Phase change for water Water can change form between gas liquid and solid To do so Heat is required or Heat is released Remember the boiling pot thought experiment Measuring Moisture Vapor Pressure Dry air O2 and N2 Water Vapor H2O Total air pressure is the sum of the pressures from individual gasses Dalton s Law On Earth the primary contributors to the dry air pressure are O 2 N2 and Argon Since water vapor is a highly variable gas in space and time it can contribute a significant amount to the total air pressure Vapor Pressure Dry air mb Moist air mb Total pressure mb Vapor pressure can not be measured directly It is calculated based on the Dewpoint Temperature Surface of the Saturation Vapor Pressure Def of SVP SVP VP RH 100 Air is saturated SVP VP RH 100 Air is NOT saturated Air has a finite holding capacity for water vapor that depends on How to measure moisture Dew Point Temperature Td Definition Relative Humidity Definition Parcel of air Simple example SVP 12 spots set by the T VP 4 spots are full set by the Td RH 4 12 100 33 What if we heat the air up What it we cool it off What if we add more water vapor Moisture variability in time and space Isodrosotherm Practical Examples Bismarck ND T 35 F Td 34 F Phoenix AZ T 101 F Td 59 F Chicago IL T 55 F Td 45 F Miami FL T 86 F Td 68 F Which city has the highest relative humidity Which city has the lowest relative humidity Which city has the highest saturation vapor pressure svp Which city has the highest vapor pressure vp Which city has the highest moisture content Which city would sweating be the most least efficient way to cool off
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