Temperature measurement#Surface air temperature

Is the temperature of the lower atmosphere is referred to as air temperature, which is not influenced by solar radiation nor by soil heat or heat conduction.

The exact definition by scientists and engineers is different for each specialist something. In meteorology, the air temperature is measured at a height of two meters, for which the classic, white painted weather huts ( huts thermometer ) are used in an open environment.

In the heating technology to measure the outside temperature, it is taken to a near-wall, temperate place, without mitzumessen the wall radiation.

Influences

The main factors influencing the air temperature on the one hand the radiation budget of the Earth or its local radiation balance and on the other hand mixing effects by the wind.

Variability

The air temperature varies throughout the day, the seasons and climate variability. The highest temperatures ( Hitzepol ) of almost 60 ° C are observed in the interior of deserts, the deepest values ​​( cold pole ) occur in Antarctica on (almost -90 ° C).

Annual cycle

In the annual cycle, based on either a daily or monthly means as long-term averages, shows up for Central Europe about the following course. The month of January is the coldest month from March to May, shows a rapid increase with a maximum in July and from September to December, a rapid decrease as the temperatures.

Diurnal cycle

The diurnal variation of the air temperature is directly linked to the diurnal variation of global radiation and therefore shows a marked drop in the night, so after sunset. The minimum is reached while in the early morning and watch the sunrise around. This tendency is attenuated by a heavy cloud and wind, especially in the vicinity of large water surfaces. Falls below the air temperature while the dew point temperature, may lead to phenomena such as fog, dew or frost. After the temperature has passed her days minimum, it increases rapidly at first and then at noon at a little slower. Your maximum they reached after the high sun, in winter usually have 13 to 14 clock, in summer 16-17 clock, sometimes only before 18 clock. Then it sinks in the evenings quickly and in the night from a little slower until it again reaches its minimum in the early morning. This normal case of the diurnal cycle applies to both the summer and for winter. However, dynamic effects such as an intrusion of warm or cold air can lead to some significant deviations, and possibly a reversal of the temperature gradient. Near the coast, the sea breeze is responsible for ensuring that the daily maximum temperature is often reached much earlier by 12 to 13 clock, the temperature in the rest of the day no longer increases.

Depending on the amount

The change of air temperature with height is the most widely used criterion for division of Earth's atmosphere in different layers. The troposphere as the bottom layer has over central Europe has an extension of about 11 kilometers. It shows a linear temperature drop approached from an average of 10 ° C at the bottom to 0 ° C in two kilometers, around -20 ° C in five kilometers and finally -55 ° C in ten kilometers high. For these atmospheric temperature, there are two dynamic model cases, the feuchtadiabatischen and trockenadiabatischen. On average, the static temperature decrease is about 0.65 ° C per hundred meters, which is called geometric temperature gradient. If there is no further decrease in temperature, so you reach the tropopause. If this particularly high as in the tropics, and minimum temperatures of -80 ° C can form in the troposphere.

Subsequently, the temperature rises to a stationary phase again, usually about from 25 km altitude. This is due to the relatively high concentration of ozone and the resulting radiation absorption in this layer of the atmosphere, known as the stratosphere. The maximum temperature is reached at about 0 ° C at the height of the stratopause. In the adjoining this mesosphere the temperature drops and reaches to the mesopause at -100 ° C, a new minimum. It follows the thermosphere and finally the exosphere with a turn increasing temperature to at these altitudes but can hardly speak of air and they actually already belong to space. The particle density is so low that even a temperature of several thousand degrees Celsius would cause no significant heat transport processes.

Measurement methods and instruments

The measurement of the air temperature is usually done by thermometer or probe. The former are usually filled with alcohol or mercury, while the sensors are working mainly with semiconductors or thermal effect in the design as expansion thermometer. For less accurate measurements also bimetallic strip can be used.

Generally corresponds to the temperature measurement of a dip measurement that is often accelerated in the art by means of ventilation. Therefore, the spin thermometer is used for rapid but accurate scientific measurements. When reading one must however - as with other scales - sure, Right Angle to look at the scale, Otherwise, a parallax error of 1 ° and more arise. Even when Aspirationspsychrometer ( Assmann psychrometer ) ventilated is measured ( in the air stream of a small paddle wheel ), thereby obtaining the wet bulb temperature very accurately.

Many measurement error caused by the fixation of the thermometer at an unfavorable location. So an external thermometer should always be installed in the north of the building, but could be in the summer for 2x 1 hour in the sun it here. In addition to the sensible heat of the building ( to the already few centimeters distance from the window glass help ) can also distort the reflection of a neighboring building measuring around 1-2 °.

The approximation of a thermometer at the air temperature needs a certain amount of time, which can last from several minutes to half an hour. If, for example, with a relatively sluggish room thermometer give a fast result needed, one can speed up the reading by turning the thermometer with stretched hand. The half-life is about 20 seconds, that is after this time, the "artificial wind " approximated the display on the scale to 50 % of the true value.

The estimate of the air temperature can succeed exactly in calm and appropriate experience to 1-3 °. The wind chill temperature with wind but judged significantly colder by wind chill.

For comparison of temperature values ​​that were measured at different locations and heights, use is made of the potential temperature. Is the focus directed to the humidity and air density, so one uses the virtual temperature.

Influences on the measurement accuracy

A measurement to one decimal, ie 0.1 degrees Celsius exactly, is the extreme accuracy that is even possible or useful outdoors, because even slight air movements have an impact of some tenths of a degree. Moreover prevail even with no wind horizontal temperature gradient in the order of 0.1 ° C per meter, which can vary greatly with the sun, rocks and vegetation and near the ground also can be several degrees. The most stable, the temperature field is called in a strong cloudy to overcast skies and moderate strong winds. In fair weather, however, it is the most turbulent (see also cloudless and updraft ).

Because of these circumstances requires a reliable measurement of the air temperature at about 0.5 ° C accuracy already considerable precautions, especially a well ventilated cover of solar radiation and the thermal radiation from the ground and buildings. The best location for a temperature sensor and a thermometer is therefore a shady spot in the north of a detached building.

For laymen an accuracy of about 1 ° C is reached, if the above conditions are met and the meter are calibrated approached. Otherwise, errors can occur up to 3 ° C, with a lack of radiation protection even above 5 ° C.

The weather stations meteorologists measure the temperature at different heights, on the one hand to obtain statements about the radiation or energy balance, on the other hand to be able to take account of the effects listed above in part. The temperature is referred to as air temperature, which is measured in exactly 2 meters in a weather shelter radiation protected. In addition, the ground temperature is measured: usual, the measurement depths of 5, 10, 20, 50 and 100 cm in the ground.

In astronomy and geodesy include the inevitable anomalies of tropospheric temperature field of the most unpleasant, difficult because modeled faults. The Astronomical refraction can be, however - as average, regular refraction - relatively well calculated from 3-4 air parameters.

Astronomers call the turbulence which " distract " the direction of the star's light at 0.5 to 5 seeing " ( atmospheric turbulence ) or scintillation ( " flicker " of the stars ), the local climate in the dome of an observatory can cause a so-called Saalrefraktion. The geodesics fear these influences less because they fall out as random errors with longer measurement series. Unpleasant, however, is a systematic error caused by the Seitenrefraktion that occur especially in tunnels and sightings which pass short of a thermally different area (for example, a sunny house wall ). Also changing wind systems in the mountains or in technical large-scale projects can have critical systematic influences.

History

In Florence the daily temperature was started on December 15, 1654 with the regular measurement and recording.

Standard external temperature of the heating and cooling technology

For the purpose of heating bill do you measure the heating requirements or the required cooling capacity based on fixed predetermined temperatures. Here for instance, is:

• According to the German VDI guideline 2067/DIN 4108 T6 the heating limit is assumed at 15 ° C, for the outside temperature shall be determined by the German Weather Service values ​​based;
• In Austria, Switzerland and Liechtenstein to using a heating limit of 12 ° C, here are the temperature values ​​of ZAMG ( Austria ) and MeteoSwiss ( Switzerland and Liechtenstein ) reference value.