Record Dew Point Temperatures

By: Christopher C. Burt , 7:30 PM GMT on August 11, 2011

Share this Blog
6
+

Record Dew Point Temperatures

Just recently the Minnesota State Climate Office issued a statement declaring the 88° dew point temperature measured at Moorhead (on the Red River across from Fargo, North Dakota) between 7pm and 9pm on July 19, 2011 as a new all-time state record for the highest such reading ever observed. This would also be one of the highest dew points ever observed in the United States. Here is a brief summary of record dew point levels from both the U.S.A. and around the world.

The 88° Dew Point Measured at Moorhead, Minnesota on July 19, 2011

One might assume that the highest dew points measured in the United States would be those observed during in areas along or near the coast of the Gulf of Mexico during the summer months. Although for the most part this is true the other region that occasionally seems to record extraordinary heat and humidity is the Upper Midwest. I have not been able to discover just why the dew points in Iowa, Minnesota, and Wisconsin sometimes are higher than anywhere else in the United States during exceptional heat waves versus, say, Missouri or other states that are generally more humid and also endure exceptional summer heat. One possible factor is that the worst heat waves in the area sometimes occur following heavy rainfalls that have saturated the ground and lowest levels of the atmosphere whereas other more southern places usually reach there highest temperatures during periods of drought, as is the current situation in the Southern Plains.





Two graphics illustrating the affect of humidity on the apparent feel of the temperature: at top is NOAA’s Heat Index chart (% of humidity with air temperature) and below that the effect of dew point with air temperature. Unfortunately the dew point table does not consider dew points above 82°.

On July 19th not only did Moorhead record its highest dew point on record but so did Minneapolis with an 82° (and possibly even 84° for a five minute period between 3:21-3:27 pm) on the same day. Another site in Minnesota, Madison, also registered an 88° dew point (although this site may not be considered an official reporting station). The NWS office in Grand Forks, North Dakota issued this statement explaining how heavy rainfall the morning of the event had saturated the soil and left ponding water near the instrument site. Furthermore, the AWOS is surrounded by sugar beets and soy beans “two of the most prodigious transpiring plants”.



A photograph of the Moorhead Airport Automated Station tat registered the 88° dew point on July 19th. Photo courtesy of the Grand Forks, North Dakota National Weather Service Office.

Highest Dew Point Measurements in the United States

Last summer (2010), Newton, Iowa recorded an 88° dew point on July 14th. Chicago, Illinois’s highest dew point was 83° at 8 a.m. on July 30, 1999 as was Milwaukee, Wisconsin’s with an 82° the same day. But it was during the July heat wave of 1995 that the highest dew point of all was measured in the Upper Midwest: 90° at Appleton, Wisconsin at 5 p.m. on July 13th of that summer. The air temperature stood at 101° in Appleton at that time leading to a heat index reading of 148°, perhaps the highest such reading ever measured in the United States. Here are the METARS for Appleton that day:



METARS chart for Appleton, Wisconsin on July 13, 1995 from wunderground.com.

The July 1995 heat wave resulted in the deaths of 750 people in Chicago and other cities in the region recorded amazing dew points as well such as Cedar Rapids, Iowa: 84° (heat index 129°) and Oelwein, Iowa 85° (heat index 131°). Unusually high dew points in the region persisted for a couple days, aided by evaporation from soil and a stable layer of air aloft that prevented moist air in the lowest few thousand feet from mixing with drier air above. This was also the case in Minnesota and North Dakota last month on July 19th.

Aside from Appleton, the only other instances of 90° or higher dew points I am aware of are the following: 91° at Melbourne, Florida at 2 p.m. on July 12, 1987 (air temperature 95°) and 90° at New Orleans Naval Air Station at 5 p.m. on July 30, 1987 (air temperature 91°). Both of these records have not been vetted for accuracy.

Record high dew points seem to be a particularly difficult record to verify. On the East Coast I can find no reference to what New York City’s highest dew point might have been. Philadelphia’s highest such was 82° on July 15, 1995 (the heat index peaked at 129°). Syracuse, New York’s record was 77° on July 4, 1999. Anecdotal on-line reports claim an 86° dew point at Virginia Beach, Virginia sometime during the summer of 1998. In general, it may be assumed that the highest dew points on record for most places east of the Great Plains would be in the 77°-85° range.

Highest Dew Point Levels in The World

The hottest most humid regions of the world are the coastal areas of The Persian Gulf, Red Sea, and Gulf of Aden.



Map of the Arabian Peninsula region, the coastal areas of which record the highest dew points in the world. Boosaso (referenced below) is located on the Gulf of Aden coast of Somalia just visible at the bottom portion of the map. Dhahran, Saudi Arabia is located on the Persian Gulf just north of Bahrain.

Thanks to the shallow nature of these bodies of water they heat up during the summer season with average sea surface temperatures in the upper 80°s. In fact, the hottest sea surface temperatures ever recorded anywhere in the world have been 98° in the Persian Gulf and 96° in the Red Sea. Data compiled by the British Met Office (in its ‘Tables of Temperature, Relative, Humidity and Precipitation for the World: Part 4; Africa and the Indian Ocean’, 1967) for Boosaso (formerly known as Bender Caasim), Somalia between 1934-1946 indicate an average dew point of 83° at 2:30 p.m. during the entire month of June (104° air temperature with 61% humidity). Obviously, some days are even hotter and more humid (the record June high for the period was 113° and the record low 78°). Assab, Eritrea has an average June afternoon dew point of 84°.

The absolute highest dew point recorded in the region and therefore the world (of which I am aware) was 95° at Dhahran, Saudi Arabia at 3 p.m. on July 8, 2003. The dry bulb temperature stood at 108° at the time, so theoretically the heat index was 176°. Sharjah in the United Arab Emirates (UAE) apparently once recorded a dew point of 93.2° (date unknown) according to ‘Weather Climate Extremes’ Army Corps of Engineers TEC-0099 report.

KUDOS: Thomas Schlatter of NOAA for USA dew point records and Dr. Adnan Akyuz, North Dakota State Climatologist for Moorhead, MN information.

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 9 - 1

Page: 1 — Blog Index

9. SubtropicalHi
5:41 PM GMT on July 13, 2014
Responding 3 years later: I grew up near that airport. It was surrounded by farms and corn!
Member Since: June 27, 2010 Posts: 1 Comments: 377
8. SubtropicalHi
5:34 PM GMT on July 13, 2014
Quoting 4. macrobiologist:

While this may not hold true for Appleton (I'm not sure, many of the exceptionally high dewpoints in the Upper Midwest are recorded at sites near row-crop agriculture--in particular, corn. Almost all of the high dewpoints were recorded when high soil moisture, hot high pressure, and the peak growth of corn co-occurred. The corn creates localized areas of extremely high dewpoints, and it the Upper Midwest, there is so much corn that the effect can be seen across a lot of sites. The Red River valley where the recent 88 degree dewpoint occurred is an area of concentrated agricultural production, and the Newton (I was born there and have lived there) and Knoxville sites in Iowa are very near areas of extensive corn production. Often, on the same day at nearby sites that aren't as immediately adjacent to agriculture, the dewpoints are considerably lower (this is often true for more urban areas like Des Moines or some some of the other Iowa sites adjacent to pasturelands or woods).

As a plant ecologist this makes perfect sense to me. With ample moisture and very hot temperatures, plants that can will keep their stomata open to cool their leaves and prevent damage. The presence of corn (and perhaps other crops that have a lot of young leaves at their peak photosynthetic capacity during a heat wave)is key, because it increases the evaporational surface, and in the Midwest, this translates to a landscape-scale phenomenon.

Also, even if soil moisture is very high, after a couple of sunny days, most Midwestern loams will have a thin dry layer at the surface, or they will be under crop canopies, which increases the boundary layer and diminishes the flux of moisture into the lower levels of the atmosphere.
Member Since: June 27, 2010 Posts: 1 Comments: 377
7. milwaukeeee
3:34 PM GMT on February 28, 2013
Charleston has a highest heat index than appleton !!!

65C or more heat index was observed in many occasion in charleston !!!

Member Since: February 28, 2013 Posts: 0 Comments: 0
5. SkyOdyssey
12:47 AM GMT on August 24, 2011
macrobiologist is correct, it is the crops that account for the very high dew points localized in the Upper Midwest. In particular, corn is a profuse transpirer of water from the ground to the air and this area is known as the Corn Belt. The process of plants releasing water to the atmosphere is called transpiration and is enhanced when hot weather follows wet weather or anytime there is copious water available in the soil during summer.

Mature yet actively growing plants transpire the most and this peaks in the summer months (young plants and aging plants transpire less). Significant evaporation from the soil also occurs when heat follows heavy rains. The processes of evaporation and transpiration are together evapotranspiration and explain how such high moisture content is available in the atmospheric boundary layer of a continental landmass thousands of miles removed from its primary moisture source; in this case the Gulf of Mexico. Proximity to the Gulf of Mexico accounts for high moisture in the South and similarly for respective areas across the globe such as Iraq and Kuwait in the Persian Gulf region (which also can experience very high temperatures concurrent with the high dew point). 120F/90F is not uncommon!
Member Since: March 8, 2008 Posts: 0 Comments: 0
4. macrobiologist
12:59 PM GMT on August 14, 2011
While this may not hold true for Appleton (I'm not sure, many of the exceptionally high dewpoints in the Upper Midwest are recorded at sites near row-crop agriculture--in particular, corn. Almost all of the high dewpoints were recorded when high soil moisture, hot high pressure, and the peak growth of corn co-occurred. The corn creates localized areas of extremely high dewpoints, and it the Upper Midwest, there is so much corn that the effect can be seen across a lot of sites. The Red River valley where the recent 88 degree dewpoint occurred is an area of concentrated agricultural production, and the Newton (I was born there and have lived there) and Knoxville sites in Iowa are very near areas of extensive corn production. Often, on the same day at nearby sites that aren't as immediately adjacent to agriculture, the dewpoints are considerably lower (this is often true for more urban areas like Des Moines or some some of the other Iowa sites adjacent to pasturelands or woods).

As a plant ecologist this makes perfect sense to me. With ample moisture and very hot temperatures, plants that can will keep their stomata open to cool their leaves and prevent damage. The presence of corn (and perhaps other crops that have a lot of young leaves at their peak photosynthetic capacity during a heat wave)is key, because it increases the evaporational surface, and in the Midwest, this translates to a landscape-scale phenomenon.

Also, even if soil moisture is very high, after a couple of sunny days, most Midwestern loams will have a thin dry layer at the surface, or they will be under crop canopies, which increases the boundary layer and diminishes the flux of moisture into the lower levels of the atmosphere.
Member Since: August 14, 2011 Posts: 0 Comments: 6
3. Christopher C. Burt , Weather Historian
3:59 AM GMT on August 12, 2011
Quoting blairtrewin:
One problem with assessing extreme dewpoints is that in most cases (especially for older data) they are derived from dry- and wet-bulb temperatures and the data are therefore very prone to quality problems if the water reservoir for the wet-bulb thermometer dries out. This is fairly easy to spot if you have data at hourly or higher frequency (since the dewpoint starts to track the dry-bulb through the diurnal cycle), much harder if you only have a small number of observations per day.

The Australian database has a small number of dewpoints in the 30-31 C range but I'm not totally convinced of their accuracy (all are from sites which only had measurements at 9am and 3pm, so hard to check); nothing above 31.

One would have to imagine that crop transpiration is a major part of the picture in the Upper Midwest; I can't see any other reason for such extreme dewpoints to occur there and not further south.

I've had the dubious pleasure of experiencing a 29 C dewpoint in Beijing (close to, but not quite, a record) - not something I want to repeat any time soon. It was in August 2005, exactly three years before the Olympics; it's fortunate those conditions weren't repeated in 2008 (although it did give me some good stories to scare aspiring marathoners with).


Thanks for this Blair. I agree that dew point measurements are fraught with ambiguity.

I also spent some rather miserable days in Hong Kong (like you mention about Beijing) during summer months. These were the most uncomfortable I have personally ever experienced. So we know (from experience) that, although the actual dew point measurements may be questionable, it is likely that 85-90 dew points are in the realm of possibility.

Speaking of hot and humid Asian cities, I'd like to note that Bangkok, Thailand recorded a dew point of 89.6 at 9 a.m. on April 26, 2005 :

http://www.wunderground.com/history/airport/VTBD/ 2005/4/26/DailyHistory.html?req_city=NA&req_state= NA&req_statename=NA

Nevertheless, it is hard to say how accurate this data really was.

Dew point measurements are probably not nearly as accurate as dry bulb measurements for a variety of reasons.

Member Since: February 15, 2006 Posts: 300 Comments: 280
2. blairtrewin
1:34 AM GMT on August 12, 2011
One problem with assessing extreme dewpoints is that in most cases (especially for older data) they are derived from dry- and wet-bulb temperatures and the data are therefore very prone to quality problems if the water reservoir for the wet-bulb thermometer dries out. This is fairly easy to spot if you have data at hourly or higher frequency (since the dewpoint starts to track the dry-bulb through the diurnal cycle), much harder if you only have a small number of observations per day.

The Australian database has a small number of dewpoints in the 30-31 C range but I'm not totally convinced of their accuracy (all are from sites which only had measurements at 9am and 3pm, so hard to check); nothing above 31.

One would have to imagine that crop transpiration is a major part of the picture in the Upper Midwest; I can't see any other reason for such extreme dewpoints to occur there and not further south.

I've had the dubious pleasure of experiencing a 29 C dewpoint in Beijing (close to, but not quite, a record) - not something I want to repeat any time soon. It was in August 2005, exactly three years before the Olympics; it's fortunate those conditions weren't repeated in 2008 (although it did give me some good stories to scare aspiring marathoners with).
Member Since: October 25, 2010 Posts: 0 Comments: 35
1. rod2635
8:23 PM GMT on August 11, 2011
I see it (Appleton's Dew Point), but it is hard to believe it. The Middle East I can understand, given the extreme heat and shallow water. There was an appreciable breeze at Appleton, so it wasn't stagnant air. The dew point rose during the day! Were other nearby stations at least in the same ballpark? Thanks for researching these extremes...always fascinating to see what, and in particular, where things can happen.
Member Since: January 27, 2011 Posts: 0 Comments: 318

Viewing: 9 - 1

Page: 1 — Blog Index

Top of Page

About weatherhistorian

Christopher C. Burt is the author of 'Extreme Weather; A Guide and Record Book'. He studied meteorology at the Univ. of Wisconsin-Madison.