Archive for September, 2025

The Hottest Summer Days in the U.S. Have Barely Warmed in the Last 40 Years

Thursday, September 18th, 2025

The total warming of the hottest 3 days in each summer month averaged across 400 mostly-airport weather stations is only 1.2 deg. F over 40 years.

I recently posted about the weather observations from Reagan National Airport that showed the warmest days of summer have experienced no statistically significant warming in the last 40 years, despite this being the period of maximum radiative forcing from increasing atmospheric CO2.

Of course, you would never know this based upon media reports… in fact, most people are probably under the impression that our hottest days are rapidly getting hotter.

One commenter on my post (correctly) pointed out that what I presented was just one weather station. Well, now I have processed ~400 mostly-airport (WBAN) weather stations and over 2,000 cooperative observer (COOP) stations across the U.S.

Here’s a plot of those station locations.

The period I’m addressing is the last 40 years (1985-2024) because we have Landsat-based Impervious Surface (IS) cover data at high spatial resolution (30 m) for those years, and I’m looking at how recent warming trends are impacted by the urban heat island (UHI) effect. IS is a percentage cover of Landsat pixels by roads, parking lots, buildings, and other human development impervious surfaces.

Daily High Temperature (Tmax) Results

I don’t like “heat waves” as a statistical quantity. It is “binary”, which means it has an arbitrarily chosen threshold of temperature and number of days of duration, and those can be manipulated to give very different results for heat wave trends.

Instead, I computed a statistic which has no threshold, is always the same number of days, and occurs every month: the average of the 3 warmest (and 3 coolest) days in each summer month (June, July, August) during 1985-2024.

I can then compute trends in those, just like is usually done for the average of all daily Tmax (or Tmin). I did this separately for the mostly-airport (WBAN) stations which are well maintained for aviation safety reasons, and for the COOP stations which have varying and mostly unknown levels of quality control, siting, etc.

Since people are used to looking at time series, we will start with the multi-station average summer temperatures for 3 of the 9 U.S. climate regions as defined by NOAA/NWS. From top to bottom, these are the Upper Midwest, the Northeast, and the Southeast; I have offset the warmest-3 and coolest-3 day results for legibility:

Note how much more slowly the warmest 3 days per month are warming compared to the coolest 3 days. As an example, for the Northeast U.S. climate region (PA/MD and northeastward), the hottest summer days have been warming at an average rate of 0.10 C/decade, which equates to 0.7 deg. F over 40 years. All 9 climate regions exhibited this feature, by varying amounts. Again, these results are all for daily maximum temperatures, Tmax.

Next, I took all of the stations in the U.S., and split them into 7 equal-size groups of increasing IS growth which I am using as a proxy for urbanization for the purposes of temperature impacts of the urban environment. These plots are different: The temperature trend is on the vertical axis, while the category of urbanization growth is on the horizontal axis. Again, these results are for Tmax; the results for WBAN stations are on the left, and for COOP stations are on the right:

There is little dependence of the 40-year temperature trends on the rate of growth in urbanization (IS trend), maybe just slight upward slope with the most rapidly urbanizing stations experiencing a little higher warming trend. The generally higher trends at low values of IS growth (especially in the COOP data) are because most of those stations are in the western U.S., where warming trends have been greater. I wouldn’t put too much faith in the absolute values of the COOP trends because no time-of-observation (TOBS) adjustment has been made. But that should not affect the spread between warmest and coolest days.

What really stands out is the fact that the coolest summer days are warming much faster than the warmest summer days. The difference in warming trends is about 0.35 C/decade in the WBAN data, a little less in the COOP stations. This suggests a moderation of summer temperatures, with less variability.

Averaged over all 400 WBAN stations, the warming trend equates to only 1.2 deg. F of warming in 40 years. I would wager this weak upward trend in the warmest summer days is much less than what most people would expect, given media coverage of “heat waves”.

And if you are wondering how the trend in the average of all Tmax temperatures in the month compares to NOAA’s official homogenized, area-averaged dataset, they are about the same, to within 0.01 or 0.02 deg. C/decade

Daily Low Temperature (Tmin) Results

As seen in the next plot, the effects of increasing urbanization are much more pronounced in daily minimum (Tmin) than daily maximum (Tmax) temperatures, with the greatest warming trends occurring at stations with the fastest growth in impervious surfaces.

Note that these plots allow one to estimate what the station average warming trends would be in the absence of urbanization by just looking at where the regression lines intersect the vertical axis (IS trend = 0). Remember, the 7 IS trend groups have equal numbers of stations. If those values are used for the “climate signal” (as opposed to the increasing urbanization signal), the trends are not too different from those in Tmax

Conclusion

My main takeaway is that, contrary to what we have been told, there has been very little warming of the hottest summer days averaged across the U.S. in the last 40 years. The second takeaway is that nighttime (Tmin) temperatures are warming rapidly with urbanization, but when those statistics are extrapolated to no growth in urbanization, the average Tmin warming trend is greatly reduced, especially for rapidly growing locations.

The Hottest Summer Days in D.C. Have Not Gotten Hotter in Last 40 Years

Tuesday, September 2nd, 2025

…but the coolest summer nights have warmed by 5 deg. F.

John Christy and I continue to examine U.S. air temperature trends, especially those in summer, and John has recently been looking at “heat wave” statistics.

My interest is in determining how much the urban heat island (UHI) effect has impacted reported warming trends. Last year we published a paper using population density as a proxy for urbanization, and found that about 60% of U.S. urban and suburban warming trends in Tavg (the average of the daily maximum [Tmax] and minimum [Tmin] temperatures) since 1895 in the “raw” (non-adjusted) temperature data could be accounted for by urbanization.

But we also found that relationship largely disappeared by the 1970s, with little warming since then being accounted for by increases in population density.

Landsat Impervious Surface Data

We used population density in that study because the datasets are global and extend back to the 1800s (and even earlier). But the most direct physical relationship to UHI warming would be the coverage of the area around the thermometer by impervious surfaces (IS). Those data are now available at 30 meter resolution from Landsat for each year between 1985 and 2024 (40 years). IS might well reveal UHI effects in cases where population density is no longer increasing but wealth has increased (more air conditioning, Dollar Generals, etc.)

But I’m not going to show IS data today, that’s for another time. I’m only explaining how I got here.

D.C. Urban Warming Trends: The Difference is Like Day and Night

For now I’m examining metro areas (which is what the EPA Heat Wave papers also do), using airport ASOS measurements which is what the National Weather Service and FAA mostly rely upon. These systems are well-maintained since their primary purpose is to support air traffic safety.

I started with the center of America’s universe, Washington D.C. And I also decided that something better than a “heat wave” index was needed.

The heat wave (like pornography) is difficult to define, but you know it when you see it. How many days in a row constitute a heat wave? And how hot do those days have to get? Above the 85th percentile? 90th percentile? Those questions do not have definitive answers.

Also, by choosing a binary variable, there is no gray area available for days that are almost a heat wave (oh, sorry, there were only three days above 100 deg. F, so you didn’t meet the 4-day threshold). Such definitions lead to dodgy statistics, such as computed trends in heat waves,

So, I decided (as a meteorologist) that the hottest days in each month make more sense to keep track of for climate trends. I decided on the average of the 3 hottest daily maximum temperatures in each summer month (June, July, and August) as a potentially useful metric, which is approximately the hottest 10% of the days in the month. This metric always exists, every month, every year, and it always has 3 days. This is good for statistical analysis.

But then I thought, why stop there? What about the 3 coolest Tmax days each month?

Which then led to, “What about the warmest and coolest 3 days minimum temperature (Tmin) measurements?”

So, I started with Washington D.C., Reagan National Airport, which is used by your favorite congresspersons and presidents (as well as the public) to keep track of how hot it’s getting.

The results surprised me. Here are the temperature trends in those different categories. What is amazing is that the coolest summer nights in DC have warmed 10 times faster than the hottest summer days:

In fact, the trend in the hottest days’ temperatures is not even statistically significant, at only +0.12 deg. F per decade, which is just under a total of 0.5 deg. F warming in the last 40 years. No Boomer would notice that in their lifetime.

But look at those nighttime temperatures! The coolest nights have warmed by almost 5 deg. F in the last 40 years. This is clearly dominated by the UHI effect, since climate models tell us that days and nights should be warming at much closer to the same rate.

Now, Washington D.C. might be an outlier for urban areas. I’m just starting down this road, so we shall see. But I’ll bet most people would not have expected these results if they have been watching the local D.C. TV stations’ weather and news coverage.

UAH v6.1 Global Temperature Update for August, 2025: +0.39 deg. C

Tuesday, September 2nd, 2025

The Version 6.1 global average lower tropospheric temperature (LT) anomaly for August, 2025 was +0.39 deg. C departure from the 1991-2020 mean, up a little from the July, 2025 anomaly of +0.36 deg. C.

The Version 6.1 global area-averaged linear temperature trend (January 1979 through August 2025) remains at +0.16 deg/ C/decade (+0.22 C/decade over land, +0.13 C/decade over oceans).

The following table lists various regional Version 6.1 LT departures from the 30-year (1991-2020) average for the last 20 months (record highs are in red).

YEARMOGLOBENHEM.SHEM.TROPICUSA48ARCTICAUST
2024Jan+0.80+1.02+0.58+1.20-0.19+0.40+1.12
2024Feb+0.88+0.95+0.81+1.17+1.31+0.86+1.16
2024Mar+0.88+0.96+0.80+1.26+0.22+1.05+1.34
2024Apr+0.94+1.12+0.76+1.15+0.86+0.88+0.54
2024May+0.78+0.77+0.78+1.20+0.05+0.20+0.53
2024June+0.69+0.78+0.60+0.85+1.37+0.64+0.91
2024July+0.74+0.86+0.61+0.97+0.44+0.56-0.07
2024Aug+0.76+0.82+0.69+0.74+0.40+0.88+1.75
2024Sep+0.81+1.04+0.58+0.82+1.31+1.48+0.98
2024Oct+0.75+0.89+0.60+0.63+1.90+0.81+1.09
2024Nov+0.64+0.87+0.41+0.53+1.12+0.79+1.00
2024Dec+0.62+0.76+0.48+0.52+1.42+1.12+1.54
2025Jan+0.45+0.70+0.21+0.24-1.06+0.74+0.48
2025Feb+0.50+0.55+0.45+0.26+1.04+2.10+0.87
2025Mar+0.57+0.74+0.41+0.40+1.24+1.23+1.20
2025Apr+0.61+0.77+0.46+0.37+0.82+0.85+1.21
2025May+0.50+0.45+0.55+0.30+0.15+0.75+0.99
2025June+0.48+0.48+0.47+0.30+0.81+0.05+0.39
2025July+0.36+0.49+0.23+0.45+0.32+0.40+0.53
2025Aug+0.39+0.39+0.39+0.16-0.06+0.69+0.11

The full UAH Global Temperature Report, along with the LT global gridpoint anomaly image for August, 2025, and a more detailed analysis by John Christy, should be available within the next several days here.

The anomaly in the tropics (20N – 20S) has dropped considerably, to +0.16 deg. C. The U.S. was below the 30-year average in August.

The monthly anomalies for various regions for the four deep layers we monitor from satellites will be available in the next several days at the following locations:

Lower Troposphere

Mid-Troposphere

Tropopause

Lower Stratosphere