ATMO 336 1st Edition Lecture 1 Outline of Last Lecture I. No previous lecture Outline of Current Lecture II. Vapor pressureIII. Evaporation and condensation in a closed systema. Rate of evaporationb. Rate of condensationc. Saturation vapor pressureIV. Evaporation and condensation in the atmosphereV. ParcelsVI. Relative humiditya. Relative humidity formulab. Mixing ratio formulac. Saturation mixing ratio formulaVII. Dew pointVIII. Difference between air temp. and dew point temperature indicate whether relative humidity is low or highCurrent Lecture- Vapor pressureo Gas pressure (force/area) exerted by water vapor molecules alone Higher the water vapor molecules (number density), the higher the vapor pressureo Vapor pressure is one way to keep track of the amount of gas water vapor.o Higher vapor pressure= greater amount of water vapor in the air- Evaporation and condensation in a closed systemo Rate of evaporation= number of water molecules that change phase form liquid to gas each second. Set by the temperature of liquid water. Higher the temp., the faster the rate of evaporationo Rate of condensation= number of water vapor molecules that change phase from gas to liquid each second. Depends on vapor pressure in the space above the liquid surface The higher the concentration of water vapor molecules above the liquid surface, the faster the rate of condensation Occurs when water vapor molecules collides with liquid water surface and chemically binds to liquid water molecules At 20 degrees Celcius, the rate of condensation is zeroThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o In closed system, dynamic equilibrium will be reached where the rate of evaporation equals rate of condensation= saturation vapor pressure (air inside closed system is saturated with water vapor) Higher the air temp,. the higher the saturation vapor pressure (strong temperature dependence) Water will boil (reach 100 degrees F) when saturation vapor pressure is equal to the surrounding air pressure!- Evaporation and condensation in the atmosphereo Near earth surface, vapor pressure is usually LESS than the saturation vapor pressure, therefore rate of evaporation is greater than the rate of condensationo Once evaporated, air rises upwards and cools. As air cools, its saturation vapor pressure DECREASES  If air rises enough and cools sufficiently, it will not be able to hold all the water vapor it contains= condensation back into a liquid (this is how clouds form!)Tracking water vapor in the atmosphere (relative humidity and dew point)- Parcelso “parcel of air” used to explain behavior of airo air tends to move together in blobs about the size of parcels (not molecule by molecule)o parcel concept important in describing could and thunderstorm formation- Relative humidityo ratio of the amount of water vapor actually in the air to the max amount of water vapor required for saturation at a particular temperatureoo one way to take the ratio of the actual vapor pressure (e) in parcel (measured) with saturation vapor pressure (es) at parce temp (RH=e/es)o We use mixing ration to keep track of the amount of water vapor in air parcels. The greater the mixing ration, the more water vapor that is in the air The higher the air temperature, the greater the saturation mixing ratioo Use saturation mixing ratio (Us) to tell us the max amount of water vapor that can be in the air- Dew Point (Td)o Dew point temperature= temperature to which an air parcel would have to be cooled (with no change in its water vapor content) in order for it to be saturated with water vaporo As mixing ration (U) increases, the dew point temperature (Td) increaseso Dew point temperature does indicate the actual vapor content of the air The higher the dew point, the more water In the airo Use the relative humidity equation and the table to saturation mixing ratios to perform simple calculations of dew point temperature - The difference between air temperature and dew point temperature indicate whether relative humidity is low or higho 1. When the air and the dew point temp are far apart= relative humidity is lowo 2. When the air and dew point temp are close to same value= relative humidity is higho 3. When the air and dew point temps are same= the air is saturated and the relative humidity is 100%o Near earth’s surface, relative humidity is LESS THAN 100% = net evaporation is occurring (rate of evaporation faster than rate of condensation)o Question: besides fog, are there ever situations near the earth’s surface when net condensation occurs? Answer is YES (dew and frost) Dew if the object is warmer than 0 degrees Celsius Frost if the object is colder than 0 degrees Celsius (forms by deposition, water