GB2353354A

GB2353354A - Monitoring cloud cover distribution

Info

Publication number: GB2353354A
Application number: GB0019896A
Authority: GB
Inventors: Stuart Mcmuldroch; Jan-Peter Albert Lothar Muller
Original assignee: BLUE SKY IMAGING Ltd
Current assignee: BLUE SKY IMAGING Ltd
Priority date: 1999-08-18
Filing date: 2000-08-15
Publication date: 2001-02-21
Also published as: GB0201829D0; GB0019896D0; WO2001013145A1; AU6459600A; GB9919405D0; GB2368121A

Abstract

Monitoring of the spatial and temporal distribution of cloud cover and the nature of the cloud cover entails forming an image of the clouds by monitoring the reception of infrared radiation over at least part of the relevant hemisphere, interpreting the resulting image as an indication of cloud density and distribution, and repeating the exercise at frequent intervals. Monitoring apparatus comprises a wide-angle infrared lens, eg lens unit 22 comprising lenses 10, 11 and 12, and means to interpret the image provided by the lens, eg an electronic unit 17 and computer 18. The lens 10 is cleaned by a wiper 14, a filter wheel 15 enables images to be obtained at different wavelengths, and a chopper 16 provides a temperature reference to enable a temperature image to be obtained. The velocity of the clouds may be obtained from successive images and their height from pairs images taken from spaced positions.

Description

1 2353354 CLOUD IMAGING METHOD AND APPARATUS The present invention is

concerned with monitoring the spatial and temporal distribution and nature of cloud cover over the observable sky area and comprises a method of carrying out such monitoring and also apparatus for putting that method into practice.

The monitoring of cloud cover and of changes in the nature and distribution of cloud cover is conventionally carried out by one of two methods, namely either by methods entailing obtaining a view of the sky at successive discreet intervals or by observations of the reflection off the clouds of a laser or collimated light beam.

Apparatus hitherto used for such monitoring by the first type of such methods has comprised visual imaging equipment in the form of mirrors and/or of optical lenses. Such mirrors and lenses have included such components able to transmit 180-degree visual images of the sky at the monitoring point but each of these approaches suffers from serious disadvantages. By way of example, such mirror-based imaging systems tend to be relatively insensitive and 2 therefore to fail to detect clouds of low optical density. Such mirror systems also suffer from having a portion of the sky occluded by parts of the device essential to its operation. Other mirror systems scan the sky and therefore require complex moving parts and substantial time to acquire an image of the entire visible sky. Thus, while optical mirror systems of the foregoing type have some general merit in'P'roducing general observations of clouds, they are unsuitable for use in carrying out unobstructed, sensitive observations of clouds at frequent intervals, such as are of vital importance in aviation and in weather monitoring.

Another approach using such methods has been to observe cloud cover by means of a wide-angle lens suitable for visible wavelength observations. Such devices cannot measure cloud cover directly during night time - as is necessary in aviation and in weather monitoring.

An alternative approach has been to measure the reflection of a laser or collimated light beam from a cloud. The observations thus made are recorded in the form of measurements of reflections from cloud particles or water droplets, which lead to the interpretation of cloud height and cloud amount at a given point. Such devices are able to measure the cloud height at only a single point in the sky. The devices rely on motion of clouds across the point of measurement to infer cloud amount over the whole sky. These methods have 3 proved to be unreliable, especially in situations of developing cloud distribution and/or in the presence of occasional and optically very thin low-level cloud.

There therefore remains a need for a reliable method and apparatus for determining the distribution of cloud cover over the entire observable area of the sky and at frequent intervals, which method and apparatus are able to operate successfully and reliably in widely-varying cloud conditions and in varying light conditions including after dark. It is an object of the present invention to provide such a method and apparatus.

The method according to the present invention for monitoring cloud cover distribution comprises forming an image of the cloud by monitoring the reception of infra-red radiation over a least a major part of the relevant hemisphere, interpreting the resulting image as an indication of cloud density and distribution, and repeating the exercise at frequent intervals.

The apparatus according to the invention comprises a wide-angle infra-red lens, and means to interpret the image provided by said infra-red lens as an indication of the density and distribution of cloud observed via said infra-red lens.

By means of the method and apparatus according to the present invention, it becomes possible to provide, at frequent intervals and throughout daylight and 4 darkness, details of the density and distribution of cloud over the monitored area of the sky. It is a particularly preferred feature of the present invention that the infra-red image of the cloud be then converted to a corresponding temperature image by calibration of the former against a temperature standard, so that the resulting temperature image may afford an enhanced indication of the density and distribution of the cloud. By assuming a profile of the air temperature below the observable cloud deck versus height, it is further possible to determine the height of the cloud above ground level.

Thus the apparatus according to the present invention may further comprise calibration target means to provide a temperature standard, sensor means to compare the image provided by the infta-red lens with the temperature standard to produce a temperature image, and means to interpret that temperature image.

In one alternative form of the apparatus according to the present invention, two of the units may be employed, spaced apart by a distance which may, by way of example, be of the order of 0.5 kilometre, to obtain two distinct stereo images and the height may then be determined using stereo photogrammetry.

As a further optional feature of the present invention, cloud distributions may be determined at measured time intervals as a result of which, by the application of machine vision technology, it becomes possible to determine the velocity of each cloud object in the sky, at the cloud-base and sometimes even at the cloud sides.

The initial observations of cloud cover distribution, upon which the subsequent interpretation and possible forecasting are based by means of the method and apparatus according to the present invention, are made by forming an image of the received infra-red radiation over a major part of the adjacent hemisphere, preferably as observed over an area equal to or approaching a 180' by 180' area, by means of an appropriate wide-angle infra-red lens. The received image is then preferably converted, by calibration against a temperature standard, to an image of temperature distribution over that same area. The calibration may readily be carried out by displaying the received infra-red image, together with an image derived from materials of known emissivity and temperature, upon an infrared sensor and calibrating the received image against the latter image.

The resulting images of temperature distribution or infira-red light intensity are themselves a reliable indication of the density and distribution of cloud over the observed extensive area of the sky. The images may readily be analysed by means of appropriate computer software, to produce the desired data with no significant delay. Thus the whole exercise may be repeated at very frequent intervals, approaching one minute or less, to give a regular and up-to-date indication of the changing cloud pattern over the whole of the observed area.

6 In turn, that information may be combined with air temperature versus height profiles or with data obtained by point measurements of cloud height, such as are already determinable by means of available instruments such as so-called ceilometers, to provide an indication of cloud height over the whole of the observed area of the sky.

In the case of all cloud information generated by means of the method and apparatus according to the present invention, that is, cloud distribution, temperature, amount, height, type or class, any geometric distortions introduced by the use of a wide-angle lens may be removed by geometric calibration and by standard photogrammetric techniques to yield an orthorectified map of the observable sky from below which may if desired be superimposed upon and directly compared with cloud observations made by aircraft or satellite from above.

The infra-red lens by means of which the initial image of the cloud pattern is obtained may advantageously be constructed of one or more elements made from optical-grade germanium. Alternative, less preferred, suitable materials include zinc sulphide and zinc selenide. The lens preferably is so designed as to produce an image of the whole sky of the relevant hemisphere - or as close as possible to an image of such an angular extent.

7 As indicated, the infra-red image produced in the foregoing manner is then preferably converted to a temperature image by calibration against a temperature standard. This conversion may readily be carried out by comparing parts or whole images obtained at the same or different times with and without the temperature standard. The target from which the target image is derived is constructed from materials of known emissivity and temperature.

The infra-red or temperature image thus obtained may be interpreted to provide an almost immediate map of the amount and height of the cloud over the whole of the observed sky area. A filter wheel which may be interposed between the infra-red lens and the infra-red sensor enables one or more samples of the observed wavelength to be measured, which may be compared to provide the optical depth of the cloud. This data, together with information about the height, structure and texture of the cloud, may provide a description of the cloud class or type. The interpretation of the image will usually be carried out automatically by a computer provided with the software necessary for that purpose.

Thus the method and apparatus according to the present invention enable the user to produce an image of the characteristics of all cloud, including its height and optical depth, across a major part of the accessible sky area, in a matter of one or more minutes or less. Software is readily developed to monitor progressive changes in the observed cloud, thereby enabling forecasting of 8 anticipated movement of the clouds, and in turn of the anticipated weather, quickly and at frequent intervals. Quite apart from the general attractiveness of all such frequent and progressive weather forecasts, the method and apparatus may afford observations of relatively thinner, lower level and possibly faster moving cloud than are readily detected by conventional methods and may therefore be of particular value to pilots in flight, at all times in daytime or after dark.

The invention will now be further described and illustrated, by way of example only, with reference to the accompanying drawing, which illustrates, in schematic view, one preferred form of cloud imaging apparatus according to the present invention.

The illustrated apparatus comprises a lens unit 22, which in this example comprises a sequence of three lenses 10, 11 and 12, designed to receive an infra-red image of the whole of the visible sky area and to project that image downwardly towards an infra-red ferro-electric sensor 13. The input lens 10 is cleaned intermittently by a computer-controlled wiper 14.

A sample of the received infra-red image is selected via optical filters contained in a filter wheel 15 and is passed, via a rotating chopper 16 which acts as the temperature standard, to the infra-red sensor 13. In this illustrated example, the image striking the sensor 13 is circular.

9 Images of the cloud and of the chopper incident upon the infra-red sensor 13 are compared, by means of an electronic unit 17 and software associated with a computer 18, to produce a temperature image. The temperature image is in turn interpreted, by software associated with the computer 18, to produce an analysis of the distribution, temperature and general type of cloud observed by the lenses 10, 11 and 12. In this illustrated example, by comparison of the foregoing data with data relating to cloud-base height provided by a ceilometer, an improved indication of the height of the observed clouds may be provided across almost the entire sky, thereby assisting further the use of the observed data for weather-forecasting purposes.

The foregoing components, with the exception of the computer 18, are enclosed within a weatherproof housing 19. The interior of the housing, and the cloud imaging components within it, are maintained at a stable temperature by means of cooling and heating devices 20 and 21.

Claims

1. A method of monitoring cloud cover distribution, which method comprises forming an image of the cloud by monitoring the reception of infra-red radiation over at least a major part of the relevant hemisphere, interpreting the resulting image as an indication of cloud density and distribution, and repeating the exercise at ftequent intervals.

2. A method as claimed in Claim 1, wherein said frequent intervals amount to one or more minutes or less.

3. A method as claimed in either of the preceding claims, wherein said infrared image is subsequently converted to a corresponding temperature image by calibration against a temperature standard, whereby to obtain an enhanced indication of the cloud density and distribution.

4. A method as claimed in Claim 3, further comprising determining the height of the cloud above ground level by assessing a profile of the air temperature below the observable cloud deck versus height.

5. A method as claimed in Claim 3, further comprising determining the height of the cloud above ground level by combining said temperature image with data obtained by point measurements of cloud height.

6. A method as claimed in any of the preceding claims, wherein said cloud distribution determinations are repeated at measured time intervals to enable the determination of the velocity of each cloud object in the sky.

11

7. Apparatus for monitoring cloud cover distribution, which apparatus comprises a wide-angle infra-red lens, and means to interpret the image provided by said infra-red lens as an indication of the density and distribution of cloud observed via said infira-red lens.

8. Apparatus as claimed in Claim 7, further comprising calibration target means to provide a temperature standard, sensor means to compare the image provided by said infra-red lens with the temperature standard to produce a temperature image, and means to interpret that temperature image.

9. Apparatus as claimed in Claim 7 or Claim 8, comprising two or more of such apparatus, spaced apart by a distance, to obtain two distinct stereo images to enable determination of the cloud height.

10. Apparatus as claimed in any of Claims 7 to 9, wherein said wide-angle infrared lens is constructed of one or more elements made from opticalgrade germanium.

11. Apparatus as claimed in any of Claims 7 to 9, wherein said wide-angle infra-red lens is constructed of one or more elements made from zinc sulphide or zinc selenide.

12. Apparatus as claimed in any of Claims 7 to 11, comprising also a filter wheel interposed between said infra-red lens and a sensor for said lens.

13.Apparatus for monitoring cloud cover distribution, said apparatus being substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.