When passing through the atmosphere, the SR is not only being attenuated, but it also change its spectral composition

Spectral	The Sun is in
area	zenith

Sun altitude is 30°

The Sun at

horizon

UV 4%

Visible 46%

IR 50%

65° Sun altitude

30° 19°

11°
O,	1	1,5 2	2,5
UV 5		IR

visible

3% 0

44% 28%

53% 72%

Maximum emittance is shifted to the longer wave side as the Sun altitude decreases.

	The shorter wave beams
	suffer the largest
	extinction. Thus, passing
	through every new layer,
	the SR becomes more and
	more enriched with longer
	wave radiation.	11

The atmosphere turbidity factor

Optical depth of the atmosphere can be presented as a sum of three items. i c a

	i is the optical depth of the ideal atmosphere.
	c is the optical depth formed by variable constituents (CO2, H2O )
	a is the optical depth formed by aerosols.
		T		is turbidity factor.				T 1
		i					i
	As we know			iT		Pi
			I I0e m			I I0e iT m			I I0 PiT m
	I I0 Pm ;

Comparing the formulas suggests how many masses of ideal atmosphere are needed to get the SR extinction produced by one mass of the real

atmosphere.

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I	Pm I PT m		m lg P Tm lg Pi
0	0 i

T lg P

lg Pi

Atmosphere turbidity factor (ATF) varies in a wider range than the transmission coefficient does.

ATF does not depend on m value as much as the transmission coefficient does.

ATF depends on physical properties of air masses

Air masses

Continental arctic air

Maritime polar air

Continental polar air Continental tropical air

ATF 2.45 2.66 3.09 3.40

Air mass is a huge air body characterized by homogeneous distribution of the air

properties such as temperature, humidity, transparency etc.

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Direct solar radiation (DSR)

The solar radiation coming on an observation point as a bundle of parallel rays is called DSR.

Fluxes of I and I ' I sinho depends on the following factors:

•Solar constant.

•Distance between the Earth and the Sun.

•Physical state of the atmosphere over the point.

•Altitude of the Sun.

Values of I and I’ have well-defined diurnal and annual variations. Maximal values is observed at the local noon. They are also influenced by turbidity of the atmosphere. They increase with increasing altitude of a locality (in this case optical mass decreases). It is why in mountain areas these quantities are larger than over planes.

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Cloudiness makes an appreciable impact on the DSR. At overcast condition it completely blocks the DSR.

The DSR fluxes falling on slanted surfaces are different of those falling on horizontal surfaces

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Scattered (diffused) radiation (DR)

Amount of scattered radiation coming on a unit of area in a unit of time is named SCATTERED RADIATION FLUX (i).

It depends on

•Altitude of the Sun

•Transparency of the atmosphere

•Cloudiness

In a certain condition, contrary to the DSR, cloudiness makes DR stronger. However, some interior clouds (St, Sc at ho<15°)can not do that.

The DR flux reaches its maximum value at medium and high level clouds. At some cases it can be 2-3 times more intensive than the clear sky does.

The maximal value of DR is observed at local noon when the Sun altitude is the highest for the given day.

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Relationship between DSR and DR

b12

b13

i/I

0,22

0,17

0,12

0,09

i b I0 I sinho

Empirical coefficient

For ideal atmosphere

For real atmosphere

c

0,67

0,54

0,43 0,34

						There are some other
		The loss of
		DSR in the					formulas
		atmosphere
						i bcI		i bc	I '
									sinho
	“c” is parameter describing the

atmosphere transparency.

From these formulas it follows:

•At c=const , the DR flux is proportional to I . The Sun altitude increases (m value decreases), DR grows up.

•The ratio i/I depends upon c value only. For ho=10..75

•The ratio i/I grows up when the Sun altitude and c value decrease

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At cloud free sky i 0,1 I

As medium and high level clouds appear in the sky, amount of DR grows up.

Snow cover also makes some contribution into increase of DR.

DSR is reflected by the snow

Atmosphere scatters the reflected DSR

A part of back scattered radiation comes back to the surface

The maximal energy of the DR falls on wavelength

m 0,425 0,450

DSR m 0,47

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