Sticky Situation: Examining Dewpoints for July 2010

The summer of 2010 has seen more than its fair share of rain, and a good amount of moisture to go with it. The average July dewpoint for the southern portion of the state of Wisconsin is about 60.9 with both Madison and Milwaukee combined, over the last 63 years. July 2010 in Milwaukee ranked #4 on the list of highest July average dewpoints with an average of 65.83 degrees, and Madison comes in #3 with an average July dewpoint of 67.06 degrees. This isn't exactly record breaking, yet most people agree that this summer is definitely more uncomfortable than usual. The #1 average July dewpoint for both cities was almost 69 degrees in Madison during 1999 and almost 70 in Milwaukee during 1995.

One of the reasons why this summer feels so humid could be because last summer, we experienced lower dewpoints (less humidity) through much of that period.  In 2009, the average July dewpoint in Madison was a very comfortable 55.98 degrees, while in Milwaukee it was only 54.13 degrees.  These numbers at are the very bottom of July averages for the past 63 years!

In July 2009, not a single day had a dewpoint above 70 degrees in either Milwaukee or Madison, and when compared to the fact that half or more the days in southern Wisconsin in July 2010 had dewpoints of 70 or higher, it makes a very big difference in how comfortable the air feels.

Top-10 Julys (number of days) with Dewpoints above 70 and Top-10 July Average Dewpoints 

Year

# of July Days above 70 (Madison)

1999

23

1987

20

1955

17

1983

17

2002

17

1949

16

1986

16

1952

15

2006

15

2010

15

Year

# of July Days above 70 (Milwaukee)

1999

22

1987

19

1955

18

1952

17

2002

17

2010

17

1977

16

1949

15

1983

15

1986

15

Year

Average Dewpoint for July
(Madison)

1999

65.96

1986

65.47

1955

65.30

1987

65.18

1949

64.98

2010

64.74

1995

64.26

1983

64.08

2006

63.90

1993

63.55

Year

Average Dewpoint for July
(Milwaukee)

1955

65.98

1987

65.95

1999

65.52

2002

65.02

1949

64.83

1986

64.49

2010

63.89

2006

63.51

1977

63.51

1995

63.51

This doesn't necessarily mean that July 2010 was more (or less) humid than the top years. Temperatures were not taken into consideration for this study. Relative humidity, the most common method of measuring how saturated the air is, is measured by finding the percent ratio of the dewpoint to air temperature. For example, a high temperature and a low dewpoint would result in an arid environment, and a low temperature and high dewpoint would result in a cooler, but more humid environment.

So far for the month of August, Madison dewpoint has averaged 67.06 degrees, which ranks as 3rd highest on record. Milwaukee has averaged 66.39 degrees, making this August the fourth highest on record. The 63 year average for this period is 60.46 degrees in Madison and 61.29 degrees in Milwaukee.

Please note that the data for dewpoints from 1962-1972 in Madison and from 1962-1975 in Milwaukee, dewpoints were only taken every 3 hours. The rest of the data for the last 63 years was taken every hour.

A special thanks is extended to Ed Hopkins of the Wisconsin State Climatology Office, on the UW-Madison campus, for supplying the hourly dewpoint data for both Madison and Milwaukee.

Below are the graphs for the average dewpoints in Madison and Milwaukee for June, July and August from 1948 to 2009.

Milwaukee Madison
June Dewpoints
July Dewpoints

August Dewpoints

By examining the trends, you can see that June and August dewpoints are slowly increasing, and July dewpoints have been increasing until about 2000, and then have decreased over the past 10 years.  Each month has a variation of about 10 degrees throughout the past 60 years.  July has been the month with the most flucuations, since there was a decrease in dewpoint temperature from 1948 until around 1975, then an increase until around 2000, followed by another decrease until 2009.

By Penny Zabel, Michael Hansen (Student Volunteer) and Laura Schutte (Student Volunteer)



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