Terms in this set (101)

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.
When measuring wind need to know both the speed and direction. A wind sock is visual indicator of bother speed and direction. Points the way the wind is blowing. Shape and movement indicates the strength of the wind.

Wind vane-measures wind direction, arrow points into the wind.

Anemometer-measures wind speed.

Cupped anemometer-Wind blows the sups around that are attached to vertical shaft. Rate of spin proportional to wind speed. Depends on dimensions of cup and arms. Used in ASOS.

Windmill anemometer-Axis of rotation parallel to direction of the wind (horz). Must change orientation if wind changes since needs to be pointing into wind. Aerovane-Combines propeller and wind vane(tail) so will move with wind.

Sonic anemometer-Measures ultrasonic sound waves to determine wind speed and direction (turbulence). Measures how much sound waves traveling between a pair of sensors and sped up or slowed by the wind. These measurements are made by using short pulses of ultrasonic sound in the three different directions. A 3-dimensional view of the wind can be determined. Used in weather stations, nuoys, aviation, wind turbines, aviation. Have a very fine temporal resolution. Few moving parts, so good for long term use. Have a lower accuracy due to precipitation and distortion of flow due to structure.

Weather balloons carry up instrument packages (radiosondes) that can be tracked. Measuring their movement provides wind information. Balloon pops near stratosphere (30km). Satellite cloud drift winds (from GOES) can measure both wind speed and direction, but only when clouds are present. Doppler radar can measure the speed of winds toward and away from the radar (radial velocity) by measuring the movement of particles.
Wind tower measures wind speed. Wind speed from cup anemometer-Photo-interrupter device (beam of light) on shaft counts the number of pulses in a given time. Wind direction from wind vane-A precision potentiometer is connected directly to the shaft of the wind vane (measures change in voltage).

Wind speed- measured by rotating three cup anemometer.

Wind speed and wind directions are recorded every five seconds. ASOS reports A 2 minute average of the 5-second average of wind speed. A 1 minute average of the 5-second wind direction.

Wind gust-Reported every 10 minutes if a wind value exceeds the average wind by 3 knots.

Peak wind-The greatest 5-second average wind speed exceeding 25 knots in the past hour.

Precipitation Identification Sensor-Light emitted diode weather indicator (LEDWI)-Identifies precip as rain, snow or undetermined. Will only repot if fall rates are .01 in per hour or greater. Categorizes as light, moderate or heavy. Rapid interruption of beam=rain, slower interruption of beam=snow. Sometimes sensor gets confused-sleet reported as rain, hail as rain, blowing snow as snow or rain.

Rain sensor-Heated tipping bucket method-Heated collector melts and frozen precip. TIpping cant keep up in very heavy rainfall. Missing up to 40-50% for measurement of liquid equivalent of snowfall. Snow/ice gets stuck to part of gauge and melts long after event has ended.

Freezing rain sensor-Uses an ultrasonically vibrating probe to detect the presence of icing conditions.

Temp/dew point sensor-Hygrothermometer-Measures dew point sensing condensation on a chilled mirror. Another thermal sensor detects the air temp.

Cloud height sensor-Used to measure the height of the bottom of the clouds. Laser projects a beam of infrared radiation upwards and measure the elapsed time until the beam is reflected back to the sensor.

Celiometer-Senses and reports cloud heights, vertical visibility and sky coverage. Only reports celling to 12,000ft. Cant report more than 3 cloud height levels at a time.

Lightning sensor-Measures thunderstorm activity. Thunderstorm reported when an optical flash and an electrical field change occur within milliseconds of each other. Can detect cloud-to-ground (used to generate an estimate of the range) and cloud-to-cloud lightning strikes.

The ASOS thunderstorm algorithm-Reports a thunderstorm when two lightning reports occur within 15 minutes of each other in a an attempt to reduce false alarms.

Visibility sensor-Determined by measuring the amount of light from a pulsed Xenon flash lamp that is scattered into a photodiode receiver.

Pressure sensor-Most reliable and accurate sensor in ASOS.