hemisphere and a land hemisphere  Only 2.78% of water is freshwater, o Ice sheets and glaciers contain 77% of freshwater o Adding subsurface water amounts for 99.36% of freshwater o Remaining freshwater amounts to under 1% of all water in lakes, rivers and streams  Greatest single volume of lakewater resides in Siberia’s Lake Baykal o 70% of lakewater is in Asia, North America and Africa; over 3 million lakes exist in Alaska; Canada has around 32,000  Not connected to the ocean are saline lakes and slaty inland seas, contain 104,000km^3 of water Unique Properties of Water  Hydrogen and 2 oxygen atoms  Oxygen bond gives the hydrogen side of a water molecule a positive charge and the oxygen side a negative charge. As a result of this polarity, water molecules attract each other: the positive side of a water molecule attracts the negative side….hydrogen bonding  Polarity of water molecules causes water to “stick” to things and dissolve many substances  The effects of hydrogen are visible in everyday life o Hydrogen bonding creates the surface tension that allows floating of denser materials (i.e. steel), allows overfilling of a glass  Hydrogen bonding also causes capillarity, which you observe when you “dry” something with a paper towel o The towel draws water through its fibers because hydrogen bonds make each molecule pull on its neighbour o Capillary action is an important component of soil-moisture processes Heat Properties  For changes between solids, liquids and gases, HEAT ENERGY must be absorbed or liberated  In fact, the heat exchanged between physical states of water provides more thjan 30% of the energy that powers the general circulation of the atmosphere o Sublimation: direct change from vape to ice OR vice versa  Ice: goes from 4 to -29 degrees C; up to a 20% increase in volume is possible o Pure ice has 0.91 times the density of water, so it floats o Icebergs: 14% mass is exposed, the rest is hidden beneath ocean surface o Ice has affected Earth’s landforms  Water: the liquid phase o It takes 100 calories to boil 1g of water o Heat energy involved in the phase change is latent heat and is hidden within the structure of water; it becomes liberated whenever the phase reverses and a gram of water freezes  Water Vapour: the gas phase o An invisible, compressible gas in which each molecule moves independently of the others o The phase change from liquid to vapour at boiling temperature under normal sea-level pressure requires the addition of 540 calories for each gram, the latent heat of vaporization o When water vapour condenses to a liquid, each gram gives up its hidden 540 as the latent heat of condensation- remember the steam when draining pasta? o Takes 720 calories to turn 1g of ice into water vapour; takes 720 to LIBERATE from gas to ice Heat properties of water in Nature  In a lake or stream or in soil water, at 20 degrees C, every gram of water tjhat breaks away from the surface through evaporation msut absorb from the environment approximately 585 cal as the latent heat of evaporation; this is slightly more energy than would be needed if the water were at a higher temperature such as boiling  The process reverses (LIBERATION) when air cools and water vapour condenses vack into the liquid state, forming moisture droplets and thus liberating 585 cal for every gram of water as the latent heat of condensation  Estimated: moisture in Katrina totaled over 30T metric tons at peak  Latent heat of sublimation: absorbs 680 calories as a gram of ice transforms into vapour Humidity  Refers to water vapour in air  The capacity of air for water vapour is a function of temperature- the temperatures of both the air and the water vapour, which are usually the same  There are several ways of expressing humidity  North Americans spend money to adjust humidity: Air conditioning (EXTRACTING water vapes and cooling) or air-humidifying (ADDING water vapour) Relative Humidity: a ratio of actual water vapour in air to maximum water vapour possible in the air at that temperature.  Warmer air increases the evaporation rate from water surfaces, whereas cooler air tends to increase the condensation rate of water vapour to water surfaces  Relative humidity tells us how near the air is to saturation and is an expression of an ongoing process of water molecules moving between air and moist surfaces o 11am: evap rate > condensation, though not by as much since daytime temperatures are not as high, so water vapour now occupies half the maximum possible capacity o 5am: in the cooler morning air, saturation eq’m exists and any further cooling or addition of water vapes produces net condensation o When the air is saturated with maximum water vapour for its temperature, the relative humidity percentage is 100%  Saturation: at 100% relative humidity, when the rate of evaporation and the rate of condensation- the net transfer of water molecules- reach eq’m o Saturation indicates that any further addition of water capour or any decrease in temperature that reduces the evaporation rate results in active condensation o Dew-point temperature: the temperature at which a given mass of air becomes saturated and net condensation begisn to form water droplets; when dew-point temperature and the air temperature are the same o Frost Point: when temperatures are below freezing o A cold drink in a glass provides a common example: the water droplets that form on the outside of the glass condense from the air because the air layer next to the glass is chilled to below its dew-point temperature and thus becomes saturated. Walking to class you might notice damp lawns, an indication of dew-point conditions in cool morning air o Satellites using infrared sensors now routinely sense water vapour in the lower atmosphere o Water vapour absorbs long wavelengths (infrared), making it possible to distinguish areas of high and low water vapour  Daily and Seasonal Relative Humidity Patterns: An inverse relation occurs during a typical day between air temperature and relative humidity: as temperature rises, relative humidity falls o Relative humidity is highest at dawn during lowest temps o Relative humidity is lowerst in the late afternoon, when higher temps increase the rate
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