Textbook Notes (362,820)
Canada (158,064)
Chapter 5

chapter 5.docx

7 Pages
Unlock Document

McMaster University
Environmental Science
Sean Mc Master

Chapter 5 The Hydrologic Cycle  The amount and timing of precipitation vary markedly from one region of Earth to another , the total amount of precipitation for the entire globe is relatively constant from one year to the next at about 104 cm ( 41 in) per year  There is continual replenishment of water lost through precipitation  Water is constantly added to and removed from the atmosphere  Hydrologic cycle: the movement of water between and within the atmosphere and Earth  The hydrologic cycle is amongst the fastest of all geochemical cycles; atmospheric residence time of water vapor is only 10 days or so  The hydrologic cycle is a continuous series of processes that occur simultaneously  The entire process has no real end or beginning  Evaporation can occur directly from the oceans or from water bodies on land surfaces such as lakes and rivers,  It can also occur indirectly through plants through a process called transpiration  The water that goes into the air eventually becomes water droplets or ice crystals in the form of clouds or fog  Many fog and cloud droplets or crystals will evaporate back into the air but others pericipiate down to the surface  The shortest root the cycle can take is from the ocean to the atmosphere and back to the ocean  Interception: process where water goes onto land and then vegetation , etc .--- it might drip or trickle down the plant on the surface or evaporate back into the air  Rainfall that doesn’t reach the land surface ,either directly or after interception might then flow above the surface into rivers which then transport the water into a lake or ocean or it can evaporate directly back into the atmosphere  If the precipitation falls as ice, it might temporarily remain on the ground before melting or it might be locked away for eons as part of a glacier  Liquid water at the ground doesn’t always flow along the along surface but instead penetrates into the ground in the process called infiltration  Such water is pulled downward by gravity and can collect in the pores of underlying soil or rock as groundwater  Much of the groundwater eventually seeps into rivers for eventual transport toward the ocean , where the cycle continues  But much is almost immediately withdrawn by plants and transpired to the atmosphere  Still smaller quanitites enter the animal kingdom as plants are consumed by browsers of all sizes , from elephants to microorganisms Water Vapor and Liquid Water  Although matter in the gaseous phase is highly compressible, the density of a gas cannot be increased to an arbitrarily high level  At some point a limit is reached forcing a change to liquid or solid state  For one atmospheric gas, water vapor, that limit is achieved at temperatures and pressures found on Earth  Air that contains as much water as possible is said to be saturated and the introduction of additional water vapor results in formation of water droplets or ice crystals Evaporation and Condensation  Evaporation: the process whereby molecules break free of the liquid volume  Condensation: opposite process where water vapor molecules randomly collide with the water surface and bond with adjacent molecules  As evaporation begins water vapor starts to accumulate above the surface of the liquid  At the early stage of evaporation, the low water vapour content prevents much condensation from occurring and the rate of evaporation exceeds that of condensation which leads to an increase in the water vapor present  With increasing water vapor content, however the condensation rate likewise increases  Eventually the amount of water vapor above the water surface due to offsetting gains and losses by evaporation and condensation  Saturation : The resulting equilibrium state , so when equilibrium exists in the atmosphere , the air is said to be saturated  Saturation can occur whether or not the air exist in the container  Water vapour is a gas, just like the other components of the air and it doesn’t tend to be held by air any more than the oxygen , nitrogen, argon and other gases of the atmosphere need to be held by water vapor  Sublimation: the direct change of phase from ice to water vapor without passing into the liquid phase  Deposition: the reverse process of sublimation which is water—to ice ,no liquid phase Indice of Water Vapor Content  Humidity: refers to the amount of water vapor in the air o Can be expressed in terms of the density of water vapor, the pressure exerted by the water vapor, the percentage of the amount of water vapor that can actually exist or several other methods o All of them apply exculively to water vapor , and not to liquid droplets or ice crystals suspended in or falling through the air Vapor Pressure  Each gas that makes up the atmosphere contributes to the total air pressure , with the most abundant permanent gases accounting for most of the pressure  Because water vapour accounts for more than 4 % of the total atmospheric mass , it exerts only a small percentage of the total air pressure  vapor pressure : The part of the total atmospheric pressure due to water vapor  like the atmospheric pressure, vapor pressure is expressed in units of millibars (mb) , in US and as kilopascals (kpa) in Canada but 10000 pa=1 mb= 0.1 kPa , in most scientific applications the Pascal is preferred  vapor pressure of a volume of air depends on temperature and density of water vapor molecules  if the air temperature is high, water vapor molecules along with all the other gaseous constituents of the atmosphere move more rapidly and exert a greater pressure  a greater concentration of water vapor molecules means that a greater amount of mass is available to exert pressure  temperature influences are small compared to density changes so vapor pressure closely follows changes in the density or abundance of water molecules  saturation vapor pressure: maximum vapor pressure; is an expression of the maxiumum that can exist ; depends only on TEMPERATURE  the increase in saturation vapor pressure with temperature is not linear,  at low temperatures there is only a modest increase in saturation vapor pressure but at high temperatures saturation vapor pressure grows rapidly  at temperatures normally econountered near Earth’s surface the saturation vapor pressure approximately doubles for every 10 * C increase in temperature Absolute Humidity  absolute humidity: measure of water vapor , which is simply the density of water vapor, expressed as the number of grams of water vapor contained in a cubic meter of air (the amount of moisture contained in a volume of air, so value changes when air expands or contracts ) Specific Humidity (q): represents atmospheric moisture  the mass of water vapor existing in a given mass of air  normally expressed as the number of grams outside the tropics usually is less than 2 % of the mass of the air near the surface , specific humidiites are normally less than 20 grams  q=m /m +m = m /m v v d v  where m rvpresents the mass of water vapor and m is the mass of atmosphere and m is thd mass of dry air  doesn’t change as air expands or contracts  is not temperature dependent  we express the mass of water vapor in the air as a proportion of all the air Mixing Ratio (r):  is very similar to specific humidity  r=m vm d  the mixing ratio and specific humidity will always have nearly equal values Saturation Mixing Ratio: is the maximum possible mixing ratio Relative Humidity: most familiar measure of water vapor content, relates the amount of water vapor in the air to the maximum possible at the current temperature RH= (mixing ratio/ saturation mixing ratio) * 100 %  water vapor exisits in warm air than in cold air , the relative humidity depends on both the actual moisture content and the air temperature  if the temperature of the air increases, more water vapor can exist and the ratio of the amount of water vapor in the air relative to saturation decreases  therefore, the relative humidity declines even if the moisture content is unchanged Dew Point temperature: a useful moisture index that is free of the temperature relationship, depends on the amount of water vapor present  when dew point is high, abundant water vapor is in the air,  when dew point is much lower than the air temperature the relative humidity is very low  when dew point is nearly equal to the air temperature the relative humidy is high  unlike relative humidity, the dew point doesn’t change because air temperature changes  dew point is always equal to or less than the air temperature, under no circumstances does it ever exceed the temperature Measuring Humidity  sling psychrometer: is the simplest and most widely used instrument for measuring humidity , which contains a pair of thermometers one of which has a cotton wick around the bulb that is saturated with water and the other thermometer hs no such covering and simply measures the air temperature  web bulb and dry bulb thermometers : are the 2 thermometers, that are mounted to a pivoting device that allows them to be circulated through the surrounding air  if the air is unsaturated water evaporates from the wet bulb, whose temperature falls as latent heat is consumed  the difference between the dry and wet bulb temperatures are called wet bulb depression , which depends on the moisture content of the air  if the air is completely saturated, no net evaporation occurs from the wet bulb thermometer , no latent heat is lost and the wet bulb temperature equals the dry bulb temperature  some psychrometers are equipped with fans that circulate air across the bulbs of the 2 thermometers o aspirated psychrometers: save the user the effort needed to sling the thermometers through the air as the aggravation of cleaning up the mess after accidently striking nearby objects o Hair hygrometer: whose basic part is a band of human hair. Hair expands and contracts in response to the relative humidity . By combining the strands of hair to a lever mechanism , we can easily determine the water vapor content. Often the gygrometer is coupled with a bimetallic strip and rotating drum to give a continuous record of temperature and humidity  Hygrothermogrpah: instrument that keeps track of humidity at major weather platforms such as airports and often housed in instrument shelters Distribution of Water Vapor  Water vapor at a point in the atmosphere is the result of either local evaporation or horizontal or vertical transport of moisture from other locations or altitutdes  The effect of horizontal transport on the distribution of water vapor is clearly  The effect of distance from the source of moistures is also evident in the West , with water vapor generally decreasing from the Pacific Coast to the Rocky Mountain , the most rapid drop occurs very near the coast because local mountains block off substantial amounts of moisture from inland areas  There is an increase in the amountof water vapor in the air in July over that in January  Horizontal distribution of water vapor varies with distance from the surface generally decreasing upward Processes that Cause Saturation  Air can become saturated by any 1 of the 3 genral proceses o Adding water vapor to the air--- ex bathroom taking a shower o Mixing cold air with warm; moist air o Lowering temperature to the dew point  Precipitation fog: forms beneath the cloud, when enough water vapor is added to the air to saturate  Although the toal water content in the mixed parcel is exactly between the amounts of the original parcels, some of the moisture occurs in the form of al iquid rather than as water vapor  The surplus therefore condenses to form fog droplets  The air is extremely turbulent in the wake of the aircraft, so the hot moist exhaust from the engines rapidly mixes with the cold surrounding air vapor directly forms into ice crystals or into liquid droplets that eventually freeze to form the contrail  Steam fog: lakes can stay warm into the fall or early winter even as air temperatures become low  because of evaporation and the upward transfter of sensible heat, a thin, transitional layer of air exists just above the water surface that is warmer and moister than the air above  if a mass of cold air abruptly passes over teh warm laek, the warm, moist transitional air mixes with the overlying layer of cold air to form a year of fog a metre or 2 thick  cloud formation --- by atmospheric cooling is the most common process Factors affecting Saturation  cloud and fog droplets are not purely made up of water  curvature o water doplests exist as microscopically small spheres o smaller objects are more curved than bigger objects o water doplets curve more remarkably have lesser semblance to a flat surface o curvature has an effect on evaporation from cloud droplet surfaces and therefore on the vapor pressures necessary for saturation o effects arising from surface tension lead to differences in the saturation point o for curved water surfaces, the evaprotion rate is greater o highly curved droplets of pure water require relative humidities in excess of 100 % to keep them fro mevaporating away o the degree of superstaureion decr
More Less

Related notes for ENVIRSC 1A03

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.