Low temperatures this Wednesday morning dipped into the 30s over parts of the state. The featured map displays the frequency of a long term climate site having the first fall sub freezing temperature (ties are shared). The map displays three preferred locations for the first fall freeze: Northwest Iowa, Northeast Iowa, and Westcentral Iowa in the Nishnabotna Valley (Atlantic is the site at 15).

Voting:
Good = 33
Bad = 3

Tags:   freeze  

The featured chart shows the frequency of low temperature thresholds for the fall period before the first freezing temperature. For instance, on average we would expect to see around 10 days with a low temperature at or below 45 degrees before the first freeze (blue dots). The maximum and minimum values are shown as well along with the current observation this fall. The moral of the story is that we probably should see a few more cool mornings before the first freeze with a day or two in the 30s and then finally that first fall freeze.

Voting:
Good = 25
Bad = 7

Tags:   freeze   climate  

It is commonly said that the first fall season freeze occurs during a full moon in late September or October. The featured plot shows that this simply is not the case in Iowa. The horizontal axis contains the number of days from the nearest full moon and the vertical axis is the number of days that observation was from the 6th of October (the median first freeze date). The contours indicate the relative frequency of the data. The important portion of the plot is the histogram in red at the top of the page clearly showing no full moon date dependence. Here is an Excel file with this data.

Voting:
Good = 20
Bad = 7

Tags:   climate   moon   freeze  

The featured chart presents the frequency of having a daily high temperature above freezing with a low temperature below freezing for Ames. The chart shows two seasonal maximums in late fall and early spring. Our weather this week looks very warm to start with values returning to near average by the end of the week.

Voting:
Good = 17
Bad = 7

Tags:   freeze   thaw  

The featured plot is from the Humbolt SchoolNet for Thursday morning showing the flat lining in temperature around freezing before more cooling occurs just before sunrise. You will often see this as special processes occur when water begins to freeze, which releases heat and therefore slows the cooling process some. Having the dew point near 32 degrees gives an indication that frost formation was a necessary process for the temperature to cool below freezing.

Voting:
Good = 39
Bad = 21

Tags:   freeze  

The featured chart shows the percentage of years since 1893 that a certain temperature threshold was exceeded by a certain date in the fall for the Ames climate site. For instance, the chance (observational frequency) of a sub-33 temperature by this time of year is roughly 50%. This means half of the years prior to this one have seen a sub-33 temperature by October 4th.

Voting:
Good = 42
Bad = 24

Tags:   freeze   climate  

The featured chart is a temperature composite of the time period before and after the first sub 32 degree temperature in the fall from the Des Moines Airport. The three lines represent the temperature range and average of the observations 3 days before thru 1 day after the event. The 12 hours before the first event show a rapid cooling which is often the case with clear skies during the night time hours. Those clear skies are then typically around for the next day when temperatures quickly recover.

Voting:
Good = 23
Bad = 4

Tags:   fall   freeze