The featured graph is a time series of recent temperature and dew point observations from the Des Moines airport. This plot clearly illustrates the importance of the dew point temperature as a floor to overnight low temperatures. Keep in mind that the dew point is not a function of dry air temperature. The upward pointing arrow shows how on Monday morning, the air temperature only was able to cool to a few degrees above the dew point near 70. Since then, a front passed with much drier air (dew points dropped ~15 degrees) and you can see what temperatures have done since yesterday evening. The floor has been lowered and with clear skies, air temperatures fell like a rock into the 50s.

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