More about the hot weather in North America, and more

In the comments today, under the article about the extreme weather around the world these past few days, there was a claim that "It was warmer in the US in 1955". There was nothing else except a link to two maps of the USA. The top map was labeled 2015 and the bottom map was labeled 1955. The maps were coloured but there was:

• no legend
• no date
• no information that would explain anything about what the charts were meant to represent.

That sort of behaviour is more common on denier blogs than it is here at HotWhopper. Anyway, it prompted me to do some reading and research, and in the process I got diverted a bit into US temperature records, and trends in diurnal temperature range. So this article is a bit of a wander, and a bit long.

How hot was it in 1955 in the USA?

First, though, let's see about the very short and somewhat cryptic comment from HotWhopper reader, Andy Wilkins. He wrote that it was warmer in the US in 1955. But was it? No, it wasn't. At least not if you are looking at mean annual surface temperatures.

Below is a chart showing the annual mean surface temperature for the contiguous USA from 1895 to 2015 (average to November). I've marked the mean temperatures for 1955 and 2015, and this year so far is 1.31 °C hotter than the annual mean temperature in 1955.

Data source: NOAA ClimDiv

Now if Andy had somehow mistaken 1955 for 1954, then the difference between then and now would have still been 0.41 °C . That is it's been 0.41 °C hotter this year so far than it was back in 1954. (Note: The US temps and chart were corrected shortly after posting.)

I dug a bit deeper into Andy's comment, which took me around deniersville. I found myself tripping from the blog of a UK denier to a video from a US denier. The image that Andy linked to turned out to be from a YouTube video from Joe Bastardi, the science denying weather forecaster from Weatherbell. He's the chap who four years ago said that we are heading for mini ice age. Oops!

Data source: GISS NASA

Anyway, here is the basis for the claim that it was hotter in 1955 in the USA. It's a map of the NCEP/NCAR Reanalysis of temperature around North America for 24 December in 1955 and 24 December in 2015. I've animated it to show the same day for both years. Yes, it's just a day - not the whole year.

Source: Joe Bastardi on YouTube

There is a huge blob of very cold air over the north west in 1955, which is smaller and warmer in 2015. There's also a big blob of cold air to the east that isn't there this year. The red blob of warm air is also bigger and positioned differently in 2015 than it was in 1955. It stretches right the way up to the Arctic this year. In 1955 it was positioned over the middle of the USA. More of the contiguous USA (not including Alaska) was warm, but in different parts to this year. Averaged overall, though, North America and its surrounds were warmer this Christmas Eve than in 1955.

By the way, the image that Andy linked to had a lot of the cold bits cropped out from 1955 and warm bits cropped out from 2015. Here it is, so you can compare it to the images from Joe Bastardi's video as shown just above:

Source: denier blog of Paul Homewood who cropped it from a video by Joe Bastardi

I wonder if what Andy was trying to say was that there was nothing unusual about the temperatures in the USA recently, despite the records being broken. And despite the fact that Joe Bastardi had to trawl all the way back to 1955 to even get close to the recent temperatures - and he could still only find one day! Even then the denier map had to chop off the surrounding regions, because they spoilt the story. (Or maybe he was wanting to say that climate science is a hoax because ... Christmas Eve.)

To complete the 1955 to 2015 picture, here is an animation using the NCAR reanalysis, comparing December 24 in 1955 with December 24 in 2015. The plots are in Kelvin. It was quite a bit hotter on Christmas Eve this year pretty well everywhere. Notice the polar regions, and the tropics.

Data source: NCEP Reanalysis

The point is...

Whether or not the mean daily temperature on 24 December for the contiguous USA was as warm or nearly as warm as it was this year on 24 December might be interesting if you play Trivial Pursuit. However it tells you next to nothing about the overall warming trend in the USA, or North America, or the entire world.  And neither does it detract from the fact that this December has seen some very unseasonal warmth across eastern North America (and much of western Europe, too).

It's been warm in much of the USA

Here is how Bob Henson and Jeff Masters at Wunderground.com described the unseasonal warmth this December:

Hundreds of records were buried by sunshine, warmth, and humidity instead of white-Christmas snowfall all across the eastern U.S. during the holidays, especially on Thursday and Friday. Christmas Day was the apex for the north-south breadth of warmth, with record highs set from Florida (82°F in Jacksonville) to Maine (62°F in Portland). Many records on Thursday and Friday were smashed by margins of 10°F or more. The Christmas Eve readings of 72°F at Albany, NY, and 68°F at Burlington, VT, both set all-time records for December. As noted by WU weather historian Chris Burt, these are truly impressive records given the late date in a month that gets progressively colder, not to mention the long periods of record at both sites (since 1883 in Burlington and 1874 in Albany). Chris adds that Philadelphia has seen eight days this month through Sunday with record daily highs: “Not since records began in Philadelphia back in 1874 has any other month of any single year experienced as many daily record highs as this December!” The capital of Christmas commerce, New York City, basked in record warmth of 72°F on Thursday and 66°F on Friday. As of Sunday, Central Park had yet to get below 32°F this fall or winter; its monthly average (12/1 – 12/26) of 52.0°F was running at an astonishing 13.8°F above normal and 7.9°F above the previous December record, going back to 1871. A cooldown this week will reduce that value, but a warmest-on-record December is all but certain for much of the eastern U.S. It’s no wonder that flowers and shrubs are blossoming from Washington to New York.

Daily records USA

Below is a table from NOAA, listing the daily records broken in the last week and month.

Period	Hi Max	Hi Min	Low Max	Low Min	Precip-itation	Snow-fall
Last 7 Days	1,210	1,412	29	12	534	127
Last 30 Days	3,902	5,364	319	276	2,778	663
Last 365 Days	28,682	40,491	18,527	11,384	29,967	6,874
Month to date	3,879	5,301	166	159	2,572	610
Year to date	28,637	40,425	18,098	11,192	29,881	6,736

The table below shows the number of daily low maxima and low minima as a percentage of the daily high maxima and minima. Over the last week and month, the lows were all less than 10% of the highs. In the last seven days, the number of record low minima was only 0.8% of the record high minima

:

Period	Low Max	Low Min
Last 7 Days	2.4%	0.8%
Last 30 Days	8.2%	5.1%
Last 365 Days	64.6%	28.1%
Month to date	4.3%	3.0%
Year to date	63.2%	27.7%

Monthly records USA

The tables above are the daily records. Below are the monthly records.

Period	Hi Max	Hi Min	Low Max	Low Min	Precip-itation	Snowfall
Last 7 Days	26	145	0	0	20	1
Last 30 Days	56	283	9	10	238	19
Last 365 Days	1,437	2,081	512	451	1,170	476
Month to date	56	283	1	0	232	19
Year to date	1,437	2,079	504	451	1,168	469

The chart below shows the number of monthly record low maxima and minima as a percentage of the monthly record high maxima and minima, which in all cases were more than the record lows. In the last seven days, there were no recorded record monthly low minima.

Period	Low Max	Low Min
Last 7 Days	0.0%	0.0%
Last 30 Days	16.1%	3.5%
Last 365 Days	35.6%	21.7%
Month to date	1.8%	0.0%
Year to date	35.1%	21.7%

It's easy to see how unusual the most recent week and month have been. Given global warming, it's not unexpected that there are more high records than low records.

Fewer low minima records than low maxima records

Over the past year for the USA the low maxima average out at around 64% of the high maxima, while the low minima are only around 28% of the high minima.

I've had a look at the literature to see why night time minima would be rising more than daytime maxima. Sometimes I read about how a reduction of the diurnal range of temperature is a sign of global warming, or at least that nights should warm faster than days. After reading up a bit, I discovered that it's not that simple. (An extensive study was done a few years ago (in 2006), which documented changes in extreme temperature (and precipitation) across the world.)

There have been a number of papers on the subject of trends in diurnal temperature range (DTR). The classic paper is from 1999, by Aiguo Dai, Kevin E. Trenberth and Thomas R. Karl - the title is self-explanatory: "Effects of Clouds, Soil Moisture, Precipitation, and Water Vapor on Diurnal Temperature Range". Cloud cover makes a difference. As the authors wrote: "Clouds can reduce Tmax by reflecting the sunlight and increase Tmin by enhancing downward longwave radiation".  Clouds are not all the same, though. Different clouds have different effects. Soil moisture and precipitation also make a difference, for example there can be a cooling effect of evaporation at the surface in the daytime, which is less at night.

If you're interested in exploring the subject, I've listed some references below. As I said, it's not straightforward. Here is something to whet your appetite, from the concluding section of Dai99. It's quite long, and you'll probably see why I decided not to try to summarise. It's complicated:

Our analysis of the daily data from the FIFE site and the global weather stations shows that clouds, soil moisture, and precipitation can reduce the surface diurnal temperature range by over 50% compared with clear sky days, while atmospheric water vapor increases both the Tmin and Tmax and has small effects on DTR over most land areas except the northern high latitudes where DTR tends to be larger in high humidity days in winter and autumn. Changes in wind directions, such as those associated with the passing of synoptic systems, can greatly alter the daily mean temperature but generally do not affect DTR significantly at the FIFE site. Clouds, which contribute most of the DTR reduction and largely determine the mean magnitudes of DTR over most regions, reduce DTR by sharply decreasing surface solar radiation and thus the daytime maximum temperature, while soil moisture increases surface latent heat releases and slows down the temperature rise during the day in warm seasons. However, surface sensible heat fluxes tend to offset a large part of the latent heat release anomalies and make soil moisture less effective in damping DTR. Precipitation affects DTR mostly by increasing the soil moisture content while its direct damping on DTR is relatively small. The nighttime minimum temperature is largely controlled by the greenhouse effect of lower atmospheric water vapor, while the daytime maximum temperature depends heavily on the surface solar heating, which is strongly affected by cloud cover, and the amount of it that is released into the air by sensible and latent heat, which depends on soil moisture content. Stronger winds tend to reduce DTR over islands, western Europe, and some other coastal areas, but have a small effect on DTR over most inland areas.

Correlations between the nighttime minimum temperature and total, low, middle, and high cloud amounts are weak in all seasons in low latitudes and in summer in high latitudes. This suggests that, except for the winter high latitudes where solar radiation is at a minimum, the nighttime greenhouse warming effects of clouds tend to balance their daytime solar cooling effects on afternoon temperatures, resulting in small net effects on the nighttime minimum temperature. This further suggests that clouds damp DTR mainly by reducing the daytime maximum temperature over most land areas while their downward longwave radiation contributes little to the DTR reduction mainly because it has a relatively small diurnal asymmetry. Our results are consistent with those of Power et al. (1998), who find that over Australia annual precipitation, which is highly correlated with cloud cover, negatively correlates with Tmax but generally not with Tmin.

Clouds with low bases are found to be most efficient in reducing the daytime maximum temperature and DTR. High and middle clouds have only moderate damping effects on DTR mainly because they are usually optically thinner than clouds with low bases.

The reduction of DTR by clouds is largest in warm and dry seasons (e.g., SON for many northern midlatitude regions such as the United States, southern Canada, and Europe) and smallest in the winter high latitudes where sunshine is largely absent. This is expected because during warm and dry seasons surface latent heat release is limited so that the daytime maximum temperature depends more on the solar heating and thus clouds.

A later paper by Ryan Eastman, Stephen G. Warren and Carole J. Hahn (2011) showed that total cloud cover increased from the middle of last century through to the late 1990s, but then decreased, mostly because of a reduction in low level cloud. Over the same period high clouds increased. That paper also looked at the relationship between cloud cover and sea surface temperature, which was interesting and showed some differences in different parts of the oceans.

In 2012 there was a paper published in 2012 by Ryan G. Lauritsen and Jeffrey C. Rogers, in which they discussed different factors affecting the diurnal temperature range in different regions across the USA. Just as the other papers demonstrated, the relationships are not simple, with different factors dominating in different regions.  That is, different regions have different patterns of cloud cover, precipitation, and soil moisture. The diurnal temperature difference can also be affected to some extent by atmospheric circulation teleconnections.

I don't know if there's any consensus about what the future will hold for clouds of different types in different regions. It's hard enough for scientists to figure out cloud cover from observations. (Read this paper by Seiji Kato et al to get an idea of some of the difficulties, and how different estimates of cloud cover will lead to very different estimates of radiative forcing.)

References and further reading

Alexander, L.V., Zhang, X., Peterson, T.C., Caesar, J., Gleason, B., Klein Tank, A.M.G., Haylock, M., Collins, D., Trewin, B., Rahimzadeh, F. and Tagipour, A., 2006. "Global observed changes in daily climate extremes of temperature and precipitation". Journal of Geophysical Research: Atmospheres (1984–2012), 111(D5). doi:10.1029/2005JD006290. (open access)

Dai, Aiguo, Kevin E. Trenberth, and Thomas R. Karl. "Effects of clouds, soil moisture, precipitation, and water vapor on diurnal temperature range." Journal of Climate 12, no. 8 (1999): 2451-2473. doi: http://dx.doi.org/10.1175/1520-0442(1999)012<2451:EOCSMP>2.0.CO;2 (open access)

Ryan G. Lauritsen and Jeffrey C. Rogers, 2012: "U.S. Diurnal Temperature Range Variability and Regional Causal Mechanisms, 1901–2002." J. Climate, 25, 7216–7231. doi: http://dx.doi.org/10.1175/JCLI-D-11-00429.1 (open access)

Ryan Eastman, Stephen G. Warren, and Carole J. Hahn, 2011: Variations in Cloud Cover and Cloud Types over the Ocean from Surface Observations, 1954–2008. J. Climate, 24, 5914–5934. doi: http://dx.doi.org/10.1175/2011JCLI3972.1 (open access)

Kato, Seiji, Sunny Sun‐Mack, Walter F. Miller, Fred G. Rose, Yan Chen, Patrick Minnis, and Bruce A. Wielicki. "Relationships among cloud occurrence frequency, overlap, and effective thickness derived from CALIPSO and CloudSat merged cloud vertical profiles." Journal of Geophysical Research: Atmospheres (1984–2012) 115, no. D4 (2010). DOI: 10.1029/2009JD012277 (open access)