Agriculture Reference
In-Depth Information
The N values would be (for latitude 37°N):
21 December, 9.4 h
21 March, 12.0 h
21 June, 14.5 h
A.1.6
Hellman's equation
To calculate the wind velocity ( U H ) at a cer-
tain height ( H ), as a function of the wind
velocity at a height of 10 m ( U 10 ), Hellman's
equation may be used (Wacquant, 2000):
A.1.4 Wien's law
(
)
H UU
=
0.233 0.656log
+
H
+
4.75
10
The product of the temperature of a radiant
surface (in K) and the wavelength of the
emitted radiation expressed in micrometres
(microns) is constant, and equal to 2897
K·mm (Rosenberg et al ., 1983).
As the Sun's temperature is of the order
of 5800 K, the dominant wavelength will be
2897
(2.11)
A.1.7
Saturation vapour pressure
Water vapour pressure in the atmosphere at
saturation ( e s ):
17.27
×
T
5800 , this is, around 0.5 mm, equivalent to
500 nm.
The temperature of the greenhouse is
of the order of 300 K (or 27°C). So, the
dominant wavelength in the radiation
emitted by the greenhouse would be 2897
e
=
0.61078 exp
×
(2.12)
s
T
+
237.3
where:
T = Temperature (°C)
e s = Water vapour pressure in the atmos-
phere at saturation in kPa
As the temperature rises, the air can
contain more water vapour, as the water
vapour saturation pressure e s increases (see
Fig. 2.17).
300 ,
this is, around 9.5 mm, equivalent to
9500 nm.
A.1.5 Wavelength and frequency
A.2
Chapter 3
The wavelength L , expressed in metres, and
the frequency f , expressed in Hertz, are
related by:
A.2.1 Thermal integral
L × f = constant (2.10)
This being constant for the speed of light
(2.9979 × 10 8 m s −1 ).
The thermal integral, thermal time or physi-
ological time is the summation of the tem-
peratures above a certain threshold or base
temperature (Villalobos et al ., 2002):
( )
Table 2.3. The electromagnetic spectrum (Bot and
Van de Braak, 1995).
=Σ −
(3.1)
IT
T T
i
1
where:
IT = Thermal integral
T i = Temperature of the process (daily
average)
T 1 = Threshold or base temperature
The thermal integral is expressed in
degree days (°C·day). A degree day is equal
to 1°C above the threshold temperature,
through a 24 h period.
In order to accurately use the concept of
the thermal integral, the response of the
development velocity versus the temperature
must be linear, and the temperatures consid-
ered must be above the base temperature
Type of
radiation
Wavelength
(nm)
Frequency
(Hertz)
Gamma rays
<0.01
>3 × 10 19
X-rays
0.01-10
3 × 10 19 -3 × 10 16
Ultraviolet
10-390
3 × 10 16 -7.7 × 10 14
Visible light
380-760
7.9 × 10 14 -3.9 × 10 14
Solar radiation
300-2,500
10 15 -1.2 × 10 14
Infrared
760-3 × 10 5
3.9 × 10 14 -10 11
Thermal
infrared
(300 K)
2,500-25,000
1.2 × 10 14 -1.2 × 10 13
Microwaves
3 × 10 5 -3 × 10 8
10 12 -3 × 10 9
Radio waves
>10 8
<3 × 10 9
 
 
 
 
 
 
 
 
 
 
 
Search WWH ::




Custom Search