DES MOINES, IA–Looking at past climate data can tell a lot about what a certain location has been experiencing and what is changing. Data show Des Moines is experiencing fewer 100° days than in decades past. But there is still an apparent warming trend with many more warm records than cold records.

100° days are becoming few and far between

The 1930s hold the record for the most 100° days in Des Moines and no other decade even comes close to matching it. If you combined all of the 100° days from the past 51 years you still wouldn’t reach the amount of 100° days recorded in Des Moines during the 1930s. There were 78 days with 100° highs in Des Moines during the 1930s. That may not seem like a lot over the span of 10 years, but the decade with the next highest is the 1980s with 38 100° days.

In 2021, Des Moines only experienced one day above 100°. It was June 17th when highs climbed to 101°, but it was Des Moines’ first time in the triple digits since 2017, before then it was 2013, however in 2012 there were 11 days at or above 100°. There’s a reason why those years ended up with 100° heat.

In June 2021 about 95% of Iowa was considered at least abnormally dry. In July 2017 about 60% of the state was abnormally dry. In 2012 and 2013 the entire state of Iowa was experiencing some level of drought. When Des Moines reached 105° on July 23, 2012, 27% of Iowa was in extreme drought.

Why does drought mean hotter temperatures?

To say it simply, dry land heats up faster than moist land. So where there are more rain and clouds, temperatures tend to stay lower. Days with more sunshine and drought mean temperatures can heat up more.

Take the 1930s, for example. There were 78 days in which temperatures reached 100° or more. Nearly two-thirds of that number came from 1934 and 1936.

“1934 and 1936 were right in the middle of the Dust Bowl, so there was pervasive drought across much of the United States and we were locked into a stagnant pattern. It was a long-term heatwave, but with very low dew points creating very dry conditions. Because there was also no cloud cover, there was a lot of incoming solar radiation on bare soils. You’re essentially baking the surface, and creating extended periods of time in which the temperature is in the triple-digit range,” said State Climatologist Justin Glisan.

While extreme levels of drought are bound to happen again across parts of our state, it’s unlikely we’ll ever see something as bad as the 1930s. “The wets are getting wetter faster than the dries are getting drier,” said Glisan. Because Iowa has experienced an increase in temperature of about 1.3°F since the 1890s, the atmosphere can hold about 4% more moisture.

“When you have more water vapor availability in the atmosphere, you’re more prone to have more cloud cover and more rain events. That results in cooler daytime highs, however, the clouds lock in the overnight low temperatures, so you see warmer temperatures then. We’ve seen a two-tenths of a degree increase in the overnight low temperature per decade since 1895,” said Glisan.

That means Iowa has seen a 2.4°F increase in the average overnight low since 1895.

In the summer months alone, Iowa has seen a 2.1°F increase in the average overnight low since just 1970. So while the daytime high has flatlined during the summer, overnight lows continue to warm.

Record Warmth vs Record Cold

Despite less 100° highs in Des Moines, there are still plenty of warm records being broken, but when it comes to cold records, warmth dominates.

What’s a warm record and a cold record?

Each date has four temperature records. We commonly talk about the record high and the record low in the almanac. That’s two of the four: the record warm maximum and the record cold minimum, essentially the hottest it’s ever been and the coldest it’s ever been on a specific date.

What we don’t talk about as much are the record cold maximum and the record warm minimum temperatures. Think of a day with an exceptionally warm morning and another day with a exceptionally cool afternoon.

June 14th, 2022, for example, broke a record warm minimum temperature. That day, the low was 78°, beating the old record set in 1994 by 3°. On June 14th, 2022’s high was 95°, and while 13° above average, it was still 7° away from breaking the record warmest maximum temperature of 101° set in 1886.

Record warmth vs record cold since 2000.

From 2000-2009 there were 60 record warm or record warm minimums set in Des Moines vs only 14 record cold maximums or record cold minimums set. That means we were nearly 5 times as likely to see record warmth than record cold.

Between 2010 and 2022, so far, there have been 92 record warm maximums or record warm minimums set vs only 21 record cold maximums or record cold minimums set. That means there were more than four times the number of warm records than cold records.

The large difference between these two numbers comes back to the extra moisture in the atmosphere due to a warming Earth. “The disparity is because of the the temperature behavior we’re seeing. We have flatlining daytime highs in the trend, but an increase per decade in the overnight lows. And that’s because of more cloud cover, more rainfall, more subsoil moisture. That prevents the Earth from radiating heat at night.”

Not all of the record warmth comes from the increasing temperature of overnight lows. There have still been 69 record warm maximums set since 2000, however, the majority did not occur during summer. There were only 3 in summer while there were 25 in winter, 28 in spring, and 13 in fall.

Between 1970 and 2019 winter was Iowa’s fastest warming season followed by fall, spring, and finally summer.

So while extreme cold is still likely to occur from time to time here, extreme warmth is way more likely.