On the verge of a major heat wave across the central US, here’s a look at recent trends in Iowa regarding summertime temperatures and heat indices.

Since 1973 the number of 90-degree days in a year has dropped by eight.

The bottom line is that the overall maximum temperatures have actually come down over the years. The number of 90-degree days and 100-degree days has really fallen. At the same time, dewpoint values have shown a similar increase. The combination of warm temperatures and high dewpoints creates the dangerous heat index.

The yearly maximum dewpoint value has increase by 2.1 degrees.

Since 1973 (the earliest dewpoint data is available) the dewpoint trend has been steadily increasing in Cedar Rapids. Over 45 years, the trend for maximum dewpoint in a year has been an increase of 2.1 degrees.

The change in number of days with more extreme dewpoint values has been a steady increase since 1973.

When looking at the number of days with a 68, 70 and 72 degree dewpoint, the trend has been consistently higher. Using a linear trend, there have been 22 more days in a year with a 68 and 72 degree dewpoint. The 70-degree mark has surpassed 24 more times in a year since 1973.

The maximum heat index tallied in a year has increased by 1.9 degrees since 1973 in Cedar Rapids.

With the increase in dewpoint, heat indices have had an increase over the same period. Since 1973 the maximum heat index in Cedar Rapids has trended higher by 1.9 degrees.

For further research I will be looking at more cities in Iowa including Waterloo, Dubuque and Des Moines to increase the overall coverage of the state.