Warmer and muggier weather is back in Iowa with dewpoint temperatures in the upper 60s and lower 70s. The increase in moisture means we are done with the overnight lows in the 50s as the dewpoint puts a floor in on how low the temperature can drop. The reason being that once the air reaches saturation, fog will form and temperatures will be stabilized as long wave radiation is absorbed and heat is released as water vapor condenses. The featured chart looks at the relationship between the 4 PM dewpoint temperature and then the morning low temperature for the next day. The top chart shows the percentage of days the low temperature is within 3 degrees of the 4 PM dewpoint. The middle chart shows the frequency of the low temperature being colder than the 4 PM dewpoint. The bottom chart shows the overall bias of the dewpoint and the next day low temperature. The reasons that this relationship does not always work include: winds can create mixing that prevents temperatures from cooling to the dewpoint, clouds prevent heat from escaping, and/or advection processes can transport more humid or drier air masses changing the dew point overnight.

Same plot for Ames and Cedar Rapids

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Tags:   dewpoint   forecasting