A measure, with dimension [L2 T] and units such as km2 d, of the spatiotemporal extent of surface melting. The melting index, usually obtained by remote sensing, is the integral over a defined region and time span of the time-varying melt extent, and is approximated in practice as a regional sum of products at local scale (such as that of the pixels of a passive-microwave sensor) of the melt extent and the duration of melting. The accuracy of the duration is principally determined by the frequency of imaging, which tends to be high at high latitudes because most orbital sensors are in polar orbits. The melting index is a valuable proxy indicator in the absence of more direct measures of melting. The melting index is sometimes called the melt index or the surface-melt index, and is formulated in slightly different ways by different authors.