Presentation on theme: "Unit 11: Atmospheric Moisture and the Water Balance"— Presentation transcript:
1 Unit 11: Atmospheric Moisture and the Water Balance Properties of WaterWater Vapor & itsMeasurementsThe Hydrological CycleEvaporationCondensation, CloudsPrecipitation ProcessesSurface Water BalanceThe hydrological cycle in action.
2 OBJECTIVES• Discuss the various forms of water and understand the important heat transfers that accompany changes of these physical statesExplain the various measures of atmospheric humidity, how they are related, and the processes responsible for condensation• Outline the hydrologic cycle and the relative amounts of water that flow within this cycle.Understand the process of evaporation.Examine the conditions necessary for the formation of clouds.• Introduce the concept of precipitation• Describe the Earth’s surface water balance and its variations
3 Phase Changes of Water, Latent Heat Heat is consumed in evaporation, melting; heat is released in condensation, freezing (sublimation).Schematic view of the molecular structure of water in its three physical states and heat-energy exchange among those states. The latent heat-exchange numbers between the arrows are explained in the text (values are for 0°C).
4 Measurements of water vapor Vapor pressure is the pressure exerted by water vapor moleculesSaturated vapor pressure is maximum pressure of water vapor at that temperatureDew point is the temperature the air must be cooled to reach saturationRelative humidity is ratio of water vapor in airto maximum the air can hold at thattemperature.RH is usually highest when daily temp-erature is lowest.Specific humidity is the mass of waterVapor in the air per unit mass of air.Mixing ratio is the mass of water vapor inThe air to the mass of dry air containing theWater vapor.Variation of saturation vapor pressure (mb) with temperature (C). The curve is nearly a pure exponential. At temperatures below 0C saturation values over supercooled water are greater than over ice.
8 Evaporation, Evapotranspiration The rate of evaporation depends on: temperature, humidity, wind speed and water quality (salinity)Plants lose water to the atmosphere through transpirationEvapotranspiration is the loss of water from the soils and plants to the airPotential evapotranspiration (PE) is the maximum evapotranspiration lost when abundant water is availableActual evapotranspiration (AE) is the amount of water lost in any actual amount of soil moistureAE is usually less than PE, except when there is abundant water available, like a swamp or water surface. In deserts, PE might be very high, while AE is very low.
9 Fig 12.2Hydrologic cycle. The numbers attached to the stages express each value as the volume of water divided by Earth surface area. Thus the values shown represent the depth of water (centimeters per year) associated with each mass transfer. All can be directly compared to the global average precipitation rate, which is about 100 cm/year.
10 Water in the Hydrosphere Most of the water is salt water in the oceanMost of the fresh water is locked up in ice sheets and glaciersMost of the liquid fresh water is in the groundFig 12.3Distribution of water in the hydrosphere. The middle and lower bars show the percentage distribution of the 2.8 percent of total hydrospheric water that is fresh. Of that freshwater component, only about one-tenth is easily available to humans.
11 Water Usage in the United States, 2005 Which states use the most water?Total water withdrawals (millions gallons per day) for the United States in 2005.
12 Condensation and Clouds For water vapor to condense into liquid, there must be-condensation nuclei for water molecules to condense upon-sufficient water vapor to reach saturation by either cooling or through evaporation of more water vaporCondensation at the ground is dew or frost (if temperature is below freezing)Condensation near the ground can form fog, if the cloud is in contact with the surfaceCondensation above the ground can form clouds
13 Condensation near the ground forms fog, a cloud in contact with the ground. As the ground and surface air cools to the dewpoint, water vapor condenses into a radiation fog (cooling by longwave radiation overnight) here in East Africa.
18 Cloud TypesSchematic diagram of the different cloud types – stratus, cumulus, and cirrus, arranged by their typical altitude.
19 Precipitation Processes The Ice-Crystal Process-requires the coexistence of ice and super-cooled water droplets in the cloud. Ice grows at the expense of water droplets that evaporate water molecules which adhere to the ice until they are large enough to fall as snow. If the air is warm enough, the snow melts and rain occurs.The Coalescence Process-requires different sizesof water droplets within warm clouds. Largerdroplets grow by falling faster and sweeping upsmaller droplets by coalescing until they arelarge enough to fall as rain.Source:
20 FIGURE 7. 6 The distribution of ice and water in a cumulonimbus cloud.
21 FIGURE 7.9 The ice-crystal (Bergeron) process. (1) The greater number of water vapor molecules around the liquiddroplet causes water molecules to diffuse from the liquid droplettoward the ice crystal. (2) The ice crystal absorbs the water vapor andgrows larger, while (3) the water droplet grows smaller.
22 FIGURE 7. 4 Collision and coalescence. (a) In a warm cloud composed only of small cloud droplets of uniform size, the droplets areless likely to collide as they all fall very slowly at about the same speed.Those droplets that do collide, frequently do not coalesce because of thestrong surface tension that holds together each tiny droplet. (b) In acloud composed of different size droplets, larger droplets fall faster thansmaller droplets. Although some tiny droplets are swept aside, somecollect on the larger droplet’s forward edge, while others (captured inthe wake of the larger droplet) coalesce on the droplet’s backside.
23 Types of Precipitation Besides rain and snow, there is:Sleet-melting ice that refreezes before reaching groundFreezing rain-melting ice that freezes on contact with a frozen surfaceHail-ice particles that grow withinclouds that have strong updraftsGraupel-soft, partially melted hailSource:Four major forms of precipitation. (A) A rainstorm douses the Ponderosa Pine Forest near Flagstaff, Arizona. (B) Falling snow accumulates in south-central Alaska. (C) Freezing rain forms an icy coating on pine needles on a golf course in Wawona, near Yosemite National Park, California. (D) Golf-ball-sized hailstones litter the countryside following a storm in northern Texas.
24 FIGURE 7. 22 A heavy coating of freezing rain (glaze) covers Syracuse, New York, during January, 1998, causing tree limbs to break andpower lines to sag.
25 FIGURE 7. 28 This giant hailstone — the largest ever reported in the United States with a diameter of 17.8 cm (7 in.) — fell on Aurora,Nebraska, during June, 2003.
26 Four Forms of Precipitation BAA-rainstormB-snowC-freezing rainD-hailCD
27 The Water BalanceBased on methods devised by climatologist C. Warren Thornthwaite, calculates inputs and outputs of water at the Earth’s surface based on simple formulae using monthly temperatures and precipitation of a station.When PE is greater than P (precipitation), there is water lossWhen P is greater than PE, there is a water gainBy calculating monthly PE values and comparing with P, one can calculate amounts of surplus (runoff) and deficit (not enough soil moisture) in surface water.Different climates exhibit different water balances
28 Water Balance for Different Climates Range of water balance conditions found at the surface of Earth. (A) Baghdad, Iraq, experiences a constant deficit because potential evapotranspiration normally exceeds precipitation. (B) At Tokyo, Japan, the situation is reversed, and a constant water surplus is recorded. (C) At Faro, Portugal, the intermediate situation occurs, with a combination of surplus and deficit at different times of the year.
29 Water Balance Averages by Latitude Average annual latitudinal distribution of precipitation, evapotranspiration, and runoff in cm per year. The arrows show the direction of the water vapor flux by the atmospheric circulation.
30 Global Distribution of Annual Evaporation, Evapotranspiration Fig 12.10?Global distribution of annual evaporation and evapotranspiration in centimeters, with land elevations adjusted to sea level. Red isolines show the pattern over land; blue isolines over the oceans.
31 Global Distribution of Annual Precipitation Source:Global distribution of annual precipitation in millimeters/day.