Cloud detector: first data

Annotated plot for 2013-05-30.

The cloud detector has been operating outside for a few days now. The plot above is an example of the data I am getting, annotations added manually! The ambient temperature is the temperature of the sensor. In reality this may be a little warmer than ambient when it is exposed to direct sunlight. The sky temperature is the object temperature from the sensor. It is an average over its field of view, and in the absence of clouds it is also averaged in height. From the plot a number of features can be identified:

Clear sky
With no clouds to reflect back thermal radiation the sky temperature for a clear sky is very low, typically < -10°C. This results in a large difference (> 20°C) between ambient and sky temperatures.


Overcast sky
With the presence of clouds most thermal radiation is not lost to space but reflected back to Earth and the sensor. The difference between ambient and sky temperatures is small (< 10°C). With a uniform cloud coverage the variation in the difference is small.


Cloudy sky
A sky with partial cloud cover has an intermediate difference in ambient and sky temperatures. The cumulus clouds resulted in a large variation of the temperature difference, caused by the more sensitive central viewing area observing alternately clear sky and cloud. Complete coverage by a thin layer of cloud is expected to give an intermediate temperature difference but low variability.


Wet sensor
When the sensor is wet with rain the sky temperature is measured incorrectly since the water is not transparent to long-wave infra-red radiation and matches the ambient temperature. As wet weather implies cloud this is not a major problem although I have concerns over the time taken for the sensor to dry off.


Dew
The formation of dew is observed in the evening as the ambient temperature falls. The difference between ambient and sky temperature is reduced to less than 10°C.

Future work

Early operation shows that dew is a problem and can indicate cloud when the sky is actually clear. I have considered creative solutions such as painting the top of the box white and the bottom black in order to minimise heat lost to the sky and maximise heat absorbed from the ground. I expect this will only delay the onset when dew becomes a problem, not eliminate it. The only viable solution is to heat the sensor slightly (~2°C) above ambient, which means moving away from battery-powered operation. If a cable is used for power then wireless communications makes little sense, power-over-ethernet seems the way forward. A heated sensor will also reduce the drying time after rainfall.

I still have some concerns regarding waterproofing around the sensor. I have ordered the MLX90614ESF-DCH-000-TU-ND which features a 12° field of view. This narrower view is obtained by fitting a refractive germanium lens above the sensor. The TO-39 package has been modified to be taller than standard to accommodate the lens. The additional height should mean that the top of the sensor is above the cable gland. Water should not collect so easily and it should be possible to use silicone sealant without accidentally coating the sensor window.

Previously I tried pointing the sensor downwards and using a metal surface as a mirror to view the sky. The experiment was not successful and I think this was in part due to the wide field of  view of the sensor (90°) preventing a clear upward view without also observing the ground or enclosure. It will be worthwhile repeating the experiment with the narrow field of view sensor.

I only have basic plotting routines at present. The goal is to produce a cloud cover index, varying between 0 indicating no cloud and 1 indicating complete cloud cover. The index will be used as the basis for sending alerts.

Cloud detector progress

Microcontroller and radio communications

To minimise the time and effort required to test the cloud detector concept I am following the approach used for the AuroraWatchNet magnetometer system and reusing as much hardware from it as possible. The MLX90614 non-contact infra-red temperature sensor is located outside. To minimise the infrastructure requirements the sensor unit is battery-powered and transmits its data over a bi-directional radio link. The sensor is controlled by one of my Calunium v2 microcontroller development boards. I'm using a pair of RFM12B radio modules operating at 433MHz to emulate a transparent serial connection. One is fitted on  the Calunium board and the other radio module is connected to a Raspberry Pi via my RFM12B shield. The Pi records the data and is responsible for uploading it for general access. I am also reusing most of the AuroraWatchNet firmware, which means I already have signed data communications and the capability to deploy over-the-air firmware updates - quite a good starting position for a new project!

Waterproofing the sensor

The biggest challenge I expect to face with this project is waterproofing the sensor. Although it is hermetically sealed there is no easy way to deploy it outside. As explained in a previous post, only a few materials are transparent to long-wave infra-red emissions and the better ones are toxic, water-soluble or expensive. I am therefore trying to avoid covering the sensor window and instead keep the electrical contacts dry. My first attempt was to drill a hole in the box and attach the sensor with silicone sealant. The mechanical fixing lacked strength and I wasn't convinced I had completely sealed around the sensor. To make matters worse I ended up with sealant on the sensor window which had to be cleaned off before it set. Another approach was required.

The new approach is to fit the sensor inside a cable gland. Before doing so I had to attach wires to the sensor connectors and insulate with heat-shrink sleeving. The result is shown below.

The next step is to fit the sensor in the cable gland before fitting the cable gland to the box.

The other end of the wires can then be soldered. For the prototype the wires are soldered to a small piece of stripboard. The stripboard has two pull-up resistors for the I2C bus and a decoupling capacitor. There is also an IDC connector to link the board to the unused JTAG interface. This was the easiest way to connect to a stock Calunium v2 board. For the lowest power operation the sensor is powered from logic-level output and I am using software I2C so all connections can be grounded when the sensor is not in use. The real-time clock on Calunium is connected the the hardware I2C interface on the microntroller and can be operated independently of the sensor.

The box and cable gland are rated IP65. Hopefully the result will be waterproof  

Results

The sensor is now deployed outside and reporting back to the Raspberry Pi base station. The transmitted packets are logged by the Raspberry Pi but as yet I don't have a convenient way to extract the ambient and object temperatures for analysis. That is the next task.

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.