Atmospheric Moisture WX-201 Henry Robinson Water is an Odd Molecule • Boiling point much higher than one would expect from molecular weight. (Lighter molecular weight moving faster for given temperature so have colder boiling temperature) Subtance Molecule weight Boiling Temp Oxygen 32 -183 oC Nitrogen 28 -196 oC Water 18 100 oC Carbon Dioxide 44 -79 oC Water is a Polar Molecule Net positive charge on one side and a negative side on the other. Polar Molecule • Electrical negative will attract positive charge. • The electrical attraction of the polar molecules is quite strong. • The attraction makes water molecules want to join together, raising the temperature of the boiling and freezing point. It is often referred to as a Hydrogen Bond. • Polar attraction also makes water a good liquid solvent. Other molecules bind to water in solution. Ice Crystal • 105 degree separation of hydrogen causes six sided figure. Snow crystals six sided Reason why water is most dense at +4 degrees Celsius and ice floats Temperatures Colder than Boiling Temperature • At temperatures colder than boiling temperature, water can exist as a liquid (or solid if below the melting temperature). Molecules in liquid attract each other strongly. Held together by Van der Wals forces. • Some water molecules in the air are far enough apart that they don’t feel the attraction of other water molecules so they can remain a gas even though the temperature is below the boiling point. Water Vapor • As the number of water vapor molecules in the air increase in number, they will have more chance of being close to another molecule and being attracted to each other. • After there are a certain number of molecules in the air, any more molecules will cause the molecules to clump together forming a liquid. • This point is the saturation point. Saturation Point • Saturation point dependent upon: • temperature (higher temperature, faster molecules get away from one another) • pressure (number of molecules present) Evaporation/Condensation • Liquid-gas interface (surface of water) will exchange molecules. • Some molecules go from liquid to gas, some go from gas to liquid. When air is at saturation the number of molecules going equal the number of molecules coming. Water Vapor Pressure • Water vapor molecules are only a fraction of the molecules in the air. • Pressure is the sum of the weights of all the molecules (Dalton’s Law of partial pressure) • The water vapor pressure is the pressure caused by the water vapor molecules in the column above you. Saturation Vapor Pressure • Saturation vapor pressure is the pressure when air is at saturation for water molecules. (temperature dependent) Moisture Indices • A number of indices have been developed to describe the amount of water vapor in the air. • Different indices are useful for different applications. Water Vapor Variables • Absolute – Vapor Density – Specific Humidity – Mixing Ratio – Dew Point Temperature – Vapor Pressure • Relative – Relative Humidity – Heat Index or Effective Temperature Vapor Density • Vapor density r • Mass of water vapor per unit volume r = mass of water vapor unit volume Specific Humidity • Specific humidity= mass of water vapor total mass of air • Only depends on the amount of water vapor • Not dependent on pressure or temperature • Frequently used in weather forecast model calculations. Mixing Ratio • Mixing ratio =mass of water vapor mass of dry air • Also conservative in that it only depends on amount of moisture. Does not depend on temperature or pressure. • Frequently used in calculations of transport of air parcels, such as in thunderstorms, storms, etc. Relative Humidity • Relative Humidity=actual vapor pressure * 100% saturation vapor pressure • Is the ratio of the air’s content to its capacity. • When relative humidity reaches 100% condensation will occur. • Saturation vapor pressure is dependent upon temperature, so RH is dependent on both the amount of moisture in the air and the temperature of the air. • Frequently used by public. Relative Humidity Calculation Relative Humidity will vary during the day as temperature changes even if the amount of moisture remains constant Dew Point Temperature • Dew Point Temperature is the temperature where saturation occurs. • Dependent only on moisture content. • Used by the NWS for observations of moisture. • When dew point temp=air temperature, 100% RH Wet Bulb Temperature • Lowest temperature that can be reached by evaporating moisture into the air. • Related to air temperature, moisture content. • Table in Appendix D for conversions. • Used to measure cooling potential of evaporation. Used to measure moisture content with sling psychrometer instrument. Heat Index • The “feels like” temperature. • Function of air temperature and relative humidity. • Used to measure potential for heat related health problems. Table in Appendix G Moisture Index Conversions • All of the moisture indices are related to one another through mathematical formulas or tables. (see Appendix D and G, Table 1 and figure 5.10 in Chapter 5) • Application determines which index is used. VAPOR PRESSURE VP RH = SVP VAPOR PRESSURE VP A= SVP 8 mb A= = 67% 12 mb VP B= SVP 24 mb B B= = 57% 42 mb A DEW POINT Td = dew point “The temperature to which air must be cooled in order for condensation to occur” Dew point and RH Dew Point (DP) is temperature where the RH is 100% for a given amount of moisture in the air. RH is the actual vapor pressure divided by the saturation vapor pressure for the current temperature. Example 1: Air temperature 70F = 21C Dew Point 10F =-12C SVP=25mb (from graph 70F go up to curve and over to 25mb) Actual vapor pressure=2.4mb (go up from 10F to curve and over to 2.4mb) RH=2.4/25=9% Example 2: Air T=70F; DP=50F; SVP=25mb actual VP=12mb. RH=12/25=48% Moisture Measurements Hair Hygrometer • Human hair gets curly (changes length) with humidity. Levers amplify the change in length. • Measures Relative Humidity directly. • Relatively cheap instrument. Moisture Measurements Sling Psychrometer • Two thermometers; one wet, one dry. Move through the air (either fan or swing around). • Wet thermometer measures wet bulb temperature Moisture measurements Dew-Point • Cool mirror until dew (or frost) forms on mirror. • Used recently by NWS at all automated surface observing stations. New NWS Humidity Sensor Moisture Measurements Electrical Hygrometer • Uses a chemical film that absorbs moisture which changes the electrical resistance. • Used in radiosonde balloon measurements. Summary • Water is a unique molecule. It is strongly polar with + and – sides that attract other molecules. • Can have free water vapor molecules in air at temperatures colder than boiling point of water. • When number of water vapor molecules exceed a certain number, water will start to condense out of the air. This is the saturation point. • The vapor pressure of water vapor is the weight of all the water vapor molecule above the point. Summary (cont.) • Saturation vapor pressure • Moisture indices: • Specific Humidity, Mixing Ratio, Relative Humidity, Dew Point, Wet Bulb temperature, Heat Index. • Moisture measurements: Hair hygrometer, sling psychrometer, chilled mirror, electrical hygrometer.
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