Atmospheric humidity

Water vapour is part of the hydrological cycle, and is a key agent in both weather and climate. Humidity affects many properties of air and of materials in contact with air. It’s no wonder, then, that we want to measure and understand it.

Often humidity is intangible in daily life, but we sometimes see its effects. We find condensation on a bathroom mirror, or we notice dew outdoors. As a point of interest, measured dew-point temperature during the day typically corresponds to the overnight minimum temperature. In cold weather, we see condensation inside windows, if they are poorly insulating. Conversely, condensation on the outside of double glazing is a good sign that indoor heat is not being ‘leaked’ outwards through the glass.

Exactly how is water vapour an environmental issue?

Water vapour is the earth’s main greenhouse gas - and usefully so. Without it, surprisingly, average temperatures would be an uncomfortable 31° C colder. On the other hand, carbon dioxide (CO2) is the greenhouse gas more commonly associated with evidence of global climate change. Because of this, man-made emissions of carbon dioxide are targeted for reduction. But unlike CO2 emissions, water vapour emissions generally can’t be controlled: surface water evaporates everywhere, in amounts governed mostly by temperature, but also by air circulation and other factors. Atmospheric water vapour forms clouds and of course falls as rain, when temperature, pressure and air movements dictate. Cloud is just one of a complex set of influences that affect the earth’s radiative heat balance. Water droplets (or ice particles) in clouds have effects quite different from water vapour. High altitude cloud tends to reflect solar radiation, reducing the radiant heating of the earth. In contrast, low cloud tends to raise temperature at ground level. Daily, every point on the earth’s surface is warmed during daytime, and cools during darkness. We all know how much milder it is on a cloudy night than a clear one; this is a direct experience of how clouds affect the heat emitted by the earth at night, when the sun’s radiation is elsewhere on the planet. While natural clouds are out of our control, the man-made contrails of water vapour left by aircraft act in the same way as clouds, and sometimes trigger additional ‘natural’ cloud formation1. These are significant enough to contribute to the radiative heat balance. Clearly, atmospheric humidity and temperature are critical to these processes. Only by studying the presence of water in the atmosphere can these effects be fully understood. And clouds are just one of many atmospheric phenomena mediated by water vapour. Not all humidity interests are global in scale. In building environments, one concern is comfort, and humidity plays a part in that. Elsewhere, a huge variety of manufacturing, storage and testing processes are humidity-critical, wherever people need to prevent condensation, corrosion, mould, or other spoilage of products. This is highly relevant for foods, pharmaceuticals, chemicals, fuels, wood, paper, and many other substances. Building management systems often control humidity, and significant energy may go into cooling the air to remove water vapour. Humidity measurements contribute both to achieving correct environmental conditions and to minimising the energy cost.

Humidity measurements - how

Because humidity has such diverse effects on materials, it follows that a wide variety of sensing principles can be used to measure it. Hygrometers exploit changes in electrical impedance (capacitance or resistance), condensation temperature, dimensional change of materials, evaporative cooling, electrolysis, infrared absorbance and, less commonly, other phenomena. Atmospheric research balloons tend to carry the most lightweight and compact humidity sensors - typically electrical impedance types. These are also commonly used in ground-based weather stations, although other hygrometers can be suitable if they are sufficiently simple, robust and low in power consumption. Infrared spectrometers are also increasingly of interest in atmospheric water vapour measurements, because of their capability for extremely rapid response, which other instrument types often don’t achieve. Among these hygrometer principles, many are non-absolute measurements - they give an output (for example electrical capacitance) that varies with humidity, but it needs to be scaled, or calibrated. In other cases, instruments may seem to measure humidity ‘absolutely’ using a fundamental principle (for example by measuring dew point or condensation temperature). Users are sometimes surprised to find that even ‘fundamental’ instruments can show measurement error or bias, so these too need calibration. A complication is that humidity is expressed in terms of various measured quantities: relative humidity; dew-point temperature; partial pressure of water vapour; volume concentration; fraction of water (in mass or moles) and others. Each is favoured by different user communities because of the way it is applied: for example, air-conditioning engineers are interested in the mass of water (and sometimes the associated heat) per unit volume of air. Specialists in storage and packaging of foodstuffs and organic materials measure relative humidity, because this directly relates to the water activity of substances. Upper atmospheric humidity may be discussed in amount fraction, but measurements may be made in terms of frost point, because of the direct relationship with water vapour pressure, and with condensation. Some of these quantities are strongly affected by change in temperature or pressure of the measured gas, so careful interpretation is required. Measurement of air temperature is particularly critical alongside relative humidity data, since a change in temperature of 1 °C causes relative humidity to change by about 6% of value, at room temperature.

Measurement traceability and calibration

Traceable calibration of hygrometers is essential to accurate humidity measurement. Calibration is the comparison of an instrument against a reference value. For a hygrometer, this comparison might be against a calibrated reference hygrometer, using a chamber or other stable source of humid gas. The reference hygrometer should itself have a calibration traceable directly, or in multiple steps, back to an authoritative standard. In the UK, primary humidity standards are held at the National Physical Laboratory, providing calibrations and tracebility to laboratories and hygrometer users throughout the UK, and more widely. Standard humidity generators are developed and validated using the most rigorous techniques to establish definitive reference humidity values, and uncertainties in these values. So what does this do for the instrument user? A calibration certificate reports any instrument errors, and gives the uncertainty in these. However, calibration only makes a measurement traceable if the calibration corrections (or calibration functions) and their uncertainties are applied to the instrument readings. With any measurement, the practical uncertainty in using the instrument is always more than on the calibration certificate. This is because all instrument types have some intrinsic drift and irreproducibility. And of course, good measurement practice is essential to minimise additional errors2.

Quantifying the humidity measurement challenges

In the area of climate and weather, the benchmark requirements for measurement are set by bodies such as the World Meteorological Organisation (WMO), and national meteorology organisations (the Met Office in the UK, and counterparts worldwide). These needs were recently detailed in a report by the WMO Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN)3. Among the top priorities, they identified requirements for improved measurements for air humidity and temperature - humidities known to better than of 2% of reading, and long term instrument stability better than 1% of reading. Similarly, the UK body operating aircraft-borne atmospheric studies FAAM (Facility for Airborne Atmospheric Measurements) requires frost-point measurements with uncertainty of 0.1 °C to 0.2 °C. Taking into account the challenging conditions of these measurements, the calibration uncertainty, and some instrument instability, this is a lot to ask. It is widely recognised that valid studies of climate (especially of slow, long term climate change) rely most critically on the consistency of measurements. This depends greatly on stability of instruments, and in principle less on the absolute accuracy. This has led to some debate about the importance in this field of measurement traceability to ‘absolute’ reference standards. However, the most reliable way to obtain evidence of ongoing instrument stability is by long term reference to established standards - and regular traceable calibration is the ideal way to do this.

The MeteoMet Project

A new international project is set to stimulate new developments in humidity measurement, and better application of the results in studies of weather and climate. This work combines the disciplines of metrology (the science of measurement) and meteorology (the science of climate and weather) in an undertaking known as MeteoMet (‘Metrology for Meteorology’). The €4.4M project addresses improvements in measurements - humidity measurements particularly, but also in pressure, flow, and temperature in the atmosphere - both at ground level and at altitude. This is jointly funded by the European Metrology Research Programme (EMRP) and 22 project partners comprising national measuremnt institutes and universities. The project is supported by collaboration from some 29 other organisations involved in weather and climate research. These bodies will contribute vitally by both steering and implementing the outputs of the project. In the humidity field, the project aims to provide many developments: improved airborne spectrometers and spectral data for water vapour; improved airborne ‘in-situ’ traceable calibrations; a comparative study of hygrometers for ground-level and airborne use; improved calibrations for balloon-borne radiosondes; new fundamental data for the vapour pressure curve of water; novel microwave and acoustic sensors for humidity and temperature; a suite of improvements to lab-based and site-based calibrations and better analysis techniques for field data. This highly ambitious programme of work is only made possible by the combination of European and national funding, and the teaming of partners with unique expertise. Key issues are how to integrate measurement tracebility and realistic recognition of measurement uncertainty, while preserving the transparency and consistency and continuity of the meteorological record.

Forward look

Measurement of water vapour will of course continue to be of widespread interest, especially in atmospheric science. This is certainly a global as well as a national and local issue. International projects are a logical approach in a field such as this, where measurement activity is diffuse and widespread, often crossing national boundaries. Collaborative projects are key to maximising the impact of developments in this area. Wherever links can be strengthened between measurement experts and the climate and meteorology community, this can only help to disseminate developments in techniques and in best practice, and to support meaningful interpretation of climate measurement data. ?

References

1 Global radiative forcing from contrail cirrus, Ulrike Burkhardt and Bernd Kärcher, Nature Climate Change, Vol 1, APRIL 2011, www.nature.com/natureclimatechange (accessed 31 March 2011) 2 A Guide to the Measurement of Humidity Institute of Measurement and Control, 1996, 68 pp, ISBN 0-904457-24-9 3 GCOS - 112 (WMO/TD No. 1379) GCOS Reference Upper-Air Network (GRUAN): Justification, requirements, siting and instrumentation options. April 2007. (WMO, Geneva.)

Author

Dr Stephanie Bell is the lead scientist for humidity metrology at the National Physical Laboratory, with more than 20 years of expertise in humidity measurement, standards, calibration, measurement uncertainty and accreditation. She is a Fellow of the Society of Environmental Engineers, and she chairs key national and international humidity committees: BSI CPI/29 Humidity and Temperature conditioning requirements, and CIPM CCT WG6 Humidity measurement. In the field of humidity, NPL’s measurement capabilities are among the best in the world. NPL provides standards for the UK covering dew- and frost- point temperature, relative humidity, mixing ratio and related quantities, in a wide measurement range. Associated research programmes develop and improve the humidity standards, currently extending these to a range of gases and pressures. The group’s research also covers measurement and tracebility of moisture content of materials, and water vapour flux from and though surfaces. Closely aligned work covers temperature calibration techniques specific to air temperature sensors. NPL provides standards, calibrations, and measurement expertise for all types of physical measurement, in support of UK industry. NPL’s measurement interests span a variety of environmental areas, including reference gases, balloon-borne atmospheric research, ground-based environmental monitoring networks, and many more. NPL’s national role, measurement facilities, services and training courses are detailed at www.npl.co.uk , including a large number of publications on good measurement practice, downloadable free of charge. Subsidised consultancies are available under the Technology Innovation Fund scheme at http://www.npl.co.uk/commercial-services/products-and-services/technology-innovation-fund/. Initial details of the MeteoMet project are at http://www.meteomet.org/links.html . The project launch date will be in Autumn 2011, after which the website will show regular updates on project progress and outputs. Contact: stephanie.bell@npl.co.uk © Crown copyright 2011. Reproduced by permission of the Controller of HMSO and the Queen’s printer for Scotland. Published: 10th Jun 2011 in AWE International