ATMS 120 1st Edition Lecture 5 Outline of Last Lecture I Phase change for water A Energy associated with phase changes II Measuring moisture vapor pressure B Crushed can example III Saturation vapor pressure Outline of Current Lecture I How to measure moisture II Relative humidity III Upper air data Current Lecture I Just a quick clarification of some terms Vapor pressure the pressure exerted by water vapor it tells us exactly how much water vapor is in the atmosphere it is found by measuring the dewpoint temperature Saturation vapor pressure the value of the vapor pressure when the atmosphere is saturated with water vapor it is set by the temperature the hotter it is the more water vapor the air can contain Now moving on to a couple new terms Dewpoint temperature the temperature to which the air must be cooled for condensation to begin form clouds dew frost and fog when temperature is close to dewpoint temperature humid air when temperature is far from dewpoint temperature dry air II Relative humidity RH a measure of the actual amount of moisture in the air relative to its maximum capacity Relative humidity Vapor Pressure Saturation vapor pressure x 100 These 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 Example Saturation vapor pressure 12 Vapor pressure 4 RH 4 12 x100 33 In order to change the relative humidity either add or subtract water vapor or raise or lower the temperature If we heat the air up RH drops If we cool the air off RH rises If we add more water vapor RH rises It is easier to saturate cold air because it doesn t have the ability to contain lots of water vapor Isodrosotherm line of constant dewpoint temperature 5 degree F increment III The primary instrument to measure air data is called a Rawinsonde weather balloon These instruments typically measure temperature dewpoint temperature pressure wind speed and direction as a function of height There are two Rawinsondes released twice per day at 0Z and 12Z explained below The international standard for time universal coordinated time UTC UTC Zulu Z and a 24 hour clock is used In Illinois during daylight savings time March through November we are 5 hours behind UTC In Illinois during central standard time November through March we are 6 hours behind UTC
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