Cloud Cover Sensor

This invention relates to apparatus for measuring the degree of cloudiness, by measuring the amount of light received from various areas of the sky.

The extent of cloud cover is of interest to meterologists as an aid in making weather predictions and for evaluating climatic factors, in evaluating the performance of solar energy collectors and converters, and in other applications. The degree of cloudiness can be evaluated by several available techniques, but they are time consuming and often do not provide a simple number that can be utilized in comparing the cloudiness of different days or different areas. For example, a photograph of the hemisphere can be taken by the use of a fish eye lens, but interpretation of the photograph requires determination of the relation between darkness or opacity of each area of the photograph and the intensity of the light causing the opacity, as well as a measurement of the areas of different opacities of the photograph. A relatively simple device which provided an indication of cloudiness with little effort and at low cost, even if with only moderate accuracy, would be of value in many applications.

In accordance with one embodiment of the present invention, a cloud cover sensor apparatus is provided which enables the generation of an indication of cloudiness using relatively low cost and easily operated equipment. The apparatus includes a frame holding several light sensors such as photovoltaic cells, including a group of indirect sensors positioned so that when a predetermined axis of the frame is directed towards the sun, the indirect sensors face most of the sky area not covered by the sun. A shield is provided to prevent direct sunlight from reaching the indirect sensors. The total output of the group of indirect sensors, adjusted for the amount of direct sunlight available, provides an indication of the cloudiness of the sky. The amount of sunlight available can be determined by providing a direct sensor mounted to face along the axis which is directed towards the sun, and by constructing the shield to prevent light from most of the sky lying away from the sun from falling on the direct sensor. Continuous operation of the apparatus can be achieved by mounting the frame on an equitorial mount and providing a motor to turn it so that the axis of the frame follows the sun in its movement across the sky.

FIG. 1 is a perspective view of a cloud cover detector apparatus constructed in accordance with one embodiment of the present invention;

FIG. 2 is a partial sectional view taken on the line 2--2 of FIG. 1;

FIG. 3 is a partial schematic diagram of the apparatus of FIG. 1.

FIG. 1 illustrates a cloud sensor 10 which includes five photovoltaic cells 12-16 mounted on a frame 18. One of the cells 12 (which is formed of four sub-cells) serves as a direct sunlight sensor, because it is positioned on the frame to face along a predetermined axis 20 that extends towards the sun S. The other four sensors 13-16 (which are each formed of two sub-cells) serve as indirect sensors, in that they are positioned to face along lines of sight 22-25 extending in different compass directions so as to collectively face most of the sky area illuminated by indirect sunlight, and to face most of the clouds C in the sky. A shielding apparatus 30 is mounted on the frame to shield the direct sensor 12 from most of the sky area around the sun, and to shield the indirect sensors 13-16 from direct sunlight. The frame 18 is mounted on an equitorial mount 32 tilted from the horizontal so that simple rotation of the frame about the axis 34 of support enables the direct sensor 12 to follow the sun in its movement across the sky. An adjustment mechanism 36 is provided to adjust the incline so as to account for changes in the sun's maximum declination with time of year and longitude.

The five sensors 12-16 are connected through a flexible cable 40 to an indicating apparatus 42 which can provide an indication of the cloudiness of the sky. The output 44 of the direct sunlight sensor 12 is delivered to a meter 46 which can be utilized to indicate the intensity of the sun. The outputs of the four indirect sensors are delivered over line 48 to a summer 50 which generates a signal having a value proportional to the sum of the outputs of the four indirect sensors. This signal is delivered to another meter 52 which indicates the amount of sunlight illuminating most of the sky around the sun. The output of the summer 50 as well as the output over line 44 from the direct sunlight sensor, is also delivered to a divider circuit 54 which generates an output indicating the ratio of the light sensed by the four indirect sensors to light sensed by the direct sunlight sensor. This ratio, which is delivered to a display 56, can be considered to be an adjusted cloudiness indication, which is high when there are no clouds in the sky and which decreases as the amount of clear sky area decreases. The indication is automatically adjusted for the declination of the sun, which varies with the time of day and season of the year, since the lower the angle of the sun from the horizon, which reduces the sum of outputs of the indirect sensors even on a clear day, the lower the output of the direct sensors.

FIG. 3 indicates one circuit which can be utilized for the summer 50 of FIG. 1. The circuit includes a small resistor 60 across each cell, for operating it in an almost short circuit mode. This is desirable because typical silicon photovoltaic cells produce an output substantially proportional to the amount of light falling on them when in a short circuit mode of operation (when at constant temperature). For a photovoltaic cell which is a square of two centimeters on each side, a resistor 60 is about one-half ohm can be utilized, to produce an output of about forty millivolts at peak sunlight illumination. A potentiometer 62 connected across each small resistor is set at a resistance (such as 100 ohms) which will provide about the same output from each cell when all are illuminated identically. The total current, from all cells indicated by arrow 66, is indicated on the meter 52. Obviously, a wide variety of circuits can be utilized to determine the outputs of the indirect cells, as by taking their sum, their average, or some other function of them. In FIG. 1, the divider 54 can be a digital calculating circuit with analog-to-digital inputs, and which is connected through a driver to a digital display 56 which provides a number indicating the cloudiness of the sky.

As shown in FIG. 2, the shielding apparatus 30 includes several plate-like portions such as portion 70 which serves to simultaneously shield the direct sensor 12 from most of the sky and the indirect sensor such as 14 from direct sunlight. In this example, the line of sight 25 of the indirect sensor 14 extends at an angle A of 60.degree. from the axis 20 along which the direct sensor faces. The shield portion 70 lying between the two axes 20 and 25 extends at an angle B of about 20.degree. from the axis 20. The size and orientation of the shield portion 70 is chosen to prevent any direct sunlight from reaching the indirect cell 14 despite several degrees of angular misalignment of the axis 20 with the sun, while minimizing the amount of sky area around the sun viewed by the direct sensor 12. It would be possible to utilize two shields such as those indicated at 72 and 74 between the axis 20 and each indirect line of sight such as 22. However, while the shield at 72 would effectively shield the direct sensor 12, the other shield 74 would block a significant portion of the sky area from the indirect sensor 13 while shielding it from direct sunlight. If very long shield portions such as 70 can be utilized, then the angle B can be reduced accordingly.

The degree of cloudiness can be indicated by merely sensing the outputs of the indirect sensors 14-16 and utilizing another means to account for changes in the angle of the sun above the horizon. For example, a simple table can be utilized to provide an adjustment factor based upon the time of day and season of the year, or a clock mechanism can be utilized to account for changes in the time of the day with adjustments to be made for different seasons of the year. However, so long as an electrical output is to be manipulated, the addition of a direct sunlight sensor for enabling an adjustment for the declination of the sun provides a simple means of adjusting for the time of day and season. In operating the device, it is desirable to take note of the possibility that a cloud may come between the sun and the direct sensor 12, in which case the output of the direct sensor cannot be utilized in determining cloudiness, and it may be necessary to wait for the cloud to drift away or to look up the values in a table. A filtering mechanism can be utilized to sense when the output of the direct sensor 12 changes more rapidly than would be encountered merely by change in declination of the sun during the day.

Thus, the invention provides a cloud cover detector apparatus which is simple in construction and operation, that can indicate the degree of cloudiness. This is accomplished by the use of a group of indirect light sensors mounted on a frame to face in different directions so that the group receives light from most of the sky, and by also providing a shielding means that shields each of the group of indirect sensors from direct sunlight. The group of indirect sensors can be mounted on a frame that can move to follow the sun, so that the sensors continue to sense almost all of the sky except that occupied by the sun, during movement of the sun across the sky. An adjustment for changes in the declination of the sun, as with time of day and season of the year, can be provided by utilizing a direct sunlight sensor which is mounted on the frame and which directly faces the sun. The shield means can be formed to shield the direct sensor from most of the sky area around the sun to minimize the influence of light from the rest of the sky. In one apparatus, an indication of cloudiness is obtained by the use of a circuit that indicates the ratio of indirect sunlight received from most of the sky area except the part occupied by the sun, as is sensed by the indirect sensors, to the light received directly from the sun by the direct sensor.