Note: This article has been modified from an article originally published in VegNet Newsletter (https://u.osu.edu/vegnetnews/) on April 6, 2020.
Now that we are well into spring and turning a corner toward May, warmer temperatures are hopefully on the horizon. As we have already experienced, April can be a fickle month, with both warm spring rains and lingering cold nights that bring frost and occasionally, a late-season snowfall. Cold April weather can delay the warmup of soils (see CFAES Ag Weather System Near-Surface Air and Soil Temperatures/Moisture) jeopardizing early planted corn and wreaking havoc on horticultural interests, especially following early season warmth where phenological conditions may be advanced for this time of year. Winter (December 2019 – February 2020) averaged 2-8°F above average compared to climatological normal (1981-2010; Fig. 1). This warmth continued throughout March as well, with temperatures 4-8°F (west to east) above average.
Frost and Freeze Potential
With many areas in Ohio experiencing hard freeze conditions (air temperatures ≤ 28°F) last week (see How cold is too cold (for winter wheat)?), how unusual is an event like this in Ohio? What are Ohio’s typical expectations regarding freeze conditions in April and May?
On average, locations throughout Ohio experience their last seasonal freeze (temperatures ≤32°F) from mid-April (southern Ohio) through mid-May (northeastern Ohio). Figure 1 shows the climatological date of a late last hard freeze, meaning that only 1 out of every 10 years does a hard freeze occur after this date. Across southern Ohio, a 28°F day occasionally occurs as late as April 20, with much later dates (as late as May 10th) across northeast Ohio.
For a state analysis, we have selected 8 locations from around Ohio to compare typical last seasonal freeze conditions (Figure 2). Figure 3 shows the probability of experiencing a later freeze in spring than indicating by the line graphs. All locations show probability based on the most recent 30-year period (1990-2019) except for 7-Lancaster (1996-2019). For each location, five temperatures are displayed (20°F-purple, 24°F-blue, 28°F-green, 32°F-yellow, and 36°F-red). For the purposes of this article we will focus on 32°F and 28°F. The bottom (x-axis) shows the probability that each of these temperatures will occur after a given date (indicated by the left or y-axis).
For 1-Wauseon, we see that there is a 50% climatological probability of experiencing a 32°F temperature (yellow) after April 27, and this probability decreases to 20% by May 10. The colder, more damaging temperature of 28°F occurs 50% of the time after April 16, with only a 20% chance of seeing 28°F after April 27. For a southern location like 8-Marietta, these dates occur earlier in the season. Here, there is a 50% climatological probability of experiencing a 32°F temperature after April 18 with 28°F occurring 50% of the time after April 2.
Besides latitudinal (north of south) position, what other factors can influence springtime minimum temperatures? Colder air is more dense than warmer air, meaning it wants to remain close to the ground and will flow over the terrain like a fluid to settle in areas of lower elevation. If your location is in a valley or low-lying area, the climatological dates will likely be shifted later to account for more freeze potential later in the spring. Water bodies are typically colder than the surrounding land areas in spring which may keep temperatures in the immediate vicinity a little colder. For 2020, water and soil temperatures are above average, so they are likely to have a moderating impact this year. Cloud cover and higher humidity in the spring will keep air temperatures warmer due to their absorption of terrestrial (from the surface) radiational effects. Finally, late season snowfall combined with clearing skies overnight can also cause the surface to cool rapidly and lead to damaging freeze potential as well. All of these factors should be considered when comparing your location to those selected in Fig. 3.