Changing Weather Patterns

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The weather pattern has been quite strange for a while. We had a very warm winter last year (2022-2023), it was followed by a near drought condition in the usually rainy May, and the winter of 2023-2024 has also been unusually warm. Often, resources like the Farmer’s Almanac list April 17th as the last frost date (https://www.almanac.com/gardening/frostdates/zipcode/60638)

So, I wanted to look at the data myself. The Almanac uses the same weather data from NOAA, so I wouldn’t expect the takeaway to change much.

I took the data of Midway Airport from 1928 to 2023 as they had a dataset that spans the longest time in the Chicago area. O’Hare Airport is closer to my home, but their data range is limited because O’Hare was built much later.

Looking at the days when the temperature was above frost (>32), light frost (>28), and hard frost (< 28), you can see the danger zone of hard frost is less likely in the 2nd week of April. I think light frost is fine for many plants such as peas, onions. As long as you provide frost cover, many leafy plants such as Kale can survive 28 degrees without issues.

Now, there’s a reason why many sites talk about the last frost day. Because a single frost is enough to sometimes kill plants. Then a more important metric is the last day in April when the temperature was below 32.

It appears that the danger of frost is much smaller after April 17th or so, and you can see a clear trend of the last frost finishing earlier. Some fragile plants such as Basil, Tomatoes, and Potatoes, can wait a bit longer - maybe until early May, but many plants can be out in mid-April.