A topic of great interest to me is the perception of weather and climate. A person in one location may have a completely different interpretation of the weather and climate than someone in a different location. Ask someone in Phoenix how the weather feels when it is 90°F outside and you will certainly get a different response than if you ask someone in Minneapolis how it feels at 90°F. Much of this is due to acclimation and expectation. In Phoenix, you don’t wake up on a July day and wonder whether or not it will be hot. In Minneapolis, a 90°F day is a fairly low probability event – not rare by any means though.
In the absence of asking people in every location what their interpretation of the weather is, across-the-board generalizations must be made. The Washington Post recently ran a story with an analysis of the number of Nice Days per year based on climatological data. A single set of criteria was applied everywhere, and the results displayed in map form.
Taking this concept a different direction, what if we want to look at the amount of time with oppressively hot and/or humid conditions? Obtaining a consensus as to what constitutes oppressive is not dissimilar to a consensus on nice weather. In any event, criteria must be chosen. In benevolent dictator fashion, here are the criteria based solely on experiential and social interactions:
- Temperature 95°F or warmer or heat index 95°F or warmer
- If temp or heat index < 95°F, dew point 75° or higher
- Wind 10 mph or less
- Between the hours of noon and 6 p.m.
Instead of looking at a daily maximum value (e.g., peak temperature or peak heat index), a tally of the hours with oppressive conditions during a particular time of day better captures the duration and intensity of oppressive conditions. A city that barely touches the oppressive category “feels” different than one that stays in the oppressive category hour after hour after hour. A note on wind. Even with hot and humid conditions, a moderate to strong breeze provides significant relief and evaporative cooling when the conditions are otherwise oppressive; therefore, any observation with a wind over 10 mph was categorically excluded from the tally of oppressive conditions.
Results
A total of 380 automated stations across the U.S. were evaluated for oppressiveness at each hourly observation between 1998 and 2017 during the noon through 6 p.m. time frame. A percentage of oppressive observations during the June-August time period was computed and the results are shown in this map.
To no one’s surprise, the Gulf of Mexico coastal areas lead the way in oppressive conditions. Louisiana is the epicenter of summertime oppressive weather. Why does Louisiana have higher areas than say Dallas, Mobile or Atlanta? Remember that winds over 10 mph disqualify an observation from counting as oppressive. Louisiana has very light winds in the summer. Also, the dew point temperature is consistently high in the Lower Mississippi River Valley. Lower dew points tend to mix down to the surface as you move away from the Gulf of Mexico. Even when temperatures are under 95°F, there is a strong likelihood that the heat index or dew point criteria will be met in the northwest Gulf coast region.
A secondary peak of oppressiveness is the desert areas of southern Arizona, Nevada and California. Unlike the Gulf coastal areas, the desert southwest area warms up so quickly by noon that the criteria are met regardless of dew point of heat index considerations. The limiting factor at those stations is whether the wind is under the 10 mph threshold.
Perhaps some people will be surprised to see that half of the Contiguous U.S. observes these oppressive conditions during summer afternoons less than 5% of the time. The areas shaded in brown on the map are reliably oppressive at some point nearly every day; for areas north of 35°N, it is far less common.
Is 95°F too Warm?
The perspective from here in Alaska is that 95°F is pretty toasty. What if the 95°F temperature / heat index threshold was dropped to 90°F? The result is the map shown below. The patterns are more or less the same, but the magnitude changes dramatically.
All of the Deep South falls into the highest oppressive category using the 90°F criteria cut off. Much of the central U.S. has a value of 20% or greater. Under 15% of the Contiguous U.S. experiences oppressive conditions less than 5% of the time.
Top "Top" 10 stations that experience the most oppressive conditions using the 95°F criteria are shown below. The percentage of time with oppressive conditions using the 90°F criteria is also shown. [Note: the Top 10 list using 90°F is slightly different.]
| Station | State | Oppressive at 95°F | Oppressive at 90°F |
| Phoenix | AZ | 64% | 70% |
| Tampa (MacDill AFB) | FL | 55% | 64% |
| Houston | TX | 48% | 57% |
| Shreveport | LA | 45% | 59% |
| Jackson | MS | 41% | 62% |
| Las Vegas | NV | 41% | 47% |
| Beaumont | TX | 40% | 48% |
| New Orleans | LA | 39% | 52% |
| Tallahassee | FL | 39% | 57% |
| Orlando | FL | 37% | 52% |
What does it all mean? As the saying goes, climate is what you expect and weather is what you get. People living in the areas with shades of tan or brown should expect oppressive conditions a large fraction of the time in the summer. In the northern half of the U.S., oppressive conditions can occur, but those times are relatively few and far in between.
Other Selected Values. Percent of time (noon to 6 p.m.) defined as oppressive at the two different criteria thresholds:
| Station | State | Oppressive at 95°F | Oppressive at 90°F |
| Albany | NY | 1% | 4% |
| Amarillo | TX | 4% | 9% |
| Atlanta | GA | 10% | 27% |
| Austin | TX | 35% | 45% |
| Balt/Wash Intl | MD | 8% | 18% |
| Birmingham | AL | 22% | 43% |
| Boise | ID | 9% | 21% |
| Charleston | SC | 24% | 35% |
| Chicago O'Hare | IL | 2% | 5% |
| Corpus Christi | TX | 16% | 18% |
| Dallas/Ft Worth | TX | 26% | 35% |
| Denver (Dia) | CO | 3% | 14% |
| Detroit/Wayne | MI | 1% | 4% |
| Dodge City | KS | 4% | 9% |
| Honolulu | HI | 1% | 1% |
| Indianapolis | IN | 3% | 8% |
| Jacksonville | FL | 34% | 46% |
| Kansas City/Intl | MO | 10% | 17% |
| Memphis | TN | 28% | 42% |
| Miami Intl | FL | 26% | 34% |
| Nyc/Jfk Arpt | NY | 1% | 2% |
| Nyc/La Guardia | NY | 2% | 5% |
| Oklahoma City | OK | 13% | 20% |
| Omaha | NE | 5% | 10% |
| Philadelphia | PA | 3% | 8% |
| Salt Lake City | UT | 8% | 19% |
| San Angelo | TX | 18% | 30% |
| San Antonio | TX | 27% | 43% |
| St. Louis | MO | 13% | 23% |
| Tulsa | OK | 20% | 28% |
| Wash Dc/Dulles | VA | 7% | 17% |
| Washington/Natl | DC | 7% | 15% |
Addendum [August 10, 2018 at 12:00 p.m. EDT]
By request, here is a companion map showing oppressive conditions during the overnight hours (11 p.m. through 6 a.m.). Any observation with a heat index of 85°F or higher or a dew point of 75°F or higher qualifies. If the wind is over 10 mph, it is automatically excluded.
