Fair or Foul: Part 1, Observing Temperature, Humidity, and Precipitation

Our up-time characters are in Little Ice Age Europe now, and hence neither their experience with twentieth-century American agriculture nor their limited literature on twentieth-century European agriculture are a completely reliable guide as to what crops will grow where. The effect of the Ring of Fire on climate is also somewhat uncertain.


Airship lift depends on the difference in density between the lift gas and the ambient air, and thus in part on their respective temperatures. So temperature is relevant to airship pilots, not just farmers.


We need to start making accurate records of weather conditions, and we will certainly be looking at temperature, humidity, and precipitation.






In Grantville, the most common form of outdoor thermometer is the liquid expansion thermometer. Most such thermometers will probably use, as the “thermometric liquid,” an organic liquid with a red or blue dye, but it will be common knowledge that mercury may also be used.


Mercury has the advantages of being opaque, easily purified, chemically stable, not wetting or chemically attacking glass, liquid over a wide temperature range (-38.8oC to 356.7oC, thus unlikely to evaporate at the top of the column), and having clearly defined meniscus, a high thermal conductivity, low specific heat (making it rapidly responsive to changes in temperature), and a fairly linear coefficient of thermal expansion. Unfortunately, it is poisonous, the expansion is small compared to alcohol, and in very cold climates it can solidify.


Several different organic liquids have been used, but the most readily available in the 1632 universe is ethanol, with a liquid range of -114 to 78oC. My prediction is that mercury will be used for a small number of precision reference thermometers and the actual weather stations will use ethanol thermometers.


Glass composition is also significant. It is not just the liquid, but also the glass, that expands as the temperature increases, and not entirely linearly (or at the same rate as the liquid). Also, after being first heated and then cooled, the glass bulb of some compositions did not return to its original dimensions, leading to a slow rise in the zero of mercury thermometers. In the late nineteenth century, Schott developed a series of more stable glasses, notably borosilicate (Pyrex(R) glass) (Vogel 21). Hard glasses are generally preferred (EB11/Thermometry).


Most modern meteorological thermometers have the stem, with engraved scale markings, inside a protective glass sheath, and there is a white enamel backing on the stem to make the liquid movement more visible. (Srivastava 96). My “hardware store” thermometers are unsheathed, and the scale is on a separate attached metal frame. The attached scale will “inevitably move slightly with time.” (Burt 116).


The first thermometers sensed air rather than liquid expansion. The first known drawing of a thermometer is from 1611. It shows an inverted flask with a long narrow stem, fitting into the neck of a short-necked flask, the latter partially filled with water. The bottom of the stem of the first flask is below the liquid surface. A rise in temperature caused the expansion of the air in the short flask, pushing the water up the stem. Alongside the stem there was a scale divided first into eight degrees and these each into six ten-minute intervals. Its inventor, possibly living in Rome, is unknown (Middleton 11).


The basic problem with unsealed air thermometers was that the expansion of the air was a function of pressure as well as temperature. In 1632 Jean Rey (1583-OTL c1645) dispensed with the second flask, and turned the first flask stem upward, creating a liquid expansion thermometer. However, the tube was unsealed so errors could arise from evaporation of the water (27). The sealed spirit-in-glass thermometer is attributed to Ferdinand II, Grand Duke of Tuscany, and most likely invented in 1654. The first experiments with mercury were in 1657, but the Tuscan academicians deemed it inferior in performance (28-37).


Before leaving the subject of early temperature measurement, I wish to call the reader’s attention to Fitzroy’s chemical weather glass (1862), as is it the sort of curiosity that a resident of Grantville might have inherited, or picked up at a craft fair, before the Ring of Fire. It “consisted of a solution of camphor and certain inorganic salts in aqueous alcohol, sealed in a glass tube.” Negretti & Zambra used potassium nitrate and ammonium chloride. The salts formed crystalline dendrites, and Fitzroy claimed that when the crystals built up, the weather would get colder and stormier, and if they disappeared, it would be dry and clear. Studies by Mills have shown that the chemical weather glass is sensitive both to the current temperature and “any preceding regime of temperature changes.” It is thus a thermoscope. Mills comments, “A rapid fall in temperature associated with an approaching vigorous cold front could conceivably trigger … rapid crystal growth if observed at a fortuitous time, but in general any correlation between appearance and future weather patterns would be purely coincidental.”


Manufacture. In 1612, Giovanfrancesco Sangredo (d. 1620) made several thermoscopes, at a cost of four lire each. These had no scale, but the column height could be measured with a caliper. He apparently made use of “a wine glass with a foot, a small ampoule, and a glass tube,” and he could make ten in an hour.


The Grand Duke’s glassblower, Mariani, had incredible skill and was able to manufacture thermometers with a “50 degree range” (corresponding to the modern -18.75 to 55oC) with great consistency. He admitted, however, that he could not do this for the Medicean 100- and 300-degree range thermometers, because “inequalities could more easily occur in the larger bulb and longer tube” (Middleton 34-5). On the other hand, Middleton asserts that “workmen north of the Alps found it difficult enough at first to make a plain bulb and tube and fill it with spirit of wine” (132).

Roemer proposed that after forming the tube, it be examined for uniformity by examining the length of a drop of mercury as it passed down the bore. If the tube was found to be irregular, it was discarded, and if conical (the length increased or decreased at a constant rate), he took measurements and divided the bore into four equal volumes (67).


While in many thermometers the bulb was blown on the capillary tube, EB11/Thermometry recommends that it be formed of a separate piece of glass fused onto the stem.


Bimetallic Thermometers. In Grantville, there should also be thermometers with a dial readout. These have a strip with two different metals layered together, usually brass and iron. The metals have different coefficients of expansion and thus the strip bends toward the less responsive metal. The deflection is proportional to the temperature change and to the square of the length; winding the strip into a helix allows a long and thus more sensitive element to be relatively compact. A pointer is connected to the center. Generally speaking, they are less accurate than liquid expansion thermometers, and require weekly (if not daily) recalibration (Thermoworks), but they are the basis for the most common kind of thermograph.


Platinum Resistance Thermometers. These, known as RTDs (Resistance Temperature Detectors) rely on the change of electrical resistance with temperature. EB11/Thermometry provides formulae, circuit schematics, and comments on errors and corrections. The current levels must be kept very low (<1 ma) to minimize self-heating (Srivastava 135, 137).


In the twenty-first century, RTDs are available in two grades, “standard” and “industrial.” RTDs will not be found in Grantville homes or schools, but it is conceivable that the power plant has them (most likely “industrial” grade). The standard RTDs are used as primary reference thermometers. They have platinum wire of 99.999% purity wound in a strain-free configuration (MINCO). Unfortunately, the strain-free resistance element is extremely delicate (Ripple), so SPRDs are used in laboratories.


The industrial grade RTDs use platinum of lower purity and also have a simpler construction in which the resistance element is supported (or thick enough to be self-supporting). When calibrated, they have an accuracy of perhaps 0.01oC, an order of magnitude less than the SPRTDs. But they are also cheaper to make and calibrate (Fluke).


There is a small quantity of platinum available in Grantville in the form of jewelry, and it may be sufficient for experimentation. Commercial development of RTDs will have to await platinum mining (see Cooper, Mineral Mastery, Grantville Gazette 23) and purification. Developers will have to worry not only about platinum purity, but also about mounting the wire so as to minimize the strain caused by thermal expansion and contraction (Price).


Even if the wire is not subject to chemical attack, it is mechanically fragile, and the wire is typically protected from the medium by encasing it in a glass, quartz, porcelain, or metal tube (Patranabis 223). A plastic cladding might also work. In any event, the sheathing increases the lag time (Srivastava).

Platinum’s advantages are that it is a noble metal, with a high melting point, and that it has a very linear response over a wide temperature range. Copper is more responsive, and linear over the range -50 to 150oC, but subject to chemical attack. Nickel is even more responsive, and is chemically resistant, but there is no simple formula for calculation of its resistance (MINCO). One can scavenge the nichrome wire heating element from a defunct toaster or heating pad. However, nichrome actually has a rather low temperature sensitivity (Lemieux). My expectation is that the first NTL resistance thermometers will use copper wire.


Thermistors. In an automated weather station, there’s no one to go out and read the mercury (or spirit) level on a conventional thermometer. Hence, some sort of electrically based temperature sensor is needed, and the platinum resistance thermometer (RTD) is too expensive for most meteorological applications.


A thermistor is a resistor whose resistance is temperature-dependent. In 1833, Faraday discovered that the electrical “resistance of silver sulfide decreased dramatically as temperature increased;” i.e., it is a negative temperature coefficient (NTC) material (Wikipedia). The first commercial thermistor was Ruben’s (1930).


There are thermistors in Grantville; they are the sensing element in the digital clinical thermometer. They are ten times as sensitive as an RTD but their temperature response is highly nonlinear (exponential). Also, a single thermistor has a useful temperature range of not more than 100oC and their maximum temperature of operation is 110oC(Ripple) (so don’t take them into the desert). (Industrial RTDs can be used outside the thermistor range.)


I assume that one of the electrical engineers in Grantville has Dorf’s Electrical Engineering Handbook (2d ed., 1997). It discloses that NTC thermistors are “ceramic semiconductors made by sintering mixtures of heavy metal oxides such as manganese, nickel, cobalt, copper and iron” (14). It is known that the automation engineering department in the power plant and public works department have copies of Instrument Engineer’s Handbook Third Edition, edited by Béla Lipták, and The Instrumentation Reference Book, Third Edition, edited by Walt Boyes. Both have extensive sections on thermometry and temperature measurement instrumentation. So that gives us a starting point, but I suspect that these must be purified to very high purity and we must also experiment to find which combinations provide strong temperature dependencies. The simplest type of thermistor to make is probably a bead; the metal oxide powders are combined with a binder (to be determined!) to make a slurry and this is applied to a pair of platinum alloy wires held parallel. The beads are dried and then fired in a furnace at 1100-1400oC to sinter the particles (Lavenuta). Given the infrastructure and experimental requirements, I am doubtful that a practical thermistor can be built before the NTL late 1640s.


Scale, Range, and Calibration. For the thermometer to be useful in meteorology, we needed to have a way of assuring the comparability of observations made with different thermometers.


If the scale were an arbitrary one, then the only way of calibrating the scale of a new thermometer would be to place it next to a reference one, expose them to several markedly different temperatures, and then mark the tube of the new one to correspond to the temperatures displayed by the reference one.


It was realized at a quite early stage that the temperature scale should be defined according to reference points corresponding to readily reproducible laboratory conditions. Then a reference thermometer is not needed at all. By 1702, Roemer proposed a scale in which 7.5 was the melting point of ice and 60 the boiling point of water. A decade later, Fahrenheit experimented with several scales, of which the final one had 32 as the melting point of ice and 96 as human body temperature (now known to be 98.6oF). He extrapolated that on that scale, the boiling point of water would be 212oF, and it was only later that others adopted that as the “hot reference” for his scale (Middleton 78-9). Celsius, in 1742, proposed the melting point of snow as the cold reference and the boiling point of water when air pressure was 25.25 Swedish inches as the hot reference, with 100 degrees in between. Other inventors proposed other scales, and a mid-eighteenth century thermometer featured eighteen scales.


The modern thermometers found in Grantville are likely to be marked in both Fahrenheit and Celsius, and it is very likely that the scientists and engineers in Grantville will push very hard for one or both of these scales to be universally adopted.


The portion of the standard temperature scale that is marked on the thermometer is its range. Typically, the bigger the range, the less accurate the reading; for ordinary thermometers, an error equal to 1-2% of the maximum range is not unusual. The outdoor thermometers I own have a range of -50 to +50oC.


Calibration has three aspects: (1) marking the thermometer scale so as to correspond exactly to the reference scale at the two points and at least roughly at in-between points, (2) tabulating the remaining errors in the thermometer scale, and (3) checking the thermometer from time to time to determine the necessary adjustments for physical changes in the instrument.


When matter is changing phase (between solid and liquid, or liquid and gas, or solid and gas), as long as both phases are present, the temperature should remain constant. Hence, the melting point of ice and the boiling point of water are, at least theoretically, “fixed points.”


In 1777, the British Royal Society reported on “the best method of adjusting the fixed points of thermometers.” They had found that depending on the manufacturer, thermometers could differ by 3.25oF in their measure of the temperature of steam. Accordingly, they gave specific instructions as to the design of the vessel (a cylindrical pot with a cover and a chimney, the latter covered with a loose-fitting tin plate), the placement of the thermometer inside, the application of the heat, and the correction for atmospheric pressure. For the ice point, the Society called for the crushed ice to reach almost to the top of the column, and for provision to made for drainage of the meltwater (Middleton 128).


The vessel used in the boiling point determination is called a hypsometer, and there is a diagram and brief description in EB11/Thermometry. The boiling point needs to be corrected for differences in pressure from the reference pressure. Characters should not use the correction set forth in EB11, but rather one based on modern steam tables. (The power plant should have them.)


Some of the precautions recommended by the Society are now known to inhibit superheating, a phenomenon in which liquid water exceeds its nominal boiling point (Chang).


Modern ice slush and steam calibration baths can achieve accuracies of 0.002oC and 0.1oC respectively (Moore 614).


Even though we can use the Celsius reference conditions to define a scale from first principles, for meteorological purposes, the range -50 to +50oC is much more useful than one of 0 to 100. For that range, other reference points may prove helpful. (The accuracy that can be expected “without extraordinary attention to purity” is typically about 1oC for most of the transitions (although it is 0.05oC for melting gallium) (Moore). Unfortunately, only a few of the “standard” phase transition baths have temperatures in that range, and we don’t have access to gallium (melting point 29.7646oC) or indium (159.5985oC). Mercury is available and melts at -38.8344oC. We might be able to obtain p-xylene (13oC); this would involve isolating it from a natural source (perhaps wood tar) or synthesizing it from readily obtained chemicals. Most syntheses also produce its two isomers, which have different boiling points, and the separation isn’t trivial despite that difference.


Studying physical data on organic compounds (The CRC Handbook should be in Grantville), I have noted some common chemicals with phase transitions in the meteorological temperature range: the boiling points of acetone (56.2oC) and benzene (80.1), and the melting points of tert-butyl alcohol (25.7) or glycerol (17.8). In each case, you must be sure that the chemical is pure (so you can rely on the reporting values) and that both phases are present. In general, melting point determinations are better than boiling point ones, because the latter are also affected by atmospheric pressure.


I have also found reference to the use of crystal transition temperatures, at which a crystalline salt changes form (perhaps as a result of the loss of water of hydration). For example, the transition temperature at which both sodium sulfate decahydrate and anhydrous sodium sulfate coexist is 32.383oC (Middleton 57). Sodium sulfate (Glauber’s salt) is commonly used because its transition temperature is close to room temperature and it is easily purified by successive recrystallization. Another possibility, once we have access to chromium ores, is sodium chromate decahydrate, which transitions to the hexahydrate at 19.529oC (Magin; Richards).


Once the two reference points are marked on the scale, the intermediate points can be marked manually by geometric dividing methods (these are known to the down-timers) or ultimately mechanically by a “dividing engine.” Either way, a uniform division is achieved.


Unfortunately, the behavior of liquid expansion thermometers is not entirely linear. The liquid and glass may change expansion rates with temperature, and the bore might not be uniform.


Modern precision meteorological thermometers are calibrated by putting the thermometer in an alcohol, water, or paraffin bath that is heated to a series of set temperatures, say 10oC apart (Srivastava 108). Naturally that means that you need a calibrated and even more accurate thermometer for monitoring the bath temperature. The platinum resistance thermometer is excellent for this purpose. (Platinum resistance is highly linear over the meteorological range, Middleton 180) The NWS in 2014 uses an SPRTD (NWSRS 8), but our characters would have to settle for less. Even better, this thermometer is incorporated into a thermostat so that the heat is turned on or shut off as needed to maintain the set point temperature. A table is prepared showing the correction needed by the test thermometer to match the reference thermometer at each of these calibration marks, and the observed applies the correction as appropriate.


On early thermometers, the scale was drawn on paper that in turn was mounted on a wood board. Scales have also been engraved on metal, glass, or ivory back plates, or directly onto the thermometer tube (etched with hydrofluoric acid).


Even a calibrated thermometer needs to be recalibrated from time to time. For example, the residual strain in a glass thermometer eases slowly, causing the glass to shrink. Most of the change occurs in the first year (Bentley 2:98).


Recalibration involves carrying an “inspector thermometer” (precisely calibrated in the laboratory) to each weather station. The station thermometer and the inspector thermometer are exposed to an ice bath (the single calibration point is good enough for a liquid expansion thermometer, see Ripple) and the station thermometer’s correction table updated.


In twentieth-century practice, inspector thermometers are mercury-based. It may have a narrow bore, so the change in length of the column is greater for a different temperature change. The downside is that the inspector thermometer must either be longer than the norm, or have a restricted range (say 30oC) (Srivaslava 103).


Accuracy. Spirit thermometers typically have an accuracy of 1-2 degrees Celsius in the meteorological temperature range (Facts).


In 2014, for NWS land stations, current and maximum temperature must be measured with 1oF accuracy in the range -20 to 115oF, and 2oF in the extreme ranges -40 to 20oF and 115-140oF. Minimum temperature accuracy is 1oF for -20 to 110oF, and 2oF for -80 to -20 (NWSRS 7). The data is nonetheless reported to the nearest 0.1oF (9).


Interestingly, this performance standard is inconsistent with the WMO recommendation that in the central range the maximum error be less than 0.4oF; NWS comments, “in practice, it may not be economical to provide thermometers that meet this performance goal.”


The accuracy with which temperature is measured can be increased by using a panel of several thermometers. If the thermometers are equally inaccurate and there is no systematic bias, the average of four thermometers will be twice as accurate as just one. (The standard error is proportional to the individual standard deviation and inversely proportional to the square root of the sample size.)


Maximum and Minimum Thermometers. These indicate the extreme value since they were last reset by the observer.


EB11/Thermometry (836) describes three different kinds of maximum thermometers: the Rutherford (1790) type, in which the mercury in a horizontal tube pushes a steel (originally, glass) index and leaves it behind when the temperature drops; that of Negretti and Zambra, with a constriction in the horizontal tube pass the bulb (the mercury expands past the constriction but the “column” breaks there when it contracts); and that of Phillips (1832) and Walferdin (1855), where the horizontal “column” is divided by a bubble of air that acts as an index. Note that the physician’s thermometer is really a maximum thermometer of the constriction type. The Rutherford type was “little used” by 1911; the problem was that the mercury tended to seep past the index (Middleton 152).


Rutherford also invented the favored minimum thermometer; again, a horizontal tube, but the liquid is amyl alcohol (originally, ordinary alcohol) and the index is made of porcelain (or glass).


There was also Six’ combination minimum/maximum thermometer (1782), a U-tube with a bulb at both ends. There is mercury in the middle and spirit in the legs, but one bulb also contains spirit and the other a mixture of air and alcoholic vapor.  The mercury merely serves as an indicator, the “thermometric fluid” being the spirit, and unfortunately the alcohol can wet the glass and pass by the mercury. (Middleton 161).


Thermographs. These provide continuous records of temperature, and thus reduce the utility of minimum and maximum thermometers. Note, however, that they tend to be less accurate than thermometers.  In essence, they couple a thermometer to a readout mechanism.


If the internal thermometer is of the liquid-in-glass type, the liquid must be mercury rather than alcohol, as the latter is too sluggish. The dominant design used a photographic readout; light shining around the mercury column onto photographed paper moved by clockwork (193). The temperature record was thus a negative image (black except where the paper was shadowed by the mercury), and the device evolved, taking advantage of improvements in light sources and paper. There were ingenious alternatives of uncertain practicability; one design balanced the thermometer horizontally on a knife edge; the temperature change shifted the center of gravity, and the tilt was recorded.  It is uncertain how this would fare in a strong wind.


The other major type was that in which a bimetallic strip is deformed in response to temperature change. The strip moves a stylus that draws on paper carried by a rotating drum; the rotation is driven by a clock mechanism inside the drum and thus protected from the elements (201-4).


I was surprised to discover that in general there wasn’t much meteorological use of thermographs featuring electrical thermometers.


Thermometer Exposure. It is not easy to expose the thermometer in such a way that it displays the true air temperature; the heat exchange between the thermometer and its surroundings is complex. The down-timers know that it is warmer in the sun than the shade, and in the mid-seventeenth century Medicean meteorological network it was initially standard for thermometers to be placed at the north and south windows of each station (Middleton 208).


For its Cooperative Observer Program, the National Weather Service advises that a temperature sensor be mounted four to six feet off the ground, in a level open clearing and away from obstructions and paved surfaces.


It is also customary for meteorological instruments to be housed in an elevated shelter (“Stevenson screen” or “Cotton Region Shelter”) that shades the instruments while providing ventilation. One shelter design appears in Popular Science (May, 1935). Typically, the shelters have louvers that slope downward and outward, are painted white to reflect solar radiation, and, in the northern hemisphere, the door faces north.


There have been a couple of thermometer designs intended to increase ventilation beyond that provided by passive air movement through louvers. One is the sling thermometer; a thermometer mounted on a frame pivotably connected to an axle that terminates in a handle. This evolved into the sling psychrometer, using two thermometers (one with a wet bulb), and used to measure humidity. The other is the aspirated thermometer; with forced convection from a suction fan. This, too, evolved into a psychrometer.


Temperature readings can be perturbed, not only by solar radiation and precipitation, but also by the observer’s own heat. Hence, readings must be taken expeditiously.


As of 2014, NWS Cooperative Observer Stations were equipped with a spirit-based minimum thermometer and a mercury-based maximum thermometer, and/or certain models of thermistor-type electronic thermometers. After the maximum thermometer is read, the tube can be spun in its mount to force the mercury in the stem past the constriction, joining the mercury in the bulb, and then it indicates the current air temperature (NWCSOM A-24).






Humidity is the amount of water vapor in the air. The warmer the air, the more water vapor it can hold. Somewhat non-intuitively, increasing humidity decreases air density (because the water vapor molecules replace heavier air molecules). So humidity is relevant to airship operations.


Absolute humidity is the exact water vapor content of the air, whereas relative humidity is the current content compared to the maximum possible at the current temperature and pressure. The “dew point is the temperature at which airborne water vapor will condense to form liquid dew” (Wikipedia). The higher the relative humidity, the closer the dew point is to the current air temperature. The difference between the two is called the “dew point depression.”


To measure dew point depression (from which we can calculate relative and absolute humidity if the pressure is known), we need both an ordinary (dry bulb) thermometer and a “wet bulb thermometer.” The latter, which approximates the dew point, is a thermometer that “has its bulb wrapped in cloth—called a sock—that is kept wet with distilled water via wicking action” (Wikipedia). (Some inventors replaced the water of the classic wet bulb thermometer with a more volatile liquid; Daniell (1820) used ether.) The combination of the two matched thermometers is called a psychrometer, a type of hygrometer.


The thermometers can be ventilated by whirling (sling psychrometer) or by a fan. The so-called psychrometer coefficient (which relates the vapor pressure to the dew point depression) is 0.0008 for a naturally ventilated psychrometer inside a Stevenson screen, and 0.000667 for a force-ventilated one (Harrison 117).


Accuracy is typically equivalent to 5% relative humidity and response time to get a reliable reading is about a minute (122).


Crude gravimetric absorption hygrometers were designed by Nicolaus Cusanus (1450), Leo Battista Alberti (1470), and Leonardo da Vinci (1490). In essence, this was a balance with a hygroscopic substance (cotton, wool, sponge) in one pan and a water-repelling substance (wax) in the other. Under dry conditions, the pans are at the same level, but if humidity increases, the cotton absorbs water and that pan sinks lower (Robens 556).


Condensation (on the outside of a vessel containing snow or ice) was weighed directly by Grand Duke Ferdinand in the 1660s (Bentley 181).


Mechanical hygrometers may be constructed using a substance whose mechanical properties are altered by humidity. Such materials include hair, goldbeaters skin (also used for airship gas bags), and animal horn or antler (109). In 1614, Santorio Santorre (1561-OTL 1636) stretched a center-weighted cord between two fixed points; absorption of water vapor caused the cord to contract and lift the weight (Wiederhold 4). In 1664, Francesco Folli (1624-85) made similar use of a paper ribbon, but the weight was connected to the center of the ribbon by a cord running over a pulley, and the pulley was connected to a dial pointer. Later, ivory (de Luc 1773) and goose quills (Buissart and Retz, 1780) were used as humidity sensors (Zuidervaart).


A hair tension hygrometer was proposed by de Saussure in 1783 (Wikipedia/Hygrometer); hair increased length by 2-2.5% for a 100% change in RH. The response is not linear (but still better than the sensors used previously) and depends on the type of hair. At subzero temperatures, response time and responsiveness are reduced. The hair length changes more when humidity increases than when it decreases. Hair is very sensitive to contamination (dust, finger oils, etc.). A hair hygrometer is usually calibrated with an aspirated psychrometer. But in a pinch, you can wet the hair bundle to reach 100% RH (JMA). Likewise, to get to 0% RH, find an up-timer with a blow dryer. Trowbridge (1896) showed that the RH error didn’t exceed 3% if the true RH was 20-85%.


In the late twentieth century, hair hygrometers were still in use. In these modern iterations, a bundle of human hair of different types is used, the hairs are carefully cleaned, and in some instances the scale is nonlinearly divided (Belfort; Ambient Weather).


In a metal-paper coil hygrometer, the paper is impregnated with a hygroscopic (water-absorbing salt) and laminated to the metal. Its absorption of water changes the curvature of the coil in a manner analogous to how temperature changes the curvature of the strip in a bimetallic thermometer. Accuracy is perhaps 10% RH.


Electrical hygrometers detect the change in electrical capacitance or resistance of a sensor element as a result of the change in humidity. Typically, capacitance changes are easier to detect. The capacitance-type hygrometer (developed in the 1930s, Wiederhold 5) features a thin film of polymer or metal oxide (the dielectric) deposited between the electrodes. The change in capacitance is about 0.2-0.5 picofarads for a 1% RH change (JMA). I am doubtful that the electrical hygrometer can be made in the NTL 1630s.


Hygrometers are typically calibrated by sampling the humidity above a saturated salt solution (Potassium nitrate and chloride, magnesium nitrate, sodium and lithium chloride are all used.) within a sealed container at a controlled temperature (121).  An older method was placing the hygrometer inside a container with a known mixture of dry air and saturated air, or in air saturated at one temperature and pressure and then increased in temperature or reduced in pressure (Middleton 1960, 116).







Rain gauges date back at least to fourth-century BC India, where rain was collected in a bowl. A cylindrical shape facilitates the estimation of the volume of rainfall; such a shape was used in the Korean iron cheuguggi used from 1441 to 1907 (Strangeways). (I believe the volume was estimated by inserting a ruler and measuring the level of the rainwater.) A further improvement was made by Castelli (1639); he used a glass cylinder. The accuracy of the deduction of rain volume from level measurements of course is dependent on the goodness of the cylindrical figure, and the accuracy of the diameter and level gradations. Rain gauges of the recording type may use a float (connected to a pen), which moves with the water level in the gauge.


Rather than reading the level, one may place the rain gauge on a balance of some kind and weigh the rain. The pan in turn can be connected to a pen for recordation, or, as in the Fischer Porter rain gauge, to a punch that puts a hole at a corresponding location on a “ticker tape” at intervals.


With a standard rain gauge, if rainfall is heavy, you have to go out, measure the rain, and empty the bucket before it fills up. A single “tipping bucket” rain gauge was developed by Wren and Hooke in the late seventeenth century as part of a “weather-wiser” (a multiple element meteorograph!). The “tipping bucket” makes possible automatic operation; each time the bucket tips, the event is recorded in some way. Another self-emptying gauge design uses a siphon.


The modern NWS non-recording precipitation gauge comprises a large (8″) diameter overflow can with a small diameter measuring tube inside, and a funnel connecting the two. These are sized so that 2 inches of rain entering the funnel will occupy 20 linear inches in the measuring tube, making it practical to measure rainfall amounts to the nearest hundredth of an inch (A-6). The Fischer & Porter recording rain gauges used to have a mechanical weighing sensor and paper-tape recording assembly, but by 2014 all of the mechanical gauges were replaced with electronic ones (A-8). The NWS expects rain to be measured with an accuracy of 0.02 inches, and (melted) snow and sleet to 0.04 inches (11).


The biggest source of error for rain gauges is the wind catching droplets and wafting them away before they fall into the receptacle (or even causing eddies that remove them after they are below the lip of the container). This was combated by giving the container a funnel top (trapping the droplets) and surrounding the gauge with a wind shield. The NWS recommends that precipitation gauges be placed in a location “where the gauge is shielded in all directions (i.e., a clearing in a grove), but “the distance of the gauge to the nearest obstruction should be at least equivalent to twice the height of the obstruction” (A-5).


Snow is more difficult to measure than rain because (1) snow is more readily deflected by the wind and (2) snow compacts with time. Ideally, snow is melted by the gauge. It is not strictly true that ten inches of fresh snow is equivalent to one inch of rain. That is correct only if the temperature is 30oF, and all of the precipitation is snow (Schwartz).


The NWS requirements for measurement of rain is 0.02″ or 4% hourly accuracy, and 0.01″ resolution. For snow, it is 0.5-1″ accuracy, 1″ resolution




In part 2, I will look at measurements of pressure and wind (which is the result of pressure gradients).



About Iver P. Cooper

Iver P. Cooper, an intellectual property law attorney, lives in Arlington, Virginia with his wife and two children. Two cats and a chinchilla rule the household with iron paws. Iver has received legal writing awards from the American Patent Law Association, the U.S. Trademark Association, and the American Society of Composers, Authors and Publishers, and is the sole author of Biotechnology and the Law, now in its twenty-something edition. He has frequently contributed both fiction and nonfiction to The Grantville Gazette.


When not writing (or trying to get an “orange blob” off his chair so he can start writing), he has been known to teach swing dancing and folk dancing, or to compete in local photo club competitions. Iver adds, “I can’t get my wife to read my fiction, but she has no trouble cashing the checks.”

Iver’s story “The Chase” is in Ring of Fire II

Leave a Reply