MEASUREMENT OF METEOROLOGICAL VARIABLES
PROF. A. BALASUBRAMANIAN
DEPARTMENT OF STUDIES IN EARTH SCIENCE
UNIVERSITY OF MYSORE, MYSORE
Meteorological observations are made for a variety of reasons. They are used for the real-
time preparation of weather charts and maps, for weather forecasts and severe weather
warnings, for the study of climate, and for local weather-dependent operations.
Meteorologists use a wide variety of instruments to measure weather conditions.
These data are required for analysis in hydrology and agricultural meteorology, and for
research in meteorology and climatology.
Synoptic observations should be representative of an area up to 100 km around the station.
For small-scale or local applications, the considered area should have the dimensions of
10 km or less.
Meteorological applications have their own preferred timescales and space scales for
averaging, station density and resolution of phenomena. It may be small for agricultural
meteorology and large for global long-range forecasting.
The scale ranges of measurements are as follows:
(a) Microscale (less than 100 m) for agricultural meteorology, for example, evaporation;
(b) Toposcale or local scale (100–3 km), for example, air pollution, tornadoes;
(c) Mesoscale (3–100 km), for example, thunderstorms, sea and mountain breezes;
(d) Large scale (100–3 000 km), for example, fronts, various cyclones, cloud clusters;
(e) Planetary scale (larger than 3 000 km), for example, long upper tropospheric waves
General Requirements of a Meteorological Station:
The requirements for weather elements to be observed according to the type of station and
observing network are as follows:
1. Present weather
2. Past weather
3. Wind direction and speed
4. Cloud amount
5. Cloud type
6. Cloud-base height
9. Relative humidity
10. Atmospheric pressure
12. Snow cover
13. Sunshine and/or solar radiation
14. Soil temperature
15. Evaporation .
Instruments are available to measure all of these elements, except the cloud type.
Some meteorological stations take upper-air measurements, measurements of soil
moisture, ozone and atmospheric composition.
Automatic weather stations
Most of the elements required for synoptic, climatological or aeronautical purposes can be
measured by automatic instrumentation.
Meteorological observing stations are designed so that the representative measurements
(or observations) can be taken according to the type of the station involved.
The ground should be covered with short grass or a surface representative of the locality,
and surrounded by open fencing to exclude entry of unauthorized persons.
Within the enclosure, a bare patch of ground of about 2 m x 2 m is reserved for observations
of the state of the ground and of soil temperature at depths of equal to or less than 20 cm.
Measurement of temperature:
For meteorological purposes, temperatures are measured for a number of media.
The most common variable measured is the atmospheric temperature (at various heights).
Other variables to be measured are the temperature of the ground, soil, grass minimum
The thermodynamic temperature (T), with units of Kelvin (K), (also defined as “Kelvin
temperature”), is the basic temperature variable.
The temperature (t), in degrees Celsius (or “Celsius temperature”) is used for most
Meteorological requirements for temperature measurements primarily relate to the
(a) The air near the Earth’s surface
(b) The surface of the ground
(c) The soil at various depths
(d) The surface levels of the sea and lakes and
(e) The upper air.
These measurements are required, either jointly or independently and locally or globally, for
input to numerical weather prediction models, for hydrological and agricultural purposes,
and as indicators of climatic variability.
Local temperature also has direct physiological significance for the day-to-day activities of
the world’s population.
Measurements of temperature may be required as continuous records or may be sampled
at different time intervals.
For climate studies in particular, the temperature measurements are affected
1) by the state of the surroundings,
2) by vegetation,
3) by the presence of buildings and other objects,
4) by ground cover,
5) by the condition of, and changes in, the design of the radiation shield or screen, and
6) by other changes in equipment.
It is important that the records should be kept not only of the temperature data, but also of
the circumstances in which the measurements are taken.
Such information is known as metadata (data about data).
The major equipments used are:
2. Liquid-in-glass thermometers
3. Ordinary thermometers= mercury-in-glass-type thermometer
4. Meteorological thermometers and
5. Radiometric thermometers.
Soil thermometers :
For measuring the soil temperatures at depths of 20 cm or less, mercury-in-glass
thermometers are used. It is measured with their stems bent at right angles, or any other
suitable angle, below the lowest graduation.
The thermometer bulb is sunk into the ground to the required depth, and the scale is read
with the thermometer in situ.
Measuring grass minimum temperatures :
The grass minimum temperature is the lowest temperature reached overnight by a
thermometer freely exposed to the sky just above short grass. The temperature is measured
with a minimum thermometer.
Electrical resistance thermometers:
Resistance thermometers, also called as resistance temperature detectors (RTDs), are
sensors used to measure temperature. Many RTD elements consist of a length of fine wire
wrapped around a ceramic or glass core. The RTD wire is a pure material, typically platinum,
nickel, or copper.
Thermometer exposure and siting
Radiation from the sun, clouds, the ground and other surrounding objects passes through
the air without appreciably changing its temperature, but a thermometer exposed freely in
the open air can absorb considerable radiation and display the values.
Measurement of atmospheric pressure
The atmospheric pressure on a given surface is the force per unit area exerted by virtue of
the weight of the atmosphere above. The pressure is thus equal to the weight of a vertical
column of air above a horizontal projection of the surface, extending to the outer limit of
the atmosphere. The basic unit for atmospheric pressure measurements is the Pascal (Pa)
(or Newton per square metre).
Some barometers are graduated in “millimetres or inches of mercury under standard
Analysed pressure fields are a fundamental requirement of the science of meteorology.
For meteorological purposes, atmospheric pressure is generally measured with electronic
barometers, mercury barometers, aneroid barometers or hypsometers.
Mercury barometers /Electronic barometers
A barometer is the instrument used to measure atmospheric pressure. Pressure tendency
can forecast short term changes in the weather.
Aneroid displacement transducers
The aneroid displacement transducer contains a sensor with electrical properties (resistance
or capacitance) that changes as the atmospheric pressure changes.
Digital piezoresistive barometers
Today’s mostly preferred pressure sensor is the piezoresistive sensor. It is cheap, and still
delivers a good result. But it has drawbacks—significant power requirements, low output
signal, large offset, and temperature dependence. Many sensors are therefore internally
compensated for temperature effects.
Cylindrical resonator barometers
A cylindrical resonator barometer (or vibrating cylinder air-pressure transducer) is designed
to measure absolute air pressure using the vibrating element principle, providing a
frequency output from which pressure is computed.
Aneroid barometers have lower accuracy than mercury barometers. These are compact and
portable equipments. Aneroid barometers are easier to handle and use, and are suitable for
Bourdon Tubes are known for its very high range of differential pressure measurement in
the range of almost 100,000 psi (700 MPa). It is an elastic type pressure transducer.
Bourdon-tube barometers usually consist of a sensor element.
Automatic digital barometers
Automatic digital barometers make use of a combination of sensor and microprocessor
techniques. In fact these instruments are composed of sensors, transducers, a micro-
processor and digital electronics. Electrical signals generated by the pressure sensor(s) are
interpreted by the micro-processor using a more or less sophisticated algorithm.
The outcome of most of the sensors, however, is temperature dependent. So inside most of
the devices the temperature of the pressure sensor is determined by a temperature sensor.
Measurement of humidity:
The measurement of atmospheric humidity, and often its continuous recording, is an
important requirement in most areas of meteorological activity.
The most frequently used quantities in humidity measurements are as follows:
It is the ratio between the mass of water vapour and the mass of dry air.
The ratio between the mass of water vapour and the mass of moist air.
Dew point temperature :
The temperature at which moist air saturated with respect to water at a given pressure has
a saturation mixing ratio equal to the given mixing ratio.
The ratio in per cent of the observed vapour pressure to the saturation vapour pressure
with respect to water at the same temperature and pressure.
The partial pressure of water vapour in air.
Saturation vapour pressures :
Vapour pressures in air in equilibrium with the surface of water and ice, respectively.
Humidity measurements at the Earth’s surface are required for meteorological analysis and
forecasting, for climate studies, and for many special applications in hydrology, agriculture,
aeronautical services and environmental studies, in general.
They are particularly important because of their relevance to the changes of state of water
in the atmosphere.
Any instrument used for measuring the atmospheric humidity is known as a hygrometer.
A hygrometer is an instrument used for measuring the moisture content in the atmosphere.
Humidity measurement instruments usually rely on measurements of some other quantity
such as temperature, pressure, mass or a mechanical or electrical change in a substance as
moisture is absorbed.
This method uses the absorption of water vapour by a desiccant from a known volume of
air. The gravimetric hygrometer is used for this purpose.
The psychrometric method
The measurement of atmospheric humidity is an important requirement in most of the
areas of meteorological studies. Psychrometry is defined as the measurement of the
moisture content of air.Nowadays many humidity measuring devices are available.
Certain materials interact with water vapour and undergo a change in a chemical or physical
property that is sufficiently reversible for use as a sensor of ambient humidity.
Absorption of electromagnetic radiation by water vapour(ultraviolet and infrared absorption
hygrometers) is used for this measurement purpose.
Measurement of surface wind :
Wind velocity is a three-dimensional vector quantity with small-scale random fluctuations in
space and time.
Wind observations or measurements are required
a) for weather monitoring and forecasting,
b) for wind-load climatology, for probability of wind damage and
c) for the estimation of wind energy, and as part of the estimation of surface fluxes.
For nearly all applications, it is necessary to measure the averages of wind speed and
Wind speed should be reported to a resolution of 0.5 meters per second or in knots (0.515
meter per second) to the nearest unit for every 10 min.
Averages over a shorter period are necessary for certain aeronautical purposes.
Wind direction should be reported in degrees to the nearest 10°.
Methods of measurement and observation
Surface wind is usually measured by a wind vane and cup or propeller anemometer.
Simple hand-held anemometers are available for this purpose.
Cup and propeller sensors are commonly used for this measurement. these are called as
wind vanes. For the purpose of obtaining a satisfactory measurement, a wind vane will be
Other wind sensors:
a lot of wind sensors are used for this purpose. some of them include:
1) Pitot tube anemometers
2) Sonic anemometers Hot-disc anemometers are recently developed solid-state
3) Hot-wire anemometers
4) Remote wind-sensing techniques with sound (sodar), light (lidar) or electromagnetic
Anemometers over land:
The standard exposure of wind instruments over a levelled open terrain is 10 m above the
Open terrain is defined as an area where the distance between the anemometer and any
obstruction is at least 10 times the height of the obstruction.
Two aspects are very important.
First, the sensors should be kept away from local obstructions as much as possible.
Secondly, the local situation should be well documented.
There should at least be a map of the station surroundings within a radius of 2 km,
documenting obstacle and vegetation locations and height, terrain elevation changes, and
Anemometers at sea:
There is an increasing requirement for instrumental measurements of wind over the sea,
especially by means of automatic unattended systems.
Measurement of precipitation :
Precipitation is defined as the liquid or solid products of the condensation of water vapour
falling from clouds or deposited from air onto the ground.
It includes rain, hail, snow, dew, sleet, frost and fog precipitation.
The total amount of precipitation which reaches the ground in a stated period is expressed
in terms of the vertical depth of water (or water equivalent in the case of solid forms) to
which it would cover a horizontal projection of the Earth’s surface.
Snowfall is also expressed by the depth of fresh, newly fallen snow covering an even
The common observation times are hourly, three hourly and daily, for synoptic,
climatological and hydrological purposes.
The analysis of precipitation data is much easier and more reliable if the same gauges and
siting criteria are used throughout the networks.
Daily measurements of precipitation should be taken at fixed times common to the entire
network or networks of interest.
rain gauges are the most common instruments used to measure precipitation.
Point measurements of precipitation serve as the primary source of data for areal analysis.
Non-recording precipitation gauges -Ordinary gauges
The commonly used precipitation gauge consists of a collector placed above a funnel
leading into a container where the accumulated water and melted snow are stored
between observation times.
Recording precipitation gauges/Weighing-recording gauge
Tipping-bucket gauge/ Measurement of dew, ice accumulation and fog precipitation
The amount of dew deposited on a given surface in a stated period is usually expressed in
units of kg m–2 or in millimetres depth of dew.
Measurement of ice accumulation
Ice on pavements:
Sensors have been developed and are in operation to detect and describe ice on roads and
runways, and to support warning and maintenance programmes.
Measurement of fog precipitation
Fog consists of minute water droplets suspended in the atmosphere to form a cloud at the
Measurement of snowfall and snow cover
Depth measurements of snow cover or snow accumulated on the ground are taken with a
snow ruler or similar graduated rod which is pushed down through the snow to the ground
Snow gauges measure snowfall water equivalent directly.
Snow pillows of various dimensions and materials are used to measure the weight of the
snow that accumulates on the pillow.
Radioisotope snow gauges:
Natural gamma radiation:
The method of gamma radiation snow surveying is based on the attenuation by snow of
gamma radiation emanating from natural radioactive elements in the top layer of the soil.
The greater the water equivalent of the snow, the more the radiation is attenuated.
Measurement of radiation :
The various fluxes of radiation to and from the Earth’s surface are among the most
important variables in the heat economy of the Earth as a whole and at any individual place
at the Earth’s surface or in the atmosphere.
To analyse the properties and distribution of the atmosphere with regard to its constituents,
such as aerosols, water vapour, ozone, and so on; (c) To study the distribution and
variations of incoming, outgoing and net radiation;
(d) To satisfy the needs of biological, medical, agricultural, architectural and industrial
activities with respect to radiation.
Radiation quantities may be classified into two groups according to their origin, namely
solar and terrestrial radiation.
Solar energy is the electromagnetic energy emitted by the sun.
The solar radiation incident on the top of the terrestrial atmosphere is called extraterrestrial
solar radiation; 97 per cent of which is confined to the spectral range 290 to 3 000 nm is
called solar (or sometimes shortwave) radiation.
Terrestrial radiation is the long-wave electromagnetic energy emitted by the Earth’s surface
and by the gases, aerosols and clouds of the atmosphere; it is also partly absorbed within
the atmosphere. For a temperature of 300 K, 99.99 per cent of the power of the terrestrial
radiation has a wavelength longer than 3 000 nm and about 99 per cent longer than 5 000
nm. For lower temperatures, the spectrum is shifted to longer wavelengths.
Since the spectral distributions of solar and terrestrial radiation overlap very little, they can
very often be treated separately in measurements and computations. In meteorology, the
sum of both types is called total radiation. In the past, several radiation references or scales
have been used in meteorology, namely the Angstrom scale of 1905, the Smithsonian scale
of 1913, and the international pyrheliometric scale. Irradiance and radiant exposure are
the quantities most commonly recorded and archived, with averages and totals of over 1 h.
Direct solar radiation is measured by means of pyrheliometers, the receiving surfaces of
which are arranged to be normal to the solar direction.
By means of apertures, only the radiation from the sun and a narrow annulus of sky is
Times and location of observation
Absolute radiometers are self-calibrating
Meteorological radiation instruments
Spectral direct solar irradiance and measurement of optical depth:
Sun radiometry (photometry) and aerosol optical depth
Measurement of global and diffuse sky radiation
1. Radiant energy,
2. Radiant flux,
3. Radiant exitance,
6. Radiant exposure & Intensity,
7. Quantity of light- Luminous flux & Exitance,
8. Light exposure,
10. Luminous flux density,
12. Reflectance, absorbance,
Measurement of sunshine duration:
The term “sunshine” is associated with the brightness of the solar disc exceeding the
background of diffuse sky light, or, as is better observed by the human eye, with the
appearance of shadows behind illuminated objects. As such, the term is related more to
visual radiation than to energy radiated at other wavelengths, although both aspects are
inseparable. The physical quantity of sunshine duration (SD) is, evidently, time. The units
used are seconds or hours.
For climatological purposes, derived terms such as “hours per day” or “daily sunshine
hours” are used, as well as percentage quantities, such as “relative daily sunshine duration”.
The requirements on sunshine recorders vary, depending on site and season, according to
the dominant cloud formation. One of the first applications of SD data was to characterize
the climate of sites, especially of health resorts. This also takes into account the
psychological effect of strong solar light on human well-being. It is still used by some local
authorities to promote tourist destinations.
Campbell-Stokes sunshine recorders
Specially designed multisensor detectors (mostly equipped with photovoltaic
cells) combined with an electronic discriminator and a time counter.
Scanning method: Discrimination of the irradiance received from continuously scanned,
small sky sectors.
Measurement of visibility
Visibility was first defined for meteorological purposes as a quantity to be estimated by a
human observer, and observations made in that way are widely used. However, the
estimation of visibility is affected by many subjective and physical factors.
Visibility, meteorological visibility (by day) and meteorological visibility at night are defined
as the greatest distance at which a black object of suitable dimensions (located on the
ground) can be seen and recognized.
Luminous flux (symbol:
F (or Φ); unit: lumen) is a quantity derived from radiant flux.The meteorological visibility or
MOR is expressed in metres or kilometres. The measurement range varies according to the
Visual perception — photopic and scotopic vision
Meteorological visibility in daylight and at night
Visual estimation of meteorological optical range
A meteorological observer can make a visual estimation of MOR using natural or man-made
objects (groups of trees, rocks, towers, steeples, churches, lights, and so forth).
Instrumental measurement of the meteorological optical range
Visual extinction meters
The lidar (light detection and ranging) technique may be used to measure visibility when
the beam is directed horizontally.
Telephotometers and visual extinction meters.
Measurement of evaporation
Quantity of water evaporated from an open water surface or from the ground.
Process by which water from vegetation is transferred into the atmosphere in the form of
(Actual) evapotranspiration (or effective evapotranspiration):
Quantity of water vapour evaporated from the soil and plants when the ground is at its
natural moisture content.
Potential evaporation (or evaporativity):
Quantity of water vapour which could be emitted by a surface of pure water, per unit
surface area and unit time, under existing atmospheric conditions.
Maximum quantity of water capable of being evaporated in a given climate from a
continuous expanse of vegetation covering the whole ground and well supplied with water.
It includes evaporation from the soil and transpiration from the vegetation from a specific
region in a specific time interval, expressed as depth of water.
The rate of evaporation is defined as the amount of water evaporated from a unit surface
area per unit of time. Factors affecting the rate of evaporation from any body or surface
can be broadly divided into two groups, meteorological factors and surface factors.
An atmometer is an instrument that measures the loss of water from a wetted, porous
surface. The wetted surfaces are either porous ceramic spheres, cylinders, plates, or
exposed filter-paper discs saturated with water.
Evaporation pans and tanks
In general, a lysimeter consists of the soil-filled inner container and retaining walls or an
outer container, as well as special devices for measuring percolation and changes in the
Exposure of evapotranspirometers
Measurement of soil moisture
Soil moisture is an important component in the atmospheric water cycle, both on a small
agricultural scale and in large-scale modelling of land/ atmosphere interaction. Vegetation
and crops always depend more on the moisture available at root level than on precipitation
occurrence. Water budgeting for irrigation planning, as well as the actual scheduling of
irrigation action, requires local soil moisture information. Soil moisture determinations
measure either the soil water content or the soil water potential.
Soil water content:
Soil water content is an expression of the mass or volume of water in the soil, while the soil
water potential is an expression of the soil water energy status. The basic technique for
measuring soil water content is the gravimetric method. Unfortunately, gravimetric
sampling is destructive, rendering repeat measurements on the same soil sample
impossible. Soil consists of individual particles and aggregates of mineral and organic
materials, separated by spaces or pores which are occupied by water and air.
Soil water content:
The capacity of soil to retain water is a function of soil texture and structure. When
removing a soil sample, the soil being evaluated is disturbed, so its water-holding capacity
is altered. Indirect methods of measuring soil water are helpful as they allow information to
be collected at the same location for many observations without disturbing the soil water
Two different radiological methods are available for measuring soil water content. One is
the widely used neutron scatter method, which is based on the interaction of high-energy
(fast) neutrons and the nuclei of hydrogen atoms in the soil. The other method measures
the attenuation of gamma rays as they pass through soil.
Neutron scattering method
Soil water dielectrics
The most widely used and least expensive water potential measuring device is the
tensiometer. Tensiometers are simple instruments, usually consisting of a porous ceramic
cup and a sealed plastic cylindrical tube connecting the porous cup to some pressure-
recording device at the top of the cylinder. They measure the matric potential, because
solutes can move freely through the porous cup.
Electrical resistance blocks, although insensitive to water potentials in the wet range, are
excellent companions to the tensiometer. They consist of electrodes encased in some type
of porous material that within about two days will reach a quasi-equilibrium state with the
soil. The most common block materials are nylon fabric, fibreglass and gypsum. This method
determines water potential as a function of electrical resistance, measured with an
alternating current bridge.
Remote sensing of soil moisture
Measurement of upper-air pressure, temperature and humidity
Measurement of upper wind
Present and past weather; state of the ground
Observation of clouds
Measurement of ozone
Measurement of atmospheric composition