7 2.1.223 Sensors using thermo-electric effects - Thermocouples Thermocouples are convenient temperature sensors because they are inexpensive and easy to make. For meteorological applications, copper-constantan thermocouples are the ones most frequently used. The weak thermal electromotive force response, of about O uV/degrees centigrade, can be increased by connecting several thermocouples in series, orb busing suitable dc. amplifiers. Modern recorders provide for reference junction temperature measurements and/or compensation. With reference to the basic physical principle, thermocouple instruments are especially useful for differential measurements. - Heat-flux plates Usually these instruments are thermopiles whose output is proportional to the temperature difference between the sides of a plate crossed by the flux. (Suppliers: AB) - Diodes and transistors Diodes and transistors with outputs higher than 1uV/ degrees centigrade are used to construct sensitive and accurate thermometers for applications in plant environments. - Katathermometers To measure the cooling effect of temperature and wind, Katathermometers are used. They are spirit in glass thermometers with a rather big bulb of accurately determined area. With these, the time required fora fixed amount of cooling, after the thermometer has been warmed, is measured. 2.1.224 Radiative sensors Surface temperatures can be determined by using radiation measurements and applying the physical radiation laws (Stefan Boltzmann, W. Wien), to them. These methods are mainly applied in remote sensing from aircrafts and satellites. 2.1.225 The chemical transformation of saccarose to glucose and fructose in a solution is an exponential function of its pH value and temperature. This reaction is used to measure the average temperature (or rather the heat quantity received) over a longer period. To measure the state of transformation, a polarimeter is used. With respect to the nonlinear response of the chemical reaction to temperature, satisfactory results with this method are mainly obtained where temperature variations are small (soil, water bodies.
8 2.2. Humidity of the Air 2.2.1 General comments on air humidity measurements 2.2.11 Physical aspects Air humidity is defined by the water vapour contained in the atmosphere. 20 definitions and specifications are given by WMO. The most relevant for agricultural meteorological purposes are - Vapour pressure "e partial pressure of water vapour in moist air of the total atmospheric pressure "p. (units :mb) - Saturation vapour pressure "E maximal water vapour pressure at pressure p and temperature T. (unit : mb) - Relative humidity "U":Ratio in percent of partial to saturation vapour pressure of a given total pressure p and at temperature T. 0 = 100 = e/E (unit :%) - Dew point temperature "Td": Temperature at which the