How to Build a Weather Station in the Sahara

An in-depth look at the Fennec Automatic Weather Station (AWS) Network: how researchers built a climate monitoring system in one of the harshest environments on earth

As the world’s largest desert, the Sahara is known for climatic extremes—temperature and aridity are the two most obvious. But it’s also known for airborne dust. A lot of dust. The Sahara is the single greatest annual source of airborne mineral dust in the world, blanketing the Caribbean and North America with haze. 

In addition to dust, the Sahara has a significant influence on the regional climate of North Africa and beyond through something called the Saharan Heat Low. This is the term for a thermal low pressure system that develops over northern Mali, southern Algeria, and eastern Mauritania during summer in the northern hemisphere. The Saharan Heat Low impacts the variability and timing of the West African monsoon and the climatic circulation of a much wider region, including western Europe.

But despite the major impact that the Sahara has on the world’s weather, the how and why of these climate effects have remained mysterious. And this is in large part because the environment is so extreme that very few observations have taken place there. There have been a few operational observing stations on the margins of the desert, including the observing station at Tamanrasset (WMO station 60680). But other than this, there were few reliable observations of the Saharan Heat Low with which to evaluate models.

In addition to observations for the heat low, local observations are needed to identify wind effects and turbulence. These drive dust uplift and play a part in cloud growth, but neither of these important processes can be measured using satellite borne remote sensing. So in 2011-12, researchers from England, Germany, France, Algeria, and Mauritania created a major field campaign to go inside the climate of the Sahara. Named the Fennec project, the team worked to capture the first on the ground observations of the Saharan environment. This includes highly detailed work on the instrumentation and design for an automatic weather station network that could function in this environment. 

Now, the technological challenges for Saharan weather stations are immense. The instruments have to survive extreme heat, airborne sand and dust. Blowing sand is one of the biggest problems since it can cause significant abrasion on the instruments. But as dust builds up on the systems, it can also damage moving parts and cause data loss as it coats solar panels and radiation sensors. The instruments also have to function in very remote areas with no power grid or buildings around them, and be capable of sending data across immense distances.

The Fennec project aimed to study a variety of phenomena in the region, involving a wide range of time scales and requiring a broad choice of sensors. The team needed to gather data on pressure, temperature, humidity, wind, but do so in a way that compensated for damage by the desert, could be easily deployed, and would successfully log data. So the team considered carefully every aspect of their instruments and how to calibrate them for this extreme environment. Read on for technical details on how the team led by Matthew Hobby, from the University of Leeds, designed and built the weather stations. 

Building the Station

The Fennec AWS mechanical structure used a lightweight aluminum three-chord truss, to produce a six and a half foot mast. Instruments were attached by lightweight C-clamps. Since it was not possible to use ground anchors for the AWS because of the sand, a base of three evenly spaced six and a half foot spars was designed. These were attached by steel guys to a point halfway up the vertical truss section. Bottle screws were used to tension the guys. Each spar was weighed down with a sand bag, although the wide base provided by the spars makes it very difficult for the AWS to tip. During nearly 6 months of operation, including a number of haboobs, there was no evidence of an AWS falling over.

Powering the Weather Station

Each AWS was powered from a sealed lead acid battery that was charged using a solar panel. The AWS consumed a total of 150 watt hours per day (this is about a tenth of what an average laptop would use over the same period). Ideally, a 40 watt solar panel would have been used, to provide enough overhead for dusty panels and inaccuracies in solar modeling. However, 40 watt solar panels were extremely limited in availability. So a Kyocera KC50 solar panel was used instead. It had a maximum power output of 54 W and approximate dimensions of 650 mm × 650 mm. 

It was necessary to keep the solar panel size as small as possible to simplify installation in the remote regions of the Sahara and to reduce wind loading. An SEC TLG50 series battery was chosen to go with the solar panel because of its thick plates, which minimize the ageing effects of operation at high temperatures. A smaller battery might have been used, but a low-percentage discharge was required to help prolong the battery life in the extreme temperatures of the Sahara.

Taking Measurements

Temperature and Relative Humidity

The team chose Epluse Elektronik EE08 relative humidity sensors for this project. They are based on a capacitive element with a dielectric that responds to environmental humidity. A Pt100 temperature sensor was collocated within 2 mm. Signal conditioning and temperature compensation were applied inside the sensor probe, producing an analog voltage. The sensor was chosen for its low cost and limited power consumption while still providing a good level of accuracy. In addition to the Pt100 inside the EE08 sensor, an OMEGA TH-44007 thermistor was used with signal conditioning provided by the AWS precision ADC electronics, thus increasing redundancy and reliance on the EE08 probe.

The EE08 and thermistor probes were mounted in a passive screen. Under the extreme heat of the sun, it would have been preferential to use an aspirated screen. However, an aspirated screen operating under dusty conditions was likely to cause early failure in mechanical moving parts. This would result in no airflow and poorer measurement than a passive screen. The team looked into various solutions involving an active pump for low wind speed conditions, but this added complexity and conflicted with the requirement to minimize power consumption.

Wind Measurements

Wind measurements were taken using two different forms of sensing technology: a traditional cup anemometer-wind vane combination and a sonic anemometer.

Both sensors were impacted by dust, which had to be accounted for in the data. The cup anemometer-wind vane combination had moving parts that would be worn out by high dust concentration. The sonic retrievals were subject to distortion or complete failure under high dust concentrations.

The Mierij Meteo MW35 cup anemometer and MW36 wind vane were used since they have sealed bearings, which should limit dust ingress and extend life. Both MW35 and MW36 continuously stream data with approximately 125 samples being collected every minute. The Vaisala WMT52 sonic anemometer was chosen because of its extremely low power consumption of 36 megawatts. The WMT52 uses three transducers that “bounce” signals off a reflective base plate to other transducers. For every sample period, about 400 cup vane and about 200 sonic anemometer measurements are made.

Radiation

The Kipp & Zonen CNR4 was chosen to measure radiation. It integrates a pyranometer pair, (measuring up- and downwelling shortwave (SW) radiation), with a pyrgeometer pair, (measuring up- and downwelling longwave (LW) radiation). The pyranometer has a field of view up to nearly 180°, providing global radiation information. A thermistor is embedded within the pyrgeometer to measure its own temperature and therefore correct the longwave measurements.

Radiometers are usually operated from a manned compound, where they are cleaned on a daily basis. Because of the remote locations of the Fennec AWS, this was not possible. So the CNR4 was chosen because of its integrated centrifugal fan. The fan was provided to blow air across the radiometer domes and to minimize the possibility of dust settling and distorting the measurement. 

Pressure

The Intersema MS5534 was chosen because of its low power consumption and successful application in a number of previous field projects. The sensors integrate a piezoresistive material with the necessary signal conditioning circuitry. It is interrogated over a proprietary “three wire” digital serial interface. The sensor was integrated with the circuitry necessary for it to communicate data.

Logging the Data

To allow AWS data to be used in operational weather models, data had to be transmitted hourly by satellite to Leeds in the United Kingdom. There, the data were cleaned, and the results were sent to the World Meteorological Organization Global Telecommunications System (WMO GTS) and made available to collaborating national meteorological services in North Africa (the Office National de Meteorologie d’Algerie and the Office National de Meteorologie de Mauritanie).

Satellite transmission costs meant that data was sent every 3 minutes and 20 seconds, though the team would have liked to send at a higher frequency. This timing misses some turbulent gusts that may be important for dust uplift. For the sampling of events such as dust-laden gravity currents (known locally as haboobs), more frequent observations on pressure, winds, thermodynamic, and flux were needed. 

The Fennec AWS uses a custom built datalogger. Despite a number of excellent commercial dataloggers being available, the team wanted to construct a system that would consume less power. This also allowed them to get rid of any self-heating effects that might have been caused by high-current electronics or over specified microprocessors. Instead, they selected low-level components that could operate under extended temperature conditions—up to +85°C. Finally, by designing their own software, they could minimize complexity and potential sources of failure.

Success of the AWS Network

The Fennec AWS network took an unprecedented range of observations that were important to further understanding of the Saharan climate. During the project, four stations continuously transmitted data in near-real time, and the temperature data from these stations was successfully assimilated into forecast models by the Met Office.

A further four stations transmitted data for approximately one week during late May and early June 2011. They then failed because of a lack of power as the solar panels provided less output under high temperatures and extreme impacts from the sun. But these stations had new regulators installed during November 2011 to overcome this problem, and once again began transmitting data.

Researchers are still studying the data that came out of this project, and the observations made by the AWS network have provided the first chance to understand the mysteries of climate impacts in and from the Sahara. We look forward to more studies coming out, and future modifications to weather station design for extreme environments. 

A picture of staff from the meteorological service of Algeria downloading data from one of the automatic weather stations at sunset. The photograph is taken at a remote location, typical of most of the stations, deep within the Sahara.

This article contains information from The Fennec Automatic Weather Station (AWS) Network: Monitoring the Saharan Climate System by Matthew Hobby, et al. published in the Journal of Atmospheric and Oceanic Technology 30, 4. Any omissions and errors should be attributed to AMS staff. Copyright belongs to the AMS.