Past IEM Features tagged: densefog
Record Dense Fog Advisories
26 Jan 2024 05:30 AMThis week has seen plenty of dense fog over Iowa as a shallow layer of cooler and humid air near the surface has been difficult to mix out. A significant portion of the country has experienced dense fog this week with the National Weather Service issuing many Dense Fog Advisories. The IEM attempts to curate an archive of the NWS alerts and provides a download interface for the data. For Dense Fog Advisories, the IEM archive dates back to 1 October 2005 (there's mechanistic reasons for that date). A query of this archive indicates that at one point Thursday morning, 1,517 NWS Forecast Zones were simultaneously under a Dense Fog Advisory, which far exceeds the previous maximum value set the prior day and the day before that. You may be wondering about the difference between "Forecast Zones" vs "Counties" as the NWS issues some alerts for zones and others for counties. In Iowa and other states without much topography, these geographic areas generally overlap with identical names, but the NWS as a whole issues some alerts for zones and others for counties NWS-wide.
Voting:
Good: 16
Bad: 1
Tags: densefog
Dense Fog Advisory Duration
05 Oct 2021 05:31 AMThe National Weather Service has a Dense Fog Advisory active for portions of north central Iowa this morning, which includes Cerro Gordo County (Mason City). The featured plot presents the distribution of advisory duration for this event type and county location. The orange line presents the duration of the advisory at issuance and the blue line represents the final duration after any extensions and/or early cancellations. Sometimes the initial advisory is expanded in time, so why the blue line is sometimes higher than the orange line. Anyway, the present advisory extends seven hours, which is right at the 50% level for frequency, which implies approximately the median value. These events tend to be limited to peak overnight cooling hours as the air temperature cools to the dew point and then "burn off" with the rising of the sun causing an increase in temperature away from the dew point.
Voting:
Good: 14
Bad: 0
Tags: densefog