An open-access cloud-detection network

Introduction

Living in the North-West of England I find that my attempts to view the aurora or astronomical events such as meteor showers, lunar eclipses and comets are often prevented by cloud. On these occasions I wonder if it is worthwhile driving somewhere to find some clear sky. But where? If only I could look at a map to see where it was clear now. Publicly-available satellite images don't appear to have the resolution needed for this purpose. Maybe I should go to bed instead and wait for an alert when the skies have cleared? Could the question of where and when to find clear skies be answered by an open-access cloud detection network operated by citizen-science volunteers?

Requirements

I've together a draft list of requirements to construct a useful network:

• Open-access. While my primary concern is with viewing the aurora I can see there are many other uses of such a network. Astronomy and obviously meteorology. Open-access enables other observers to make use of the data for their own purposes.
• Near real-time. The weather here can quickly change. Ideally measurements should be taken every few minutes and be made available without unnecessary delay.
• Use open-source hardware (OSHW). More specifically, an OSHW design should exist which allows users to contribute readings with the lowest possible cost.
• Large geographical coverage. 
• Night time operation required, day time beneficial.

The most difficult requirement to satisfy is the coverage. With ground-based sensors the viewing area might be as small as 2km diameter (cloud-base 1000m, optical viewing angle ±45°); at this size complete coverage of Great Britain would require over 57000 installations. A useful network does not require complete coverage, for auroral and astronomical purposes good results could be achieved by positioning sensors in a few viewing locations selected for low light pollution.

Remotely-sensing clouds

Perhaps the simplest method to remotely-sense cloud cover is to measure the 'sky' temperature with a non-contact infra-red (IR) thermometer. In the absence of cloud cover this is very low (say -12 to -20°C for Spring in the UK) but with cloud cover the temperature measured is warmer (0 to 4°C). These temperatures vary over the course of the year but by looking at the difference between the sky and ambient ground-level temperatures an estimate of cloud cover is possible. Some of these thermopile sensors have a digital output for interfacing directly with a microcontroller.

A camera and image-recognition software is another approach but it is more complex to develop, install and operate. It is not clear how such a system would perform at night. In order to install the maximum number of sensors I plan to concentrate on cheaper methods.

Design concept

I believe there are two key factors which limit the number of sensors in the network: cost and ease of installation. To minimise the infrastructure I plan to begin with a battery-powered unit which communicates with a Raspberry Pi (or other computer) via a low-cost, licence-free radio module. This is the same concept used by the AuroraWatchNet magnetometer sensors.

Potential problems

Precipitation (rain, snow, hail) and dew on the sensor will prevent accurate sky temperature measurements, most likely indicating cloud when it may in fact be clear. A heater to prevent dew forming is an option but the temperature gradients may cause the thermopile to give incorrect measurements. The power consumption needed is incompatible with battery operation. It may be possible to point the sensor downwards, to avoid precipitation and dew, and use a reflective metal surface to observe the sky. Tests shows the sky temperature readings are partially compromised by this approach.

Sensor

I have found a number of sensors which may be suitable:

• Melexis MLX90614. This sensor contains thermopiles for sensing both the object and ambient temperature. An SMBus interface enables direct connection to a microcontroller. It is packaged in a TO-39 metal can. The standard 3.3V operation, 90° field of view (FOV) variant is readily available in single quantities from UK hobby electronics stores for around £15 each. Other variants exist for 5V operation and with different fields of view (10°, 35°, and a dual 70° FOV). The minimum object temperature is -70°C. The only irritation is that the sleep mode requirements are not compatible with low-power operation on a I2C bus.
• Melexis MLX90615. Similar to the MLX90614 but the sleep mode requirements are compatible with low-power operation on an I2C bus. Cost is similar to MLX90614 but no UK supplier exists. Package is a TO-46 metal can. Limited to 3.3V operation and a single FOV (80° or 100° variants). The minimum object temperature is -40°C.
• Texas Instruments TMP006. Small thermopile with I2C interface for direct connection to a microcontroller. Ball-grid array package. Readily available from UK suppliers. Cost is around £3 each for single quantities.
• Thermometrics ZTP-135SR. Thermopile with analogue output in a TO-46 package, with a thermistor for ambient temperature measurement. Being an analogue output additional processing is required. Around £10 from Farnell.
• Devantech TPA81. Linear array of 8 thermopiles. Cost is around £65.
• Panasonic AMG88 Grid-EYE. An 8×8 imaging sensor in a surface-mount package. The minimum object temperature is -40°C. Available from Digikey for around $40.

For the prototype system I plan to use the Melexis MLX90614 sensor. It should be possible to work around the problems with low-power operation by using software I2C and powering the sensor from a logic output, allowing power to be completely removed when not in use.

I excluded the TMP006 because the BGA package is hard to work with; not only is it difficult to solder it will require some kind of enclosure or window to protect it from moisture. Only a few materials are transparent to long-wave infra-red emissions and the better ones are toxic, water-soluble or expensive. The other sensors may be suitable but are more expensive. The TPA-81 linear array and Grid-EYE imaging sensor could provide spatial discrimination and thus distinguish between a thin layer of complete cloud cover and denser clouds with open areas of sky.

Initial results

Initial tests with the Melexis MLX90614 sensor confirm the concept is viable. I have not noticed any ill-effects from the sun within the field of view, suggesting that daytime operation is possible. Further work is required to package the system into a usable prototype which can be deployed outside permanently. Full-time operation will indicate the frequency of measurement errors from dew and precipitation.

Other work

It would be wrong to suggest I am the first to make measurements of cloud cover in this way, useful links are listed below. I am not aware of any existing cloud detection networks.

http://bildr.org/2011/02/mlx90614-arduino/

Useful code for interfacing the MLX90614 IR thermometer with an Arduino. This code works with my Calunium Arduino clone.

http://www.kwos.org/clouds_detector.htm

Good background information.

http://kcotar.org/arduino-weather-station-1/
A weather station which uses the Melexis MLX90614 sensor for cloud detection.

http://www.noao.edu/staff/gillespie/projects/cloud-detector.html
Similar in concept but using a Peltier device (in reverse) to measure the difference between ambient ground temperature and sky temperature.