3Properties of Water Physical States Gas (Water Vapor) Liquid Solid Molecules move freely and mix well with other moleculesLiquidMolecules are close together and constantly bump one anotherSolidIn ice, molecules are arranged in a hexagonal crystalOnly natural substance thatoccurs naturally in all threestates on Earth’s surfaceIce Molecule
4Phases of Water Condensation Evaporation Melting Freezing Sublimation Molecules have enough energy to escape from the surface of ice into air above and directly into the vapor phaseDepositionWater vapor molecule attaches itself to an ice crystal and changes to ice
5Evaporation Water has a very high surface tension Takes energy to break the hydrogen bonds on a water surface in order to evaporateWhat can enhance evaporation from the surface of water?When temperatures are increases, molecules move faster (gain energy) and can break the surface tension more easilyWind also enhances evaporationWind enhances: by blowing the vapor molecules already in the air away. Therefore, this prevents saturation from occurring which would allow for a greater amount of evaporation.
6Condensation Depends on temperature For condensation to be really effective,water vapor needs something to condense onto.We call these things in air Condensation Nuclei.Dust, smoke, salts, other particles…When air is warm and molecules move fast, water vapor may bounce off the Condensation Nuclei.When air is cold and molecules move more slowly, water vapor is more likely to stick.This shows, again, that you are more likely to have more water in the vapor form in warm air than in cold air.
7SaturationIf we evaporate water in a closed container, eventually the evaporated water vapor will condense back into the liquid.The air above the water is said to be saturated with water vapor when the evaporation and condensation rates reach equilibrium.With the same number of water vapor molecules in the air, saturation is more likely to occur in cool air than warm air.
8So, we have all this really important water vapor in the air all of the time. It would be really helpful if we could keep track of it. Let’s review how we measure water vapor in the atmosphere.
9Absolute Humidity = Mv/ M Absolute humidity tells us the mass of water vapor in a fixed volume of air - or water vapor densityAbsolute Humidity = Mv/ MWhen a volume of air fluctuates, the absolute humidity changes even though the vapor content has remained constantTherefore, absolute humidity is not commonly used in atmospheric studies.The volume of an air parcel is constantly changing because of it rising and sinking - so therefore even though the water vapor content has remained constant, abs. humidity changes. So, since we are trying to measure the water vapor content in the atmosphere - this type of measurement is not useful
10Specific Humidity = rv/ (1 + rv) Specific Humidity (q)When the mass of the water vapor in the parcel is compared with the mass of all air in the parcel (vapor included)Specific Humidity = rv/ (1 + rv)This measurement does not change as a parcel rises and descends.
11Zonally Averaged Specific Humidity The specific humidity is highest in warm, muggy tropics and as we move away from the tropics, it decreases. Major deserts of the world are at around 30 degrees. This figure shows that at that latitude, the average air contains nearly twice the water vapor than the air at 50 degrees. Therefore, the air of the desert is not “dry” and the water vapor content is not very low.
12Mixing RatioCompares the mass of the water vapor in the parcel to the mass of the remaining dry air.R = e / (P – e)Very similar to specific humidityUses only dry air, where specific humidity uses the dry air PLUS the water vapor itselfMixing ratio (and specific humidity) stay constant as long as water vapor is not added to or removed from the parcel.
13Vapor Pressure (e)The air’s moisture content may also be described by measuring the pressure exerted by the water vapor in the air.Dalton’s LawThe total pressure exerted by the gases in a mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.For 1000 mb of air:78% N2 = 780 mb21% O2 = 210 mb1% H2O(v) = 10 mb ---> actual vapor pressureMore air = more pressureHigher vapor pressure = Larger # of water vapor moleculesLast bullet: more water vapor molecules = more vapor pressure just like when you have a balloon and put more AIR molecules into it, it’s total pressure increases.
14Saturation Vapor Pressure (es) Recall: when evaporation and condensation are in equilibrium, the air is saturated with water vapor.Saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature.It is the pressure that that amount of vapor would exert.
15Saturation vapor pressure depends primarily on the air temperature. Exponential relationshipWhen water and ice both exist below freezing at the same temperature, the saturation vapor pressure just above water is greater than the saturation vapor pressure over ice.The statement that water has a higher saturation vapor pressure than ice means that at any temperature below freezing, it takes more vapor molecules to saturate air directly above water than it does to saturate air above ice. This occurs because it is harder for molecules to escape from ice than water. (more air = more pressure?)
16Relative Humidity (RH) RH does not indicate the actual amount of water vapor in the air, but instead tells us how close the air is to becoming saturatedRH = (e/es) * 100RH = 100% is saturated airRH > 100% is supersaturated airMost common and familiar way of determining atmospheric moister but it is also highly misunderstood.It is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure).
17Changing Relative Humidity How do we alter a location’s relative humidity?Change the water vapor contentIncrease w.v. content raise actual vapor pressure relative humidity increasesChange the air temperatureIncrease temperature increase saturation vapor pressure relative humidity decreasesWarm = faster molecules = less likely to condense = lower RHReminder: RH = (e/es)*100
18Relative HumiditySince water vapor content generally does not vary much during an entire day, changing air temperature primarily regulates the daily variation in relative humidity
19Dew PointTemperature to which air would have to be cooled for saturation to occur (with respect to water).It is a good indicator of air’s actual water vapor contentHigher dew point = higher water vapor contentAdding w.v. to the air increases the dew pointFrost point: when dew point is determined with respect to a flat surface of ice
20Dew Point & RHRelative humidity can be misleading in indicating areas with high water vapor content.Dew point is important to look at, along with RH, in order to determine the water vapor content of a location.One location has a RH of 100% and a dew point of 0F while a second location has a RH of only 35% but a dew point of 45FWhich location has more water vapor in the air?Dry air can have high relative humidity.
21July Dew Point Averages Dew points are very high during the summer. The highest dew point area (south) receives humid air from the warm Gulf of Mexico. The lowest dew points are in Nevada where it is surrounded by mountains and is shielded from moisture moving in from the southwest and northwest.
22Skew T DiagramsSince the advent of rawinsonde observations, thermodynamic diagrams have been used to plot sounding data and to assess atmospheric stability.Despite numerous advancements in technology and forecast techniques, the thermodynamic diagram remains an essential tool of today's weather forecaster.
23Skew T Diagrams Why are skew T diagrams useful? Forecasting applications:Temperature and dew point profile of atmosphereDaily maximum temperatureLevel of cloud formationStable vs. unstable airPrecipitation type (icing forecasting)Level of tropopauseCAPE (Convective Available Potential Energy)Microburst forecastingAnd many more…