The industrial world is currently considering the adoption of Wireless Sensor Networks (WSN) for industrial process and control applications. However, when using wireless sensor networks in such application scenarios, network performance, reliability, and robustness must be ensured with high degrees of precision. In this paper we study an outdoor deployment of a wireless sensor network in an oil refinery. We investigate the influence of temperature variations and mandatory ATEX casing on data delivery performance. Our experimental evaluation shows the impact that high temperature variations, such as the ones expected at the deployment location, have on network performance. We further show how the design and the deployment of wireless sensor networks in such application scenarios is influenced by temperature, and how less energy is needed to ensure reliable communications during night.