2 Review of last lecture: Components of global water cycle Ocean waterLand soil moisture, rivers, snow cover, ice sheet and glaciersSea iceAtmosphere water vapor, clouds, precipitationWater in biosphere (including human beings)
3 Water (H2O ) is unique on earth because it can exist in all 3 states (phases) An H2O molecule3 states (gas, liquid, solid) depending on how the molecules are connected togetherCan change from any state to any other state. Latent heat is consumed or released in a phase changee.g. Evaporation -> liberation of water molecules, requires energy
4 Evaporation and Condensation Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates.Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation.There is an equilibrium between evaporation and condensation during saturation. Upon saturation, evaporation rate equals condensation rate.
5 Sublimation and Deposition Water vapor (gas) can change directly into ice or snow (solid) during deposition.Ice or snow (solid) can turn directly into water vapor (gas) during sublimation.There is an equilibrium between deposition and sublimation during saturation. Upon saturation, deposition rate equals sublimation rate.
6 Indices of Water Vapor Content Humidity: amount of water vapor in airHumidity expressed in a number of ways IndicesVapor Pressure: the partial pressure exerted by water vapor. Saturation vapor pressure (SVP) – maximum amount of vapor that can exist at a given temperature, increase w/ TairAbsolute Humidity: density of water vapor expressed in g/m3Specific Humidity: mass of water vapor (g) per mass of air (kg) (in g/kg) Saturation specific humidity (qs): highest specific humidity for a given temperature and pressureMixing Ratio: amount of water vapor (g) relative only to mass of dry air (kg) Saturation mixing ratio: maximum mixing ratioRelative Humidity: the amount of water vapor in the air relative to the possible maximum.Dew point temperature: temperature at which saturation occurs in air (generally colder than Tair, equals to Tair when saturated)RH=q/qsx100
7 Saturation vapor pressure Saturation vapor pressure is temperature dependent.Saturation vapor pressure increases with temperature. Warmer air can hold more water vapor.It’s a non-linear increase. At low temperatures the saturation vapor pressure increases slowly but it increases rapidly at higher temperatures.
9 Methods to achieve saturation and condensation Diabatic processes – add/remove heatConduction (e.g. movement of air mass over a cold surface)Radiation (e.g. cooling of boundary layer air by longwave radiation)Adiabatic processes - no addition/removal of heatAdd water vapor to airMix warm air with cold airCooling of air parcel when it rises (because air parcel expands when it rises, like a balloon)1st Law of Thermodynamics expanding air cools, compressed warms (like a manual hand air pump).
10 Forms of Condensation: saturation droplets or ice crystalscondensation/deposition dew, frost, fog, clouds
11 Forms of Condensation: Dewliquid condensation on surfaceoccurs early morning on windless cloudless daysair immediately above ground cools, reaches dew pointdiabatic process
12 Frost deposits white ice crystals known as hoar frost ~ similar to dew BUT saturation occurs below 0oCdeposits white ice crystals known as hoar froste.g. car windshieldphase change from vapor directly to solid (deposition)diabatic process
13 Frozen Dewresults when saturation occurs slightly above 0oC liquid dewformed, when Temp drops liquid dew freezesforms thin sheet of ice, tightly bound to surfacedangerous – black ice
14 Fog air has either been: cooled to dew point had moisture added can be considered a cloud with base at ground levelair has either been:cooled to dew pointhad moisture addedmixed with warm moist air5 different typesradiationadvectionupslopeprecipitationsteam
15 Radiation Fog longwave radiation reaches Dew Pt occurs when near surface air chills diabatically through loss oflongwave radiation reaches Dew Ptrequires cloudless nights and light wind to create mixed layer‘burns’ off with sunrise – evaporates from below due to surfaceheating
16 Advection Fog occurs when warm moist air moves across a cooler surface air is chilled diabatically to saturationcommon on the U.S. west coast warm, moist air fromPacific advects over the cold California currentFrequently develop near boundaries of opposing ocean temperaturese.g: northeast coast of the U.S., Gulf Streamand Labrador current
17 Upslope Fogdevelops due to adiabatic coolingoccurs when air is lifted over topographic barriers, mountainsair expands and cools as it risescommon in region between Great Plains and Rocky Mountain foothills
18 Precipitation FogRain occurs and some evaporates as it falls toward EarthSometimes this will lead to saturation near surface and cause fogAdiabatic processWeather doctor almanac 2002
19 Steam Fog Adiabatic process (no net change of energy) Mixing of warm, moist air with cold airAdiabatic process (no net change of energy)e.g., common when cold air move over warm lakes/streams in autumnSteam Fog – can see plumes risingWeather doctor almanac 2002
20 Different types of fog found throughout the U.S.
22 SummaryWater Vapor Basics (names of different phase changes, latent heat)Humidity indices (there are 6 total). Saturation vapor pressure increases non-linearly with temperatureTwo methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.