Losing Sunlight and Temperatures

We are in the dog days of summer. With the sun beating down and maturing crop producing sweltering humidity levels straight out of the Amazon Rainforest, July can be a pretty tough month in Iowa. Based off of the 1981-2010 climate normals, our overall normal temperatures hit their peak for a few weeks in July.

We still have a while until the beginning of fall and all of the lovely things that come with it including the cool temperatures, football, leaves changing, etc. However, if you are sick of the heat and humidity, you’ll be happy to know that our normal temperatures are dropping.

Max Normal High Current Normal High Change
Waterloo 85 84 -1
Dubuque 82 82 0
Cedar Rapids 84 83 -1
Iowa City 85 85 0

Slight change in our normal high from its max to now

Statistically, that means that we have put our hottest days behind us and with each passing week the chance for another heat spell get smaller and smaller. It doesn’t always work out that way because the difference between our max normal high in the beginning of July to our normal high at the end of August is only a few degrees.

Max Normal High Aug. 31 Normal High Change
Waterloo 85 80 -5
Dubuque 82 78 -4
Cedar Rapids 84 80 -4
Iowa City 85 81 -4

Slow decline from peak heat in July to the end of August

The “normal” temperature is an average from 1981 to 2010; actual day-to-day temperatures can be well above or under the normal since weather and climate are on two very different time scales. The squiggly lines in the graph below of Australia’s long-term temperature change shows the variability of daily weather vs the smooth line of the overall climate average.

https://www.climatechangeinaustralia.gov.au/en/climate-campus/australian-climate-change/australian-trends/

The reason for the decline in our normal temperature is that we are now losing about 1 minute and 45 seconds of daylight each day. That loss jumps to 2:45 by the end of August. Less daylight means less energy intake from the sun – the Earth’s primary warming lamp. Below is a graph of solar radiation vs. the normal for 2018. Notice how the graph starts to drop after the summer solstice in June. The rate increases in the fall months.

The quicker the rate of daylight loss, the quicker our normal temperatures drop. You will notice this the most during the transition months of meteorological fall when our normal average temperature drops more than 10 degrees each month.

September October November
Waterloo -12 -11 -16
Dubuque -11 -12 -15
Cedar Rapids -11 -11 -16
Iowa City -12 -12 -15

Monthly change in avg. temp in degrees F

Long story short: we are losing daylight and heat with each passing day but because of the daily fluctuations in temperatures and weather patterns, it won’t be until September, especially the second half that our chances for extreme heat will become very low.

Brandon Libby

Brandon Libby

Meteorologist
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