Season of Fog...Temp Ups & Downs

What a difference a week...let alone a few days made in North Central Wisconsin!  Back on February 10th, some of the coldest weather of the winter season had invaded the region, pushing lows into the -10s to -30s.  Then on Sunday, February 13th a change in the weather pattern began, including a flow of southerly winds into the Badger State.  This lead to record highs in many locales across our area in the upper 40s to around 50.  Just a few days later on Wednesday, February 16th highs soared across the Northwoods, well into the 50s to near 60 degrees.

The one location we take particular note of is Land O' Lakes.  On Feb. 10th the morning low was a frigid -31 degrees.  Less than a week later, the high rebounded to a balmy 59 degrees.  That's a 90 degree swing!  Now I should note that we have had some dramatic drops and rises in temperatures in a 24 hour period in Wausau of 40-50 degrees, which is an even bigger shock to the system.  To spread out the time frame to about a week is a little more tolerable, of course particularly if it gets warmer down the road.

Meantime, before the arrival of milder air in the region, the snow depth in North Central Wisconsin ranged from 10-20".  However with above freezing temperatures, the snow has melted, leading to another common weather phenomenon, fog.  Fog forms when the air temperature reaches that of the dew point temperature (the dew point being when dew or condensation forms).  A lot of times this equals frost on your car windows or dew on the grass when there isn't snow already there.  Of course when that condensation happens over the lower 1,000 feet of the atmosphere it leads to the developing of low stratus clouds, aka fog.  Here is a simple series of graphics to explain this type of fog development, known as radiational fog.

As was the case in the past few days, sunshine and a mild air mass led to the melting of the snow, causing some of it to absorb into the ground or flow into storm drains.   Meantime, the moisture also evaporated into the air, causing the dew point temperatures to rise.

As the sun sets, we lose the main source that causes temperatures to rise (for the most part).  With temperatures falling and the moisture still lingering in the air, the temperature can likely fall to the dew point.

The end result, as we experienced on Feb 16th & 17th is the development of locally dense fog in the area.

The other type of fog which we have also been witnessing is advection fog.  This fog occurs when warmer air flows in over the colder, snow covered ground.  That contrast in temperature also leads to fog, which sometimes can look like it is rolling along, since it is primarily caused by the wind. The gloomy conditions that we had on Feb. 17th was a combination of both in the region.  The end result was the same, very low visibilities. If you don't believe me, here is a visibility map from Thursday, February 17th at 8am.  Most locations in the viewing area were at a quarter mile or less.

Of course a strong cold front, like the one that was forecast to roll through on Friday Feb. 18th not only kicked up the winds to scour out the fog, but also brought in a drier & cooler air mass.  However as we move closer to spring, when we do get those days well above freezing, the odds are more favorable for fog to develop overnight and linger into the morning hours.  This is most likely so long as there is still a snow pack left on the ground.

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