Isochrone maps are an established method to depict areas of equal travel time, and have been used in transportation planning since the early 20th century. In recent years, interactive isochrone maps allowed users to select areas of interest, or explore temporal mobility patterns for different modes of transport. However, conventional isochrone maps depict one traffic situation at a time. Our visualization approach unifies isochrone maps with time-varying travel data, and instead of showing multiple isolines for different travel times, we show multiple isolines for different times of day in order to reveal time-dependent spatial travel variance. In this paper, we present Isoscope, a web application that provides an interactive map for casual exploration of urban mobility patterns. Through its aesthetic visual form and its simple interface we strive to support people casually investigating travel time in their own city. We will describe our design goals, elaborate on the design and implementation of our prototype, and discuss limitations and future extensions of the system.