Location- Standard 5ft for air temp. Group temp would be at ground level. Thermometer should be placed in a shelter with good air flow. Needs to be in location that is typical of the surround area. Not on steep slope or in a sheltered hollow. Not real close to paved or concrete surfaces or tall obstructions. A level open clearing for good air flow.
Liquid Thermometer- Comprised of mercury or alcohol in a long thin tube. As liquid is heated it expands. Read temp of calibrated scale. Can also serve as max/min thermometers. Good-stable, sensitive and relatively inexpensive. Bad-slow response time, fragile and need someone to visually read it.
Bimetallic Strips- 2 dissimilar metal strips bonded together in a spiral coil. The different metals have different coefficients of expansion of metal. Coils winds or unwinds in response to temp changes. Needle connected to coils that can be rad against a scale. Common type "dial-type" used as oven, fridge thermometer, if connected to a dial its a thermostat. Good-easy to read and inexpensive. Bad-not really accurate, slow response time, calibration may change over time due to metal fatigue.
Electrical Resistance- Consists of platinum or a nickel wire and an electrical meter. As the temperature increases, the resistance of the wire increases. The resistance is measured by an electric meter and is calibrated to represent air temp. Different types-carbon resistors, film thermometers, wire-wound thermometers, coil elements. Used in ASOS. Good-very accurate and relatively in expensive. Bad-need a power source.
Thermocouple- Junction between 2 dissimilar metals that produce electrical voltage proportional to temp. Use to measure fluctuations at fraction of seconds. Very accurate, very small and expensive.
Thermistor- A type of conductor whose electrical resistance goes up sharply with temp. Used widely current limiters, temp sensors, self-resetting over current protectors, and self-regulating heating elements. Very small and inexpensive.
Radiation Thermometer- Intercepts and measures thermal energy emitted by a source. Relates to its temp by means of plank law of radiation. Optics (generally lenses) are used to collect and focus the emitted energy onto a detector. Signal from detector can be measured directly or converted to temp. Filters are used to define the wavelength or wavelength band over which the emitted energy is measured. Good-can be very accurate and precise. Bad-if used wrong can be very wrong, hard to prove temp error.
Hygrometer-any device that measures water vapor content.
Evaporative cooling-measured by psychrometer.
Mechanical expansion-measured by hair hygrometer.
Condensation-measured by dew point hygrometer.
Electrical properties-measured by dew cell carbon hygristor.
Psychrometer-Has 2 thermometers that are well ventilated so have constant stream new air. One thermometer is normal with purpose of measuring air temp, dry bulb temp. Other thermometer has cloth covering it, then is saturated with a water, wet bulb temp. Can be used to calibrate other hygrometers, example is sling psychrometer. Wet surface is exposed to air, evaporation occurs, lowers temp to wet bulb temp, difference between wet bulb air temp is used to find RH.
Aspirated psychrometer-Both thermometers are inside housing. Uses a fan for ventilation. Just moisten wick, point into wind/hang outside, turn on fan. Monitor when wet bulb bottoms out.
Hair hydrometer-Changes of length of strands of human hair. Not very accurate. Common. Some materials absorb moisture from air and expand slightly (wood, paper, hair). Expansion can be amplified and measure to record/display RH.
Dew point hydrometer-Very accurate, very expensive, used when need high quality measurements, used on ASOS. If polished metal or mirror surface is cooled the dew point, condensation occurs and surface fogs up. Condensation is detected by optical sensor. Then temp is recorded at the point of condensation.
Dew Cell-Uses wires wrapped around fiberglass wick. Absorbs water from the atmosphere. Causes an increase in conductance. Then causes additional current to run through the wires heating the sensor. Eventually an equilibrium temp will be reached (dew point).
Carbon hygristor-Small strip of plastic dipped in a mixture of carbon particles, celluloid resin, and other chemicals then dried. As electrical current passes through the strip, the carbon particles allow it to act as a resistor. The celluloid absorbs (diffuses) water vapor and expands (contracts) with changing RH. This varies the spacing between carbon particles, changing the resistance. High/low RH results in high/low resistance. Used in radiosondes.
Infrared Hygrometer-satellite instrument that measures humidity by measuring amount of IR energy absorbed by vapor. A beam of light containing 2 separate wave lengths to gauge atmospheric humidity. One of the wavelengths is absorbed by water vapor, the other is unaffected. Extremely accurate index of water vapor for paths of a few inches or thousands of feet.
Automated surface observing system. Reports sky condition, sky visibility, basic present weather information, obstructions to vision, pressure, ambient temperature, dew point temperature, wind, precipitation accumulation, variable cloud height, variable visibility, precipitation beginning/ending times, rapid pressure changes, pressure change tendency, wind shift, peak wind. Not designed to measure clouds above 12,000ft, virga, tornadoes or funnel clouds, ice crystals, snow pellets, ice pellets, drizzle, freezing drizzle, snow, dust, sand or snowfall depth. Only able to observe a single point in time and space. NOHRSC-produces estimates of snow water equivalent, snow depths, snowpack temp, SWE change, snow sublimation and evaporation, snow melt, estimates of blowing snow, non-snow precip, modeled and observed snow info, airborne snow data, satellite snow cover, historic snow data.