Despite their clear danger to humans, snow avalanches are hard to document. They occur in inaccessible and dangerous locations, often at times of bad weather. Observation instruments frequently malfunction in the harsh conditions or are destroyed. Measurements of powder snow avalanches are particularly difficult, as these occur less frequently and are usually very large. To understand the air flow in front of and inside powder snow avalanches, we have designed an air pressure sensor to survive the harsh conditions. It consists of a differential pressure transducer, with a high-frequency response, built into a specially designed housing unit. We mounted this 10 m from the ground on a measurement mast in the Vall¿e de la Sionne avalanche test site in Switzerland. Data from five powder snow avalanches over the winter of 2004 were recorded. Three of these were natural releases, and two were artificially triggered with explosives. We present an analysis of the sensor response and an interpretation of the signals in terms of simple flow fields. We show how these data can be used to deduce information about the speed, size, and location of the avalanches using a dipole approximation. Our sensor has two major limitations: The length of the internal tubing produces low-frequency resonances, and there is only one transducer, so a complete flow model is needed to deduce the three velocity components and pressure. We discuss these limitations and give a design for a new sensor to overcome them.