IEM Daily Bulletin for Jul 31 2026

[daryl herzmann]

Iowa Environmental Mesonet Daily Bulletin for 31 July 2026

News

No news is good news

IEM Code Pushes <repo,branch> on Github

Timestamp	Repository	Message	Link
Jul 30 7:41 PM	iem,main	🐛 Assign to proper array on map click	Link
Jul 30 8:35 PM	iem,main	💚 Address codereview	Link

Daily Feature

About Those 90°F Dew Points
Date: 30 Jul 05:30 AM
Votes: Good: 24   Bad: 1 Abstain: 0

It is a tale as old as time, it seems. For the most humid days of the summer, a couple of the non-federal airport weather sensors in Iowa will report incredible dew point readings, often reaching the low 90s°F. "Corn sweat" is immediately attributed to these without much considering if such readings are even physically possible. The next level of attribution claims that these sensors are located within corn fields, so they are sensing the corn field environment and since Iowa is mostly covered by corn and soybean fields, that this reading, while high, is still representative of the state. So like what the IEM did back in August 2023, today's feature attempts to simply explain those high readings as bad data / poor sensor performance. There are three main airport weather station network types with Iowa. The "Federal ASOS" sites maintained by the FAA/NWS. The "Federal AWOS" sites maintained by the FAA. The "Non-Federal AWOS" sites that are operated by the Iowa DOT. These sites have different sensor packages, siting requirements, and maintenance methods. While a number of ASOS sites set dew point records earlier this week, a few AWOS sites reported even higher dew point temperatures in the upper 80s and even low 90s. The featured chart presents some humidity metrics over the past most humid days with the colors representing the network type that the airport weather station belongs to. Some logical and charted points to why such high dew points are bad.

1. The 90°F dew point club (Clarion, Algona) are clearly having a problem with reporting too high of relative humidity (RH) and often "stuck" at 100% (first column). They are also reporting way too high RH during the afternoon hours (last column) vs all other sites over Iowa.
2. The high RH frequencies, if accurate, would also imply reduced crop transpiration as saturated air can not take on more water vapor. This creates a conundrum for the theory that "corn sweat" can drive such high dew points when the sensor indicates high RH already.
3. Ultimately, the very high dew points are just a sign of the sensor struggling to accurately measure humidity under extreme humidity levels. There is likely some water condensation within the sensor causing it to over-report humidity.

Recapping, "corn sweat" certainly drives higher dew point temperatures in Iowa and the effect is nicely documented with this Wolf and Market article. The effect can only push dew point temperatures so far into the lower 80s.

NWS Watch/Warning Summary for 12 AM 30 Jul 2026 - 12 AM 31 Jul 2026 CDT

	Summary		By WFO					Watches
Type	US	IA	ARX	DVN	DMX	OAX	FSD	US
Tornado	0	0	0	0	0	0	0	0
Svr Tstorm	68	0	0	0	0	4	0	3
Flash Flood	15	0	0	0	0	0	1	---

ARX = LaCrosse, WI DVN = Davenport, IA DMX = Des Moines, IA OAX = Omaha, NE FSD = Sioux Falls, SD

IEM Cow Report

SVR+TOR Warnings Issued:  68 Verified:  23 [33.8%]
Polygon Size Versus County Size            [17.5%]
Average Perimeter Ratio                    [21.8%]
Percentage of Warned Area Verified (15km)  [12.1%]
Average Storm Based Warning Size           [1681 sq km]
Probability of Detection(higher is better) [0.60]
False Alarm Ratio (lower is better)        [0.66]
Critical Success Index (higher is better)  [0.28]

IEMBot Social Media Posts

IEMBot can be configured to post messages to a number of social media services. More details: IEMBot Homepage.

Posts yesterday per supported service:

• ATMosphere (bluesky only attm): 2,131
• Mastodon / Fediverse: 13
• Slack: 1,542
• Twitter/X: 8,313
• HTTP Webhooks: 198
• Weather.IM Jabber Chatrooms: 11,675