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Early Autumn Fog Season


The early autumn fog season is here and the next eight months, from the fall through early spring months, will feature several reasons for fog to form more often.  Fewer hours of daylight, damp or snow-covered ground, and periods of light wind all contribute to fog development.


Sunrise on Saturday Morning

At sunrise on Saturday morning of Labor Day weekend I began a three-hour drive to northwest Iowa where I would attend one of my favorite music festivals.   My only weather concern (fairly common at the end of summer) was the potential for dense morning fog following some late day rain showers twelve hours prior.  With damp ground and light wind the stage was set for fog development.   I knew that good visibility at sunrise might not last as late summer fog can continue to develop for an hour or two after sunrise.   Happily, along my driving route only light fog formed.  The ingredient keeping the fog from thickening was a low overcast.  That layer of low clouds prevented the radiational process which often makes for thicker fog and lower visibility.


Maturing Crops Are Moisture Laden

in late summer and early fall, the maturing crops covering much of the landscape are moisture laden.  This allows ground fog to form on clear and calm nights.  Since the amount and type of vegetation — and even air temperatures — vary a lot over short distances, the visibility may vary wildly over just a few hundred feet.  This can be quite hazardous for travelers, at a time of year when tall crops limit visibility at many intersections.  In addition to reducing visibility, this fog leaves condensation on windshields making it even harder to see.  Using the car defroster with the temperature set to warm may be necessary!  Because fog can be difficult to forecast, and because its density can vary so much over a short distance, your local forecast will not usually contain an advisory for fog unless it becomes widespread.


Mid-Winter Fog Forms at Night

In mid-winter, fog forms at night following the partial daytime thawing of snow cover.  This fog may even persist during daylight hours if a mild and high-moisture air mass (usually from the south) moves over the snow cover.  This is also a weather pattern which can allow roadway frost to form as noted in my blog post from January 21, 2017. Fog forming via this process can be of long duration due to the low (weak) winter sun angle and the limited hours of daylight, especially if there is no source of drier air at the low levels.


Spring Fog will Form

In both winter and spring, fog will form north of a warm front when a humid air mass overruns the colder air mass at the surface and still-chilly ground.   This process can operate during the day as well as at night.   If it is below the freezing mark, icy drizzle or even direct condensation of frost onto colder surfaces will test windshield wipers and defrosters.  Since spring rains also develops along warm fronts, heavier showers sometimes allow a widespread fog to briefly lift – until the rain stops and visibility lowers once again.

During my driving experience described above I was grateful for the cloud layer above that kept away all but the smallest amount of fog below.  If you try to think about the various processes working to make our weather, you can be more prepared for the visibility challenges of fog at this and other times of the year.

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