<P> In a similar fashion other chemical or physical reactions can take place in the presence of water vapor resulting in new chemicals forming such as rust on iron or steel, polymerization occurring (certain polyurethane foams and cyanoacrylate glues cure with exposure to atmospheric humidity) or forms changing such as where anhydrous chemicals may absorb enough vapor to form a crystalline structure or alter an existing one, sometimes resulting in characteristic color changes that can be used for measurement . </P> <P> Measuring the quantity of water vapor in a medium can be done directly or remotely with varying degrees of accuracy . Remote methods such electromagnetic absorption are possible from satellites above planetary atmospheres . Direct methods may use electronic transducers, moistened thermometers or hygroscopic materials measuring changes in physical properties or dimensions . </P> <Table> <Tr> <Th> </Th> <Th> medium </Th> <Th> temperature range (degC) </Th> <Th> measurement uncertainty </Th> <Th> typical measurement frequency </Th> <Th> system cost </Th> <Th> notes </Th> </Tr> <Tr> <Th> sling psychrometer </Th> <Td> air </Td> <Td> − 10 to 50 </Td> <Td> low to moderate </Td> <Td> hourly </Td> <Td> low </Td> <Td> </Td> </Tr> <Tr> <Th> satellite - based spectroscopy </Th> <Td> air </Td> <Td> − 80 to 60 </Td> <Td> low </Td> <Td> </Td> <Td> very high </Td> <Td> </Td> </Tr> <Tr> <Th> capacitive sensor </Th> <Td> air / gases </Td> <Td> − 40 to 50 </Td> <Td> moderate </Td> <Td> 2 to 0.05 Hz </Td> <Td> medium </Td> <Td> prone to becoming saturated / contaminated over time </Td> </Tr> <Tr> <Th> warmed capacitive sensor </Th> <Td> air / gases </Td> <Td> − 15 to 50 </Td> <Td> moderate to low </Td> <Td> 2 to 0.05 Hz (temp dependant) </Td> <Td> medium to high </Td> <Td> prone to becoming saturated / contaminated over time </Td> </Tr> <Tr> <Th> resistive sensor </Th> <Td> air / gases </Td> <Td> − 10 to 50 </Td> <Td> moderate </Td> <Td> 60 seconds </Td> <Td> medium </Td> <Td> prone to contamination </Td> </Tr> <Tr> <Th> lithium chloride dewcell </Th> <Td> air </Td> <Td> − 30 to 50 </Td> <Td> moderate </Td> <Td> continuous </Td> <Td> medium </Td> <Td> see dewcell </Td> </Tr> <Tr> <Th> Cobalt (II) chloride </Th> <Td> air / gases </Td> <Td> 0 to 50 </Td> <Td> high </Td> <Td> 5 minutes </Td> <Td> very low </Td> <Td> often used in Humidity indicator card </Td> </Tr> <Tr> <Th> Absorption spectroscopy </Th> <Td> air / gases </Td> <Td> </Td> <Td> moderate </Td> <Td> </Td> <Td> high </Td> <Td> </Td> </Tr> <Tr> <Th> Aluminum oxide </Th> <Td> air / gases </Td> <Td> </Td> <Td> moderate </Td> <Td> </Td> <Td> medium </Td> <Td> see Moisture analysis </Td> </Tr> <Tr> <Th> silicon oxide </Th> <Td> air / gases </Td> <Td> </Td> <Td> moderate </Td> <Td> </Td> <Td> medium </Td> <Td> see Moisture analysis </Td> </Tr> <Tr> <Th> Piezoelectric sorption </Th> <Td> air / gases </Td> <Td> </Td> <Td> moderate </Td> <Td> </Td> <Td> medium </Td> <Td> see Moisture analysis </Td> </Tr> <Tr> <Th> Electrolytic </Th> <Td> air / gases </Td> <Td> </Td> <Td> moderate </Td> <Td> </Td> <Td> medium </Td> <Td> see Moisture analysis </Td> </Tr> <Tr> <Th> hair tension </Th> <Td> air </Td> <Td> 0 to 40 </Td> <Td> high </Td> <Td> continuous </Td> <Td> low to medium </Td> <Td> Affected by temperature . Adversely affected by prolonged high concentrations </Td> </Tr> <Tr> <Th> Nephelometer </Th> <Td> air / other gases </Td> <Td> </Td> <Td> low </Td> <Td> </Td> <Td> very high </Td> <Td> </Td> </Tr> <Tr> <Th> Goldbeater's skin (cow Peritoneum) </Th> <Td> air </Td> <Td> − 20 to 30 </Td> <Td> moderate (with corrections) </Td> <Td> slow, slower at lower temperatures </Td> <Td> low </Td> <Td> ref: WMO Guide to Meteorological Instruments and Methods of Observation No. 8 2006, (pages 1.12--1) </Td> </Tr> <Tr> <Th> Lyman - alpha </Th> <Td> </Td> <Td> </Td> <Td> </Td> <Td> high frequency </Td> <Td> high </Td> <Td> http://amsglossary.allenpress.com/glossary/search?id=lyman-alpha-hygrometer1 Requires frequent calibration </Td> </Tr> <Tr> <Th> Gravimetric Hygrometer </Th> <Td> </Td> <Td> </Td> <Td> very low </Td> <Td> </Td> <Td> very high </Td> <Td> often called primary source, national independent standards developed in US, UK, EU & Japan </Td> </Tr> <Tr> <Th> </Th> <Th> medium </Th> <Th> temperature range (degC) </Th> <Th> measurement uncertainty </Th> <Th> typical measurement frequency </Th> <Th> system cost </Th> <Th> notes </Th> </Tr> </Table> <Tr> <Th> </Th> <Th> medium </Th> <Th> temperature range (degC) </Th> <Th> measurement uncertainty </Th> <Th> typical measurement frequency </Th> <Th> system cost </Th> <Th> notes </Th> </Tr>

How does water change into ice or water vapor