When will we be free of frost?

May 17, 2016: We are getting close to the time of year when frost is no longer an issue, but we aren’t quite there yet.  Patchy frost may pop up again across the Stateline tonight.

Billboard 3

You may be asking yourself: “Wait a minute. How can we see frost without getting down to freezing?”  Here’s how:

Frost While Above Freezing

As the ground cools, the heat that leaves the ground floats upward to where thermometers usually are hanging. Since this is around 5 feet off of the ground, our air temperature readings measure a few degrees warmer than the ground temperature (and we have this setup because we care more about how warm the air is at our torso level than right at the ground.  How often are you lying right on the ground, that you would need the ground temperature?).

Those few degrees make a difference if we are close to freezing, resulting in frost even though the air temperature is still closer to 40°.

When will we be done with these chilly nights? Very soon! The forecast for the rest of the week has temperatures climbing into the 70’s and 80’s for highs, so our lows will climb into the 50’s. I also did a little research, just to see when our last night with 40° or colder is for the season, on average.  I found, in a typical year, that after May 21st, we usually are free of the frosty nights (we stay above 40° for the low temperature). We’ll pass that date over the weekend.

This summer should be above average for temperature, so the worst-case scenario shouldn’t happen, but just for fun, guess when the latest date we’ve ever had in Rockford with lows still below 40°? June 21, 1992! That is right around the start of summer! Can you imagine still having frost possible until the 1st day of the summer season?  We shouldn’t (hopefully) have to worry about that this year!

-Alex

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Posted under weather

This post was written by Alex Kirchner on May 17, 2016

1 Comment so far

  1. google street view October 25, 2018 1:47 AM

    Highs on Sunday and Monday barely squeaked above zero, putting those dates in the top 5 coldest in the last 5 years.

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