I ran across a fascinating set of maps on the average date of greenup on agricultural lands today. This set of maps is provided by ForWarn, which is the satellite-based tracking service of the US Forest Service. You can find them at http://forwarn.forestthreats.org/highlights/918#. One of the most interesting things to me is the extent to which cities affect the greenup dates. Cities are generally warmer than the surrounding countryside due to the “urban heat island”, which is caused by high concentrations of pavement and impermeable surfaces that help trap heat in urban areas. Cities like Chicago are most visible in this map, but urban heat islands can be seen even in medium-sized cities. The difference in greenup dates is due in part to climate (latitude, elevation, nearness to water bodies) and in part to what crops were planted in each area.