Caleb Miller – firstname.lastname@example.org
In the Reading area, December and January seem to be prime fog season. Since I’m studying the effects of fog on atmospheric electricity, that means that winter is data collection season! However, in order to begin collecting data in the first year of my PhD, there was only a short amount of time to prepare an instrument and deploy it to the observatory before Christmas.
One of the instruments that I am using to measure fog is called the Optical Cloud Sensor (OCS). It was designed by Giles Harrison and Keri Nicoll, and it is described in more detail in this paper: (Harrison and Nicoll 2014). The OCS has four channels of LEDs which shine light into the surrounding air. When fog is present, the fog droplets scatter light back to the instrument, where the intensity from each channel can be measured.
Powering the instrument
The OCS was originally designed to be flown on a weather balloon, which meant that it was meant to be powered by battery and run for only short periods of time. In my case, however, I wanted the device to be able to continuously collect data over a period of weeks or months without interruption. Then, we would be able to catch any fog events, even if they hadn’t been forecasted. That meant the device would need to be powered by the +15V power supply available at the observatory, and my first step was to create a power adapter for the OCS so that this would be possible.
Initially, I had been considering using an Arduino microcontroller as a datalogger, so I decided to put together a power adapter on an Arduino shield (a small electronic platform) for maximum convenience. I included multiple voltage levels on my power adapter and connected them to different power inputs on the OCS. Once this was completed, the entire system could now be powered with a single power supply that was available at the observatory!
I was able to find all of the required parts for the power supply in stock in the laboratory in the Meteorology Department, and I soldered it together in a few days. The technical staff of the university were very helpful in this process! A photograph of the power adapter connected to an Arduino is shown in Figure 1.
Figure 1. The power adapter for the optical cloud sensor, built on an Arduino shield
Storing data from the instrument
Once the power supply had been created, the next step was setting up a datalogging system. On a balloon, the data would be streamed in real-time down to a ground station by radio link. But when this system was deployed to the ground, that would no longer be necessary.
Instead, I decided to use a CR1000X datalogger from Campbell Scientific. This system has a number of voltage inputs which can be programmed using a graphical interface over a USB connection, and it has a port for an SD card. I programmed the datalogger to sample each of the four analog channels coming from the OCS every five seconds and to store the measurements on an SD card. Collecting the measurements was then as simple as removing the SD card from the datalogger and copying the data to my laptop. This could be done without interrupting the datalogger, as it has its own internal storage, and it would continue measuring while the SD card was removed.
I had also considered simultaneously logging a digital form of the measurements to an Arduino in addition to the analog measurements made by the datalogger. This would give us two redundant logging systems which would decrease the chances of losing valuable information in the event of an instrument malfunction. However, due to a shortage of time and a technical issue with the instrument’s digital channels, I was unable to prepare the Arduino logger by the time we were ready to deploy the OCS, so we used only the analog datalogger.
Figure 2. The OCS with its new power supply being tested in the laboratory
Deploying the instrument
Once the power supply and datalogger were completed, the instrument was ready to be deployed! It was a fairly simple process to get approval to put the instrument in the observatory; then I met with Ian Read to find a suitable location to set up the OCS. There were several posts in the observatory which were free, and I chose one which was close to the temperature and humidity sensors in the hopes that the conditions would be fairly similar in those locations. Once everything was ready, the technicians and I took the OCS and datalogger and set it up in the field site. At first, when we powered it on, nothing happened. Apparently, one of the solder joints on my power adapter had been damaged when I carried it across campus. However, I resoldered that connection with advice from the university technical staff, and it worked beautifully!
Figure 3. The datalogger inside its enclosure in the observatory
Figure 4. The OCS attached to its post in the observatory
Except for a short period of maintenance in January, the OCS has been running continuously from December until May, and it has already captured quite a few fog events! With the data from the OCS, I now have an additional resource to use in analyzing fog. The levels of light backscattered from the four channels of the instrument provide interesting information, which I am combining with electrical and visibility measurements to analyze the microphysical properties of fog development.
Hopefully, over the next year, we will be able to measure many more fog events with this instrument that will help us to better understand fog!
Harrison, R. G., and K. A. Nicoll, 2014: Note: Active optical detection of cloud from a balloon platform. Rev. Sci. Instrum., 85, 066104, https://doi.org/10.1063/1.4882318.