With a low temperature of 70°F on Tuesday, the Des Moines airport weather station was the warmest within Iowa among airport based weather stations. This situation happens frequently for the site due to the urban heat island effect being the strongest for the most urban airport weather sensor within Iowa. The featured chart illustrates this by computing the average low temperature percentile for Des Moines vs the rest of the long term climate stations within Iowa. Please note that there was no spatial weighting done, it is just a simple comparison. The bottom panel shows the bias between the Des Moines average value vs the rest of the state. The increasing impact of the heat island is readily apparent, but you may wonder what is happening during the early period prior to ~1940. This was approximately when the station moved from downtown to the present location, which was more rural at the time. Additionally, observation techniques changed with most of the rest of the state reporting 24 hour values during the afternoon hours, which tends to bias low temperatures warmer.