Observing the Atmosphere

For understanding the atmosphere, especially its current state and future evolution, we need to observe and measure its different elements to the extent possible at surface and upper levels. However, there is a difficulty here. Unlike air in a closed laboratory, the free atmosphere is constantly in motion. So what is observed or measured at a station at a particular time is not representative of what comes over the station after an interval of time. To get over this difficulty and for practical requirements, every country maintains a network of meteorological observing stations where atmospheric variables are observed and measured at some standard hours as fixed by the World Meteorological Organization by international agreement. Most meteorological stations take surface observations but some take both surface and upper-air observations. Then there are a few stations which take upper-air observations only. Surface observations which are routinely taken and recorded at meteorological stations include pressure, temperature, wind (direction and speed), humidity, cloud, rainfall, weather and visibility. While instruments are used to measure most of these elements, some observations are made visually. Some selected stations which are earmarked as climatological stations observe and measure, besides routine elements, additional parameters, such as radiation (both incoming and outgoing), duration of sunshine, ozone content of the air, concentration of pollutants, etc. Upper-air observations are usually taken with the help of balloons carrying radiosonde and/or rawinsonde instruments which measure pressure, temperature, humidity and wind at different heights and which are tracked by ground-based instruments, such as theodolites and radars. Apart from balloons, other aerial platforms which have been used for upper-air sounding include high-flying kites, aircraft, rockets, etc.

Table 2.3 List of principal meteorological elements, their methods of observation, and instruments used to observe or measure them

Elements observed

Method of observation

Instruments used

Pressure

Instrumental

Barometers

Temperature

Instrumental

Thermometer Radiometer

Humidity

Instrumental

Hygrometer Psychrometer

Visibility

Visual

-

Instrumental

Light-meters

Wind

Instrumental

Anemometer and Wind-vane, Cloud motion

Clouds

Visual

-

Instrumental

Nephoscope Satellites

Rainfall

Instrumental

Raingauge

After the introduction of polar-orbiting and geostationary satellites, remote sensing technique has been widely applied to observe and measure several atmospheric elements from space. The space-borne instruments intercept radiation up-welling from the earth's surface and atmosphere in the visible as well as infrared radiation channel at different wavelengths from which information is derived about clouds, temperature, humidity, winds, etc. at different levels of the atmosphere at frequent intervals of time every day. Prior to the advent of satellites, observations were confined to inhabited land areas only. Vast ocean areas were practically uncharted. With the help of satellites, it is now possible to observe the atmosphere over the whole globe including the oceans. However, so far as the data over the oceans are concerned, their accuracy still leaves much to be desired because of the presence of clouds and excessive water vapour in the boundary layers of the ocean. In Table 2.3, we give a list of the standard meteorological elements which are usually observed at a meteorological station, their methods of observation and the instruments used to measure them.

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