Sky Temperature Correction Coefficients

 

To deal with excessive radiation experienced during hot days / evenings, an internal model was developed which allows the measured sky temperature to be corrected as function of the ambient temperature.

 

The internal model calculates a temperature value as a function of the ambient temperature and subtracts/adds it to the measured sky temperature.

 

The model controlling constants (K1, K2, K3, K4, K5, K6 and K7) are available and the user may adjust them. The simplest model is a linear relationship with the ambient temperature (this corresponds to the default values). More non-linear relationships can be achieved by changing constants K3, K4, K5, K6 and K7.

 

To activate / deactivate the temperature correction coefficients do the following:

 

1)      Select Device Section in Setup Tab as illustrated below

 

 

 

 

Temperature Correction Factor ON

 

 

 

2)      Press key combination <CTRL>W (i.e. <CONTROL> key and simultaneously W). The screen will change to

 

 

Temperature Correction Factor OFF

 

 

 

NOTE:

To turn if on / off, press <CTRL>W.

 

 

 

3)      Use the following suggested limits for:

 

CLEAR SKY:    -5ºC

CLOUDY SKY: 0ºC

 

 

Additional comments:

 

The temperature indicated by IR sensor corresponds to the total downward radiation in the 8 -15 micron long wavelength range.

 

Under overcast conditions, this radiation has a component corresponding to the radiation caused by the cloud water droplets and another due to atmosphere between the sensor and the cloud base. Usually, the radiation above the clouds can be safely assumed to be all absorbed by the clouds.

 

On the other hand, under clear sky conditions, the radiation measured by IR sensor will also include the radiation above the theoretical cloud base which is excluded under overcast conditions.

 

The total radiation in the range 8-15 microns, is also quite affected by the water vapor present in the atmosphere, level of C02, level of 03, dust particles, etc..

 

There are quite a number of complex theoretical models which try to deal with these factors. However, for the purpose of estimating the radiation caused by clouds in terms of overcast, cloudy and clear sky conditions, one may use a more simplified model.

 

Very briefly, the internal model can approach the model developed by Idso and Jackson (1969) for night sky radiation under clear conditions but it has been altered to somehow estimate the clear sky radiation during the day and to yield reasonable values for the IR sensor used with AAG_CloudWatcher.

 

Sky Temperature Correction Factor versus Ambient Temperature

 

Note the following:

·      The software coefficient K1 mainly affects the slope of the curve in its linear section (i.e. ambient temperature below 25ºC);

·      K2 mainly affects the x-axis crossing point – it shifts the curve upwards as K2 gets smaller and vice-versa;

·      K3, K4 and K5  have a large effect on the shape of the curve for ambient temperatures above 30ºC;

·      K6 and K7 introduce an S bent around x-axis crossing point;

 

By right clicking with the mouse over cell Factors as illustrated on the LHS figure below, the system will display a window showing the curve of the sky temperature correction factor as a function of ambient temperature for values between -20ºC and 40ºC.

 

Note that there are two graphical curves displayed – the blue curve corresponds to the actual parameters in use and the red curve corresponds to the new parameters.

 

The default button resets the coefficients K1, K2, K3, K4, K5, K6 and K7 to the program default values.

 

This allows the user to test and compare the effect of different coefficient values on the sky temperature correction model.

 

 

 

 

 

An example of a curve using the default coefficient values (red curve) and a curve using another set of  coefficients where the S bent is quite pronounced around 10º (blue curve).

 

This type of curve appears to work better during the cold season in cold climate regions where the sky temperature is lower than in mild climate regions.

 

 

 

 

 

 

 

 

Another example which illustrates a shift of S bent to the left (red curve).