The exact definition by Wissenschafter and technicians is somewhat different depending upon field of activity. In the meteorology the air temperature at a value of two meters is measured, for which the classical, knows painted weather huts in free environment to serve.
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the substantial factors of influence on the air temperature are on the one hand the radiation balance of the earth and/or. its local Strahlungsbilanz and on the other hand mixture effects by the wind.
the diurnal variation of the air temperature is coupled direct to the diurnal variation of the global radiation and shows therefore a pronounced waste in the night, thus after sunset. The minimum becomes thereby on the early morning and/or. briefly before sunrise reaches. This tendency is absorbed by a strong cloudy appearance and also wind, particularly in proximity of larger water surfaces. If the air temperature falls below thereby the dew point temperature, it can come to phenomena such as nebulas , rope or hoar frost. After the temperature crossed its daily minimum, it rises first rapidly and in the noontime then somewhat more slowly. Their maximum reaches it after the sun highest level, in the winter usually already between 13 and 14 o'clock, in the summer between 15 and 16 o'clock. Afterwards it drops in the evening hours rapidly and at the night somewhat more slowly, until it reaches its minimum too again on the early morning. This normal case of the diurnal variation applies both to the summer and to the winter. Perhaps dynamic influences like a break-down of warm or cool air can lead however to partly substantial deviations and a reversal of the temperature gradient. Near the coast the Seewind is responsible for it that the daily maximum temperature is reached often already substantially in former times around 12 to 13 o'clock, the temperature in the further course of the day thus no more does not increase.
in the yearly variation, based on either daily - or monthly means as average values of many years, for instance the following process shows up for Central Europe in. January forms the coldest month, from March to May shows up a rapid increase with maximum in July and from September to December a just as rapid reduction of the temperatures.
dependence on the height
The change of the air temperature with the height is the furthest common criterion for the organization of the terrestrial atmosphere into different layers. The troposphere as lowest layer possesses an extending of approximately 11 kilometers over Central Europe. It shows thereby one approached linear drop of temperature of on the average 10 °C at the soil to 0 °C in two kilometers, approximately -20 in five kilometers and finally -55 °C in ten kilometers height. For this temperature gradient there are two dynamic cases of model, the damp-adiabatic and the drying-adiabatic. On the average the static temperature decrease amounts to about 0.65 Kelvin per hundred meters, which one calls geometrical temperature gradient. If it comes to no further temperature decrease, then one reached the tropopause. This is appropriate as in the Tropics particularly high, can in the troposphere also for minimum temperatures of -80 °C be formed.
In the further process the temperature rises after a stationary phase, under normal conditions for instance starting from 25 km height. Responsible for this the relatively high ozone concentration and the associated radiant absorption are in this atmosphere layer, which one calls stratosphere. The temperature maximum is reached with approximately 0 °C at height of the stratopause. In hieran the following mesosphere the temperature sinks itself again and reaches on the Mesopause with -100 °C a new minimum. The thermosphere and the exosphere with a temperature again increasing finally follow themselves, whereby one can to speak belong her in these heights however hardly still of air and actually to space. The number density is here so small that even a temperature of several thousand degrees Celsius would not cause considerable heat transport processes.
measuring methods and - instruments
the measurement of the air temperature takes place usually via thermometers or sensors. First are usually filled with alcohol or mercury, while the sensors usually with semiconductor - or thermal effect work. For less precise measurements also bimetal becomes - strip uses.
As a rule the temperature measurement corresponds to a dipping measurement, which is often accelerated in the technology by ventilation. With the aspiration psychrometer after ASS ASS in this kind also the damp temperature is exactly measured.
The adjustment of a thermometer to the air temperature needs a certain time, which can last from some minutes to a half hour. For example if a more rapid result is necessary with a room thermometer, one can accelerate the reading by moving with stretched hand. The radioactive half-life amounts to about 20 seconds, i.e. after this time „the artificial wind approximated “the announcement on the scale around 50% the true value.
The estimation of the air temperature can succeed with zero wind and appropriate experience on 1-3° exactly. The felt temperature with wind is substantially more coldly estimated however by the Windchill.
To the comparison of temperature levels, which were based on different places and heights, one avails oneself of the potenziellen temperature. The focus is on the air humidity and/or. Atmospheric pressure arranged, then one uses the virtual temperature.
influences on the measuring accuracy
a measurement on a decimal place, thus exactly , the extreme measuring accuracy, those is still possible for 0.1 degrees Celsius in the free one and/or. is meaningful, because already easy air movements have an influence of some tenth degrees. Besides horizontal temperature gradients in the order of magnitude of 0,1 prevail °C per meter, those with position of the sun, rock and vegetation strongly to vary to be able and at ground level also several degrees to amount to be able also with zero wind. The temperature field in such a way specified is most stable with a strongly cloudy to covered skies and medium strong wind. With fair weather it is however jerkiest (see and upwind also “cloudless “).
Because of these circumstances a reliable measurement of the air temperature on approximately 0.5 requires °C accuracy already substantial precautions, in particular a well behind-ventilated cover of the solar radiation and the radiant heat of soil and buildings. The best place of assembly for a temperature sensor and/or. a thermometer is therefore a shady place in the north of a free standing building.
For laymen a measuring accuracy of approximately 1 is attainable °C, if above conditions are approached calibrated given and the measuring instrument. Otherwise error up to 3 can occur °C, with radiation protection lacking also over 5 °C.
The Wetterstationen of the Meteoroligists measure the temperature in different heights, on the one hand over statements about the radiations - and/or. To receive energy balance to be able to partly consider on the other hand in order the effects stated above. Air temperature the temperature is called, which is radiation measured in accurately 2 m height in a weather hut. Additionally the soil temperature is measured: The chordal addenda 5, 10, 20, 50 and 100 cm are usual in the ground.
In the astronomy and geodesy the inevitable anomalies of the near-surface temperature field rank among the most unpleasant, because only with difficulty modelable disturbances. The astronomical refraction can be computed however - as average, regular Strahlenbrechung - relatively well from 3-4 air parameters.
The astronomers call the turbulences, which divert the direction of the star light around 0,5 to 5 ", „Seeing “(air unrest) and/or. Scintillation („flares “of the stars); the mesoclimate in the dome of an observatory can do a so-called. Hall refraction cause.
The geo data are afraid these influences less, because they fall out as coincidental errors with longer series of measurements. More unpleasant however a biased error is by the side refraction, which arises in particular in tunnels and with Visuren, which run by scarcely a thermally different surface (for example a besonnten Hauswand). Wind systems in the mountains or with technical large-scale projects, also changing, can have critical systematic influences.