If did not take much rain on Saturday evening to turn Iowa's roads to a sheet of ice. The featured time series is from the RWIS site on US Highway 20 near Waterloo. The green line (dew point) tells the story of what happened to cause an icing event. Moist air was surging into the state on Saturday with air temperatures hovering just below freezing. Since the cold air was shallow and near saturation, rain was able to fall to the ground without much evaporation nor phase change and then immediately freeze as pavement temperatures were below freezing. The timing of this event late in the afternoon followed by a cold front passage both conspired to prevent most any melting over night. Cold temperatures on Sunday kept nearly all of this ice around on untreated surfaces.

Voting:
Good = 17
Bad = 10

Tags:   ice   RWIS  

The featured chart is from the Marshalltown RWIS on Sunday into Monday morning showing air temperature and 3 pavement temperatures. The plot shows pavement temperatures warming nicely during the day with air and dew point temperatures creeping above freezing (implying a lot of liquid water on the road way). By later in the afternoon, colder air moved in and you can see the rapid drop off of temperatures and also notice the little bump in pavement temperatures near freezing when some heat is released by the water freezing around 4 PM. Roads remain slick this Monday morning.

Voting:
Good = 16
Bad = 8

Tags:   RWIS   ice  

The featured graph is from the Ames RWIS site for the past 24 hours. While air temperatures (red) only reached 25 degrees, pavement temperatures reached 45 degrees at a rather rapid rate. The pavement then cooled off to the air temperature late in the evening with the loss of solar heating. Then around 2 AM, there is a change in the cooling rate. This is a result of the sky clearing allowing rapid cooling. You can generate plots like these here.

Voting:
Good = 20
Bad = 6

Tags:   RWIS  

A surge of very moist air made it into the state on Sunday with dew points (green line) rising into the mid 50s in Ames (RWIS site shown). If you get up before noon on Sunday, you should have noticed most surfaces being wet. The reason was that the dew point temperature was warmer than surfaces such as pavement, so air right next to the surface will cool to its dew point and water will be condensed out. The featured plot shows this as the 3 pavement temperatures become nearly equal during the morning hours before temperatures warm well above the dew point. You can also see the effects of the cold front that sweep out the moisture late Sunday night.

Voting:
Good = 17
Bad = 7

Tags:   rwis