The Weather Guys

The growing season is often defined as the number of days between the last time the temperature was 32 degrees in spring and the first time the temperature falls to 32 in fall.

The latest frost in spring is important to gardeners as they seek to plant after a long winter, and for most plants it is wise to do so when the temperatures are likely to remain warmer than 32 degrees.

For our region, based on temperature observations between 1981 and 2010, the median date of the last frost is between May 1 and May 10. That is why many gardeners consider Mother’s Day a good day to plant.

One reason for the variation in the last frost date is how the first frost date is defined.

Giving the median date of last frost means that there is still a 50 percent chance that a frost will occur after this date. Some definitions require that there is only a 10 percent chance of a 32 or lower temperature.

Some sites use a different 30-year period, for example, between 1951 to 1980. As it turns out, our nighttime minimum temperatures have been getting warmer, so the last frost date has been moving earlier in the spring.

The change is not much for Dane County, but in northwestern Wisconsin, the last frost date now occurs about two weeks earlier than it did in 1950.

According to the Wisconsin State Climatology Office, for Madison, based again on the 1981-2010 time period, the median date for a temperature of 32 is May 2, and it isn’t until May 20 that the probability of an occurrence of 32 has dropped to 10 percent. On April 18, there’s a 90 percent chance of a later occurrence of 32 degrees.

So, if you have not yet done your planting, now would be a good time.

Lakes Mendota, Monona and Wingra opened during the second week of April.

Lake Mendota officially froze Dec. 16 and opened April 12, for an ice season  of 117 days, which is 12 days longer than the median duration.

Despite the persistent and sometimes brutal cold we experienced this winter  in southern Wisconsin, the 117-day ice season pales in comparison to the record  for Mendota, set in 1880-81, of 161 days. That’s nearly a month and a half  longer than this year’s ice season.

Lake Monona froze Dec. 10 and opened April 10 for a season of 121 days,  exceeding the median duration by 17 days.

Lake Wingra froze Nov. 27 and opened April 9 for a season of 133 days, also  17 days longer than the median.

Interestingly, the largest lake, Mendota, took only three more days to open  than did the smallest lake, Wingra.

Because there is so much more water in Lake Mendota compared to Lake Wingra,  the big lake froze 19 days later than the small lake. This testifies to the  substantially larger amount of heat energy that must be removed from the larger  Lake Mendota before it can finally freeze.

This energy is deposited in the overlying atmosphere where it can be used to  make the lakeshore temperatures warmer than those at locations far from the  lake. Such heat lost from the lake in late autumn, since it was originally  deposited in the lake by sunlight during summer and fall, represents a delay in  the transfer of energy from the summer-autumn sun to the wintertime air.