How do the pros measure humidity?

Question

I have useda wet/dry bulb setup to estimate humidity, andit appears thatthe Argentinian meteorological service uses one in Antarctica as well.

Wikipedia sez:

Humidity measurement is among the more difficult problems in basic metrology. According to the WMO Guide, "The achievable accuracies [for humidity determination] listed in the table refer to good quality instruments that are well operated and maintained. In practice, these are not easy to achieve." ...Hygrometers must be calibrated in air, which is a much less effective heat transfer medium than is water, and many types are subject to drift so need regular recalibration. A further difficulty is that most hygrometers sense relative humidity rather than the absolute amount of water present...

Question:In practice, how do weather reporting stations measure humidity? Is there a difference between remote locations and those where people visit regularly? Does there tend to be a difference between instruments used in arctic and tropical conditions?

Answer 1

The WMO reportInstruments and Methods of Observationhas a chapter onMeasurement of humidityand a separate chapter onMeasurement of upper-air pressure, temperature, humidity. The first deals with instruments used for surface measurements. The second deals with instruments used for measuring vertical profiles, which have to deal with a larger range of temperatures and pressures.

For the surface instruments they list:

• Gravimetric: take an air sample, extract all the water and weigh it (typically used for calibration);
• Chilled mirror: dewpoint or frostpoint: used for observations and for calibration;
• Salt-solution: matching vapour pressure of a salt solution to atmospheric vapour pressure: used in automated weather stations;
• 湿度:比较湿或冰灯泡temperature with actual temperature - used in observations and calibration;
• Sorption methods: using hair or an alternative which stretches when wet, or using a material with a electrical resistance or capacitance that changes when wet.

In other words, the professionals use a lot of different methods.

Answer 2

We use something like an Onset HOBO electronic sensor. They've got logging (and in some cases, networking) built in. They'll typically log both temperature and relative humidity at regular intervals specified by the user.

Humidity sensors have a tendency to wander off calibration, so you will need to take that into account when designing your monitoring protocol.

If it's a very inaccessible location, you might add extra duplicate sensors to allow you to compensate for this calibration issue.

Electronic RH sensorstypically use a moisture-absorbing (hygroscopic) material, and either: acapacitive sensor- as humidity increases, the capacitance of the sensor increases, as the sensor absorbs moisture from the air; or a resistive sensor - as humidity increases, the resistance decreases.

Sensors that measure absolute humidity, do so by combining a measure of relative humidity, and a measure of temperature.

And the suppliers will typically have different models that are rated for different conditions. The most expensive are sometimes called "milspec" - military specification - and are designed to particularly high standard for working in a wide range of extreme conditions.

Answer 3

The Bureau of Meteorology in Australiatraditionally useswet and dry bulb thermometers read by humans, when measuring temperature. However, for remote automated weather stations,electrical equipmentis used. The dry bulb temperature is measured using a resistance temperature device (RTD) and relative humidity is measured using relative humidity probe.

The Bureau has produced avideo.