Past IEM Features tagged: fog
A Dense Fog Advisory is in effect for the entire state this Friday morning. The featured chart presents some frequency metrics for how often this weather alert is issued for somewhere in the state of Iowa. The top panel shows the weekly frequency of having at least one event for that week per year. The bottom panel shows the overall total number of events. The winter months are certainly the most common time of year for this product with summer having the least. Fog is easier to maintain during the winter thanks to moist air masses over top a colder ground surface which helps to keep air temperatures near water vapor saturation.
Seeing fog this time of the year during the early morning hours is certainly not uncommon. The featured chart looks at the frequency of fog reports by the automated weather station in Des Moines. The chart is a 2-D histogram showing the observation frequencies by hour of the day and week of the year. The highest frequencies are in the early morning hours, when the best conditions for fog exist (longest duration of overnight cooling, highest humidities, calmest winds and least vertical mixing). The summer months show a clear minimum in the mid afternoon and overnight periods. These are periods influenced by heating, which lowers humidities and well mixes the lower atmosphere.
The past two mornings have seen Dense Fog Advisories issued by the National Weather Service. How common is this advisory issued? Are there favored places in the state for these conditions to warrant issuance of the product? The featured map attempts to answer both of those questions by displaying the average number of dense fog advisories issued each year. A clear east to west gradient is shown with averages ranging from 5 to 10 per year.
After the rain yesterday, skies cleared off and temperatures cooled creating dense fog this morning. The National Weather Service has much of the state under a Dense Fog Advisory this morning. The featured map displays the yearly average number of Dense Fog Advisories issued by forecast zone. The lines on the map are the edges of NWS forecast office zones. For Iowa, most of the state is in the 7-10 advisories per year range. The highest frequencies are near the Gulf of Mexico as the warm and humid air comes onshore and cools, creating fog. There is an interesting white patch over West Virginia. This is the Charleston, WV forecast office, which according to IEM databases has not issued a Dense Fog Advisory since at least 1 Jan 2006.
Tags: nws fog
Fog was common yesterday over Iowa and continues this morning as a major winter storm organizes. The featured chart presents the frequency of having the maximum afternoon temperature above the average daily high temperature when fog was reported at the 6 AM observation that day for Des Moines. The general signal is for fog in the winter time to be associated with warmer than average afternoon temperatures. Correlation is not causation and foggy conditions in the winter time are typically a sign of being on the warm side of a passing storm system. The cold side of the storm system will arrive today and take the fog with it. So enjoy the fog and slightly warmer temperatures while they last!
Dense fog has developed most every day this week as warmer air from the south keeps encountering all of this cold ice and snowpack covering Iowa. On Wednesday, the webcams caught some neat lapses of the fog lifting and then settling in again. The Nevada webcam lapse is from 150 feet above the ground, so it was above the shallow layer of fog that crept along Wednesday morning. The Newton webcam lapse shows thick forming, lifting, settling back in, and then lifting once more. We will see more dense fog today in some locations.
Tags: fog webcam