In addition to the air monitoring equipment, a meteorological MET station will be installed at the refinery. The station will provide wind speed, wind direction, temperature, relative humidity and rainfall measurements. In either open path or point detection, using the data in combination with the metrological data from a MET station located on site is helpful in determining where sources originate from and in which direction the gases are moving. The basic difference is demonstrated in Figure 1.1. In this figure, a gas plume is generated from a source and released into the air. As the gases are released, they are carried by the wind and begin to disperse and move away from the source. As seen in the figure, the concentration of gases in the plume will be highest in the center of the plume and closest to the source, while gas concentrations will be less at the edge of the plume as the distance from the source increases. The shape of the plume can be affected by changing terrain and meteorological conditions, but in general, this dispersion pattern is an accurate depiction of what happens after a gas release.