Why Was July 2011 So Hot Across The US? (Explainer)


image: Mr. T in DC/CC BY-ND

There's likely no need to remind most TreeHugger readers how hot the summer of 2011 has been in the United States. We're still in the sweaty middle of it in many places. Even so, let's take stock a bit: How unusually hot has it really been, what's causing this meteorologically, and how hot will it likely get due to our changing climate? Is this sort of summer really just becoming the new normal?Let's focus on July as representative of what's happened in the season so far.

It's Not Just You, It Really Is Abnormally Hot
The NOAA Satellite and Information Service has released an overview of the state of the climate for July, which really breaks down the extremes of the situation.

For July the average US temperature was 2.7°F above the 20th century average, at 77°F. That means July 2011 was the fourth hottest July and fourth warmest month on record--which generally goes back to the second half of the 19th century. Just seven states in the lower 48 states had July temperatures with average of below average temperatures; all others had temperatures ranging from above normal to record warmest.

In that latter category were Oklahoma and Texas, each of which had their hottest month since records began at 88.9°F and 87.1°F respectively. In the case of Oklahoma, the heat was so extreme that it now stands as the highest average temperature record for any state in any month.


images: US Drought Monitor

Keep in mind that those temperatures accompany record-breaking drought in this region as well. Every single county in Texas has been declared a disaster area due to abnormally low rainfall, worse than any in the past century. As the chart above indicates, only a very small portion of the state isn't experiencing extreme drought, as well as extreme average temperature.

In terms of specific cities, Dallas had temperatures above 100°F for all but one day in July. Newark NJ, Washington DC, and Portland ME all set new single day record highs (108°F, 105°F, 100°F). All told 78 places in the US tied or broke daily temperature records.

Nights Warming More Than Days: Important
Those record highs only tell part of the story though. Perhaps more important is the fact that nights aren't cooling off as much as they normally do.

NOAA records show that 213 places tied or broke records for the hottest minimum daily temperature in July.

On the 12th, Richmond VA only cooled off to 81°F; on the 23rd and 24th the temperature at Washington/Reagan airport only dropped to 84°F; on the 26th Dallas-Forth Worth airport so a low temperature of 86°F and Ponca City OK only dropped to 91°F at night.

That ratio between new record daily highs and record daily lows is indicative of a trend that's crucial to understanding how the climate is changing.

Over time the ratio between new record hot temperatures and new record cold temperatures should be about 1:1, but over the past decade or so that ratio has increased to about 2:1--largely due to the fact that nights aren't cooling as much as they used to. In recents months that ratio has been even higher, but that's just looking at the shorter time period.

The importance in this is that record warm nights have serious implications for crop yields (for some crops increasingly warm nights have a greater negative impact than warm days) and for the ability of both human and non-human animals to survive heat waves. High nighttime temperatures are behind many deaths in heat waves in general, and in the latest heat wave were implicated in the deaths of thousands of cattle in South Dakota.

What Was the Meteorology of This?
As for the meteorology of all this, what happened was that a vast "heat dome" formed across the central part of the nation, Huffington Post explains. This trapped heat and moisture underneath it, while pushing the jet stream's cooler drier air to the north and sucking hot, humid air from the Gulf of Mexico farther north than usual.

As for a more technical explanation, Senior Meteorologist for Weather.com Stu Ostro writes:

For extremity of summertime ridges of high pressure aloft over the United States, meteorologists look at "500 millibar heights" -- in non-technical terms, that represents how high the pressure is a few miles above the Earth's surface -- and in particular how close those heights get to 600 decameters (19,685 feet). That's the benchmark, as it's about as high as those "heights" ever get.


image: Wright Weather

Ostro goes on to explain how drought conditions across the South played into the extreme heat and how wetter than usual conditions further north boosted humidity:

In the southern states, particularly the southern Plains, afternoon high temperatures have consistently been particularly extreme, assisted by how dry the soil is. Rather than some of the sun's energy going into evaporating soil moisture, it gets efficiently converted into quickly-rising temperatures each day.

And in turn, the soil dries out even more, worsening the drought.

Immediately adjacent, it's been the opposite, with exceptionally wet conditions including record flooding.

Soil and crop moisture evaporating will boost the dewpoint, which translates to how humid the air feels. Dewpoints and heat indices are expected to rise to exceptionally high levels as far north as parts of the Dakotas and Minnesota this weekend into early next week.

Check out the juxtaposition of extremely wet conditions in the northern plains and the drought across the south just prior to the July heat wave:


image: Weather.com
How Hot Might It Get?
With all the extreme heat this summer you'd be right in thinking that we ought to be really shifting what 'normal' temperatures are. In fact NOAA has done this. From 1981-2010 the average temperature for the nation is 0.5°F warmer than the 1971-2000 period, representative of global average temperatures warming. Just looking at the 21st century so far, compared to the 1970s, it's about 1.5°F warmer now on average.

What's considered normal for any given month varies more widely. For example, the old normal average for Richmond, Virginia on July 1 was 88.7°F. It's now 90°F.

But the big question is really 'how hot will it get?'.

There are many factors still in play on how much climate change will warm the United States--and our personal and national actions in reducing greenhouse gas emissions still will have a huge influence on how hot it will get absolutely and on average.

Recently there have been a number of attempts to drive home the notion that if this summer seems hot, you better get used to it because what's abnormal in 2011 will be commonplace in coming decades.

Research published in Climatic Change concludes that by 2030 the tropics could see "the permanent emergence of unprecedented summer heat"--and more broadly, that the coolest summers then will be warmer than the hottest summers of the 20th century.

In 2010, in the midst of another record-breaking heat wave and Philadelphia experienced an exceptional 17 days above 90°F, Climate Central pointed out that by 2050 this will be normal, expected conditions.

Other research, published in Geophysical Research Letters at the beginning of July, finds that exceptionally long heat waves could be commonplace in the US by 2040.

The team analyzed temperature data from the continental US between 1951-1999 to determine the longest heat waves for that time period and then used these results in a number of climate models. They found that an intense heat wave, equal to the longest on record in the second half of the 20th century, was likely to occur as often as five times between 2020 and 2029 in the central and western portions of the nation.

The chart below shows how the nation as a whole fares.


image: Noah Diffenbaugh/Stanford University

Going back to the ratio of record highs to record lows: June's ratio was 11:1. Projections on how that could change show that if we successfully reduce emissions to a level that will stabilize the climate, by 2050 we may be able to bring the ratio down to 8:1. Should we continue with business as usual emissions, record high temperatures could outpace record lows by 25:1 by 2050 and even perhaps an astonishing ratio of 50:1 by 2100.

As for the effects of both extreme heat waves and rising temperatures, well, they are too complex to deal with in a single post--indeed much of the archive of TreeHugger deals with this in one way or another.

But at the personal, immediate level one thing is certain: We all would be well served by consciously starting adjusting ourselves to summers being hotter than what we have been used to, to stop being shocked by this sort of heat and start getting on with it as the usual summer weather.

More on Global Climate Change
Climate Change to Spur Major Rise in Heat Wave Deaths in Chicago
New Research Shows Baltimore Heatwave Was Worsened by DC's Hot Air

Tags: Global Climate Change | Global Warming Effects | United States

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