Frost Dates: First and last frost dates for Mason City Muni Ap, IA

How to read these charts for the fall
As your growing season comes to an end, the nightly temperatures for Mason City Muni Ap, IA start to go down, and therefore every day that goes by increases the chance that you'll get frost. Your risk of frost really begins around September 20, and by October 8 you're almost certain to have received at least one frost event.

The charts on this page show the probabilities of receiving a certain temperature on a certain day. Some examples that might help:

  1. You have a small 20% chance of getting 32° by September 23.
  2. There is a 50% chance of being hit by a 32° frost starting around September 30
  3. You have a 80% chance of seeing 32° by October 8
  4. Said another way, you have a 1 in 5 chance at making it to that day without a 32° night.
In the Fall
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
First 16° Oct 27 Nov 1 Nov 5 Nov 9 Nov 12 Nov 15 Nov 18 Nov 22 Nov 28
First 20° Oct 17 Oct 23 Oct 26 Oct 30 Nov 2 Nov 5 Nov 8 Nov 12 Nov 17
First 24° Oct 8 Oct 13 Oct 17 Oct 20 Oct 23 Oct 26 Oct 30 Nov 3 Nov 8
First 28° Sep 28 Oct 2 Oct 5 Oct 8 Oct 11 Oct 14 Oct 16 Oct 20 Oct 24
First 32° Sep 20 Sep 23 Sep 26 Sep 28 Sep 30 Oct 3 Oct 5 Oct 8 Oct 11
First 36° Sep 11 Sep 16 Sep 18 Sep 21 Sep 23 Sep 26 Sep 28 Oct 1 Oct 5

How to read these charts for the spring
As the growing season approaches, the days get longer, the weather begins to warm up, and the nights start being less cold. You're still getting frost, but the chance of nightly frosts gets less and less with each passing day. Eventually, the frost stops, and this is when your tender plants are fully safe. So, how do you plan for this? The probabilities on this page help you assess your risk of frost on any given day.

What's a safe temperature for tender plants? When the nightly temperature falls, frost can form, even above 32°, because the air is colder high above the ground and the frost can form up there and then fall down onto your garden and do some damage, even if it's 36° on the ground. So many factors come into play, including wind, concrete, houses, trees and other structures, etc etc etc. Because of all this, you might want to consider 36° as "the danger zone".

In your average springtime, you have a 90% chance that there will be no 36° nights by May 25. In other words, you can pretty much count on being safe from frost by that day. But we want to get those tomatoes in the ground as soon as possible, right? We see that there's still an 80% chance of 36° on May 2, so we don't dare plant that early. We wait a few days and by May 11 we are at the 50/50 point. At this point, we are close and we can start watching the weather forecast. If the upcoming week's forecast doesn't show below 40°, then it's probably okay to risk planting out your plants.

In the Spring
Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90%
Last 16° Apr 6 Apr 1 Mar 28 Mar 25 Mar 22 Mar 18 Mar 15 Mar 11 Mar 6
Last 20° Apr 14 Apr 10 Apr 7 Apr 5 Apr 2 Mar 31 Mar 28 Mar 25 Mar 21
Last 24° Apr 19 Apr 15 Apr 12 Apr 9 Apr 6 Apr 4 Apr 1 Mar 29 Mar 25
Last 28° May 4 Apr 28 Apr 25 Apr 21 Apr 18 Apr 15 Apr 11 Apr 7 Apr 2
Last 32° May 13 May 8 May 5 May 2 Apr 30 Apr 27 Apr 24 Apr 21 Apr 16
Last 36° May 25 May 20 May 17 May 14 May 11 May 8 May 5 May 2 Apr 27

Data is provided by the NOAA National Centers for Environmental Information.