1. Basics
Within the dimensions interesting in the meteorology the radiation takes an important place, because on the one hand the solar radiation is the primary energy source of the atmospheric events, and, on the other hand, the different radiation streams deny an essential part of the energy transport in the atmosphere. These radiation streams, with the physical name Radiation flux densities, i.e. in the time unity by the horizontal unit area stepping radiation energy, are given in W / m². In addition, the ultraviolet rays have won during the last years in interest.
2. In the meteorology used radiation flux densities
Radiation is dependent on wavelengths (Plancksches law) what is used, for example, with remote sensing by radiation measurement in the satellite. Because in the meteorological sphere the radiation is treated, nevertheless, always under her energetic aspects, the radiation integrated about the wavelength (Stefan-Boltzmannsches law) is of interest. With regard to the spectral distribution the meteorological significant radiation flux densities can be separated in two areas, the solar one and the terrestrial radiation.
2.1 Solar radiations
The short-wavy radiation in the area of about 0.3 … 3 µ m comes primarily from the sun and, therefore, is called solar radiation. In the upper edge of the atmosphere 1367 Wm2 (solar constants) arrives on average by which a part is absorbed in the atmosphere or is strewn upwards or below. Hence, the solar radiation down resigning from the atmosphere consists of direct and diffuse radiation which are called together global radiation.
1. The direct solar radiation S. = l sin (h)
Besides, there is l the solar radiation falling on a surface vertically standing to the radiation direction and h the solar height (corner!). Often the position of the sun also becomes about her zenith described. Then counts S. = l cos (?) and the change caused thereby of the solar radiation on the horizontal surface it is called "cosine law". With thick clouds or a cloud before the sun this is not recognizable, i.e. see = 0. In cases in those no cloud the sun is covered S. greater than D, except with the low standing sun where the direct sun is weakened by the long way by the atmosphere. (Movement of the solar color in Red).
2. The difuse solar radiation D (sky radiation)
This is that of the molecules, aerosol and clouds particles in the atmosphere scattered solar radiation.
3. Global radiation G
if the usual name is for the whole solar radiation resigning below from the atmosphere: G = S. + D, all the same around what meteorological situation it concerns. The global radiation hitting on the ground is partially reflected, the rest is absorbed.
4. The reflected global radiation R
if the radiation reflected by the earth surface, likewise a radiation flux density in W / is m². The reflecting property of natural surfaces is in the first Näherung isotrop and independent of solar state. With it the reflecting property of a surface - is attached except for water because of the shining component available there - as steady{constant}. With it changes R proportionally to G; it is worth R = a G. Besides, there is a the reflecting degree of the surface which receives the name Albedo a for the spectral average looked here about the solar spectral range. With known a can be determined R from G. Therefore, it is not necessary as a matter of routine to measure R. The values of the Albedo of natural surfaces lie between 6% and 15% for the ocean after solar state and covering degree, about 10% for wood, about 20% for meadow and asphalt and about 50% for snow-covered surfaces .ö Higher values of snow count only to fresh snow in big areas in which no growth or similar breaks through the snowy cover.
5. Solar radiation balance Qs
of the earth surface is the radiation absorbed by this surface, as a balance between what comes and is radiated what by reflection again. This is the energy per surface and time, again in W / m² which is available for the conversion into other forms of energy. Because the radiation falls on the earth surface, counts i.e. on an impervious medium, so that can be only reflected or be absorbed: a + it =1 (the solar issue coefficient is like the solar absorption coefficient) and with it
Qs = G - R = it. - G = (1 - a) G = (1 - a) (See + D)
2.2 terrestrial radiations
The long-wavy radiation in the area of about 4 … 60 µ m comes above all from earthly matter, i.e. from the ground and the components of the atmosphere which shine after the Stefan Boltzmann law according to her temperature. Therefore, it is called terrestrial radiation. One makes a distinction:
1. The atmospheric counterradiation A,
the down directed terrestrial radiation of the atmosphere, i.e. the temperature radiation of the atmospheric gases (primarily} steam and carbon dioxide) and the clouds. This gets from the whole half space and has with it no preferential direction like the direct solar radiation with the solar radiation.
2. The radiation of the earth surface.
E = et• s •TB4. Besides, the middle emission ratio is et in the terrestrial area which is a law like the terrestrial absorption degree according to the Kirchhoffschen. a =5.66956 10-8 W m2 . 8226 K-4 is the constant of the Stefan-Boltzmannschen of radiation law and TB the (absolute) temperature of the earth surface.
3. The reflected counterradiation.
If the emission ratio is not et =1, i.e. if the ground cannot be understood in its issue behavior as a " black body ", a part of the counterradiation is reflected, it proves see I the long-wavy reflex radiation, r = (1 - et) A. These forms together with the real radiation of the ground the heat radiation coming from the earth surface which consists with it additiv of two limbs} E and r which can be separated, however, only with costly measuring methods. Often one looks at the earth surface in the long-wavy area as black, because is with all natural surfaces except some rock kinds, et ~ 1. Because therefore at the same time r is neglected, the small mistake within the scope of which Measurement accuracy even compensates is still decreased.
4. The terrestrial radiation balance
st again{on the other hand} the difference between on the ground{bottom} incoming and the radiation leaving from the ground: Qt = A - E - r with the accepted simplification e> t <et = 1 surrenders:
Qt = A - E = A - s-TB4
2.3 radiation balances
The radiation balance Q as a sum of Qs and Qt brags which radiation energy is available in the earth surface all together for the change in other forms of energy:
Q = Qs + Qt = S. + D - R + A - E - r = (1 - a)-G + et (A - s-TB4) = (1 - a)-G + A - s-TB4)
Q contains about a, et and TB qualities of the ground surface: A size free of this influence is the actual radiation Qeff = S. + D + A - s TL4, that is the radiation balance of a horizontal black surface (a = 0, e = 1) with air temperature TL. As an other size becomes the actual radiation Eeff = sTL4 - A used which arises 0) as a negative value of Qeff with missing solar radiation (s + D =0) This differs from the long-wavy radiation balance as an absolutely black respectable ground Ql = A - s-TB4 except by the portent also around σTB4 - σ TL4 = as. (σB-σL), and as the radiation crossing coefficient (5.6-W m 2K~1σ B is the n-surface and σL the air temperature.
The introduced radiation flux densities change of course with the time of day and season, but also with the cloudiness. As typical annual averages for the earth can apply m² evaluates in W / everything:
|
Global radiation |
S + D |
104 |
154 |
|
reflex radiation |
R |
20 |
17 |
|
shortwave radiation balance |
Qs |
84 |
137 |
|
counter radaition |
A |
320 |
335 |
|
emission |
E |
357 |
390 |
|
longwave. radiation balance |
Qt |
-37 |
-55 |
|
radiation balance |
Q |
47 |
82 |
These annual averages show how small really to the conversion into other forms of energy on the ground{bottom} available radiation flux density Q is, above all if one compares them{her} with on average on the unit area of the earth occurring extra-terrestrial solar radiation of 342 W of m-2.
3.0 spectral weighting
The visible one and the ultraviolet rays matters to the person. The visible radiation is determined by the sensitivity with which the eye reacts to radiation. Of the radiometrical unit W of m 2 radioation flux density which describes the energetic aspect of the radiation and is used in the meteorology) suitable photo-metrical unit there is the lux (from = gek. lx - the density of light which describes the light impression or brightness impression). The bright delicacy dependent on the wavelength of the human eye has see her{their} biggest, on 1 standardized, We = rt with 0.555 µ m. Here counts: 1 Ix =3D 1.47 10 =-3W m-2. With other wavelengths are smaller the spectral brightness = mpfindlichkeit see and the conversion factor and the following. The following table{chart} gives values of mono-chromatic radiation:
| ? |
0,4 |
0,45 |
0,5 |
0,55 |
0,60 |
0,65 |
0,70 |
0,75 |
µm |
| s |
0,04 |
3,8 |
32,3 |
99,5 |
63,1 |
10,7 |
0,41 |
0,01 |
% |
| f |
0,3 |
26,0 |
221 |
682 |
432 |
75,0 |
2,8 |
0,10 |
lx(W m2) |
For a nonmonochromatical radiation one can only brag{can only act big} and the following, we = nn one knows her spectral composition. For the global radiation with = of the sun of high standing ("daylight") is and the following et = wa 100 lx / (W m 2). the "daylight factor". It should be stressed once again, that the visible spectral range, d = he possibly by 0.4 … 0.8 µ m goes, is not identical with the solar spectral range. To longer wavelengths the infrared area (> joins to the visible area 0.8 µ m) in which almost = 50% of the solar radiation lie. Indeed, the area of Ultraviolet or ultraviolet rays (<0.4 µ m), that is the area with shorter wavelengths than sichtb = are light contain only about 5% of the solar radiation, however, is important because of the photo-biological and photo-chemical processes which this radiation can release. As with the visible light this radiation also usually becomes according to her{their} spectral effect if t weights{evaluates} =. This is especially necessary in the UV because by the strong absorption of Oz = on the spectral radiotherapy density decreases within 30 nm from 320 nm up to 290 = nm by about 5 ten powers. On the other side effect increases biologisc = hey in the reverse litre of beer, so that only taking into account such weighting the effect the UV radiation can be properly described. A typical weighting is for the sunburn, the Erythem.. This is taken on behalf for many biological UV effects and gs eht also in by the WMO and WHO introduced UV index one. This size, UVI, is defined as erythem gs weighted ultraviolet rays in W / m² m² multiplies by 40 1 / W/. With it the UVI is a size without dimension the radiation flux densities UV describes and in Germany values between 0 and 8, on Zugspitze with the sun and snow up to 10, reaches.