A somewhat required step one for getting any significant amounts of precipitation is to have air that is humid enough to allow condensed water aloft to reach the ground without evaporating. We have had a number of events this fall season struggle to produce with ample dry air in place over Iowa. Such will be the case for much of today with an approaching storm system needing to overcome dry air, but rain is expected overnight Friday into Saturday although only meager totals will fall. An illustration of our recent issues with dry air can be found with the featured chart. It presents the daily climatology and 2023 values of vapor pressure deficit. This is a measure of the demand for water from the atmosphere. A higher value indicates a drier air and thus higher demand for water. A value of zero would indicate saturated air unable to hold any more water vapor. So the bottom panel shows the daily departures and you can see they are almost all red (increased atmospheric demand for water) since the first of November.

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Tags:   vpd