A look at weather extremes and the big-picture climate connections.

Record Warm Nighttime Temperatures: A Closer Look

By David Kroodsma

July was hot: Washington, D.C., Oklahoma City, Philadelphia, and Austin set records for not just their warmest July in history, but their warmest month on record. The heat prompted people to hide indoors, crank up the air conditioning, or attempt stunts such as cooking eggs on the roof. But what made this month unusual wasn't only the hot days, but rather the hot nights.

Even though repeat heat waves brought sizzling hot days, overnight temperatures broke far more records: According to the National Climatic Data Center (NCDC), in July there were 6,106 record high minimum temperatures, and "only" 2,722 record high daytime temperatures.

Curious about these numbers, I looked more into the past decade of temperature records, and also spoke with Climate Central’s staff scientists. Have nights generally warmed faster than days? And if so, why? 

I downloaded data from NCDC's database of more than 5,000 weather stations across the United States. For each day since January 1, 2000, I looked at four possible records — two for the nighttime low temperatures (record low minimum and record high minimum), and two for the daytime high temperatures (record high maximum and record low maximum).



Since 2000, in the average month, record highs (high maximum temperature) beat out record lows (low minimum temperature) by a two to one margin. This is exactly what has been found in previous peer reviewed studies — including this study, published in Geophysical Research Letters in 2009, by Climate Central’s Claudia Tebaldi and three other researchers.

But looking at these four types of records, it appears that nights have warmed even more: the average month recorded 10 percent more record high minimum temperatures than record high maximums. The record low temperatures tell an even more compelling story: it was much more likely for the daytime temperature to be colder than average than it was for the nighttime ones. There were only 1,235 record low minimum temperatures set per month, while there were 1,697 record low maximum temperatures set per month. By this measure, a record cold day was 40 percent more likely than a record cold night.

Interestingly, when I looked at different months of the year, I found that the nighttime warming was even greater during the summer. During June, July and August, record high minimum temperatures outnumbered record low minimum temperatures by about three to one.

All of this is all a fancy way of saying “nights have warmed more than days.”

I spoke with Phil Duffy, Climate Central’s chief scientist, about why nighttime lows are warming faster than the daytime highs. He replied that the answer isn’t straightforward, and then he referred me to research that has shown that an increase in cloudiness (as well as a few other factors) has warmed nights more than days. During the day, clouds both warm and cool, as they act like a blanket to reflect heat back to the surface (warming), but they also reflect sunlight back to space (cooling). At night, they only warm temperatures, acting like an insulating blanket. Thus, nights warm more than the days, and this is exactly what climate models predict. In fact, this is a good example of climate models making a prediction (warmer nights), and then having the prediction born out by the data.

We will come back to this issue in future weeks, as we compile more data on nighttime temperatures and examine climate model projections more closely.

Below is an info-graphic that lets you look at these four types of records for any month since January 2000. Notice that there is a significant amount of variation between months, but that warm temperature records, and especially record high nighttime temperatures, outnumber the cold temperature records. (I also note on the graphic how many possible records there were in a given month, which is roughly the number of stations times the number of days in the month.) All of the data in the graphic comes from NCDC.



« Extreme Planet


By Abraham ben Judea (Fl)
on August 3rd, 2011

Just wondering…How much cooler would the night time be in America if we replaced the Dawn to dusk sensor on all these street lights with smart motion sensors, after all a single street lamp burns at a temp above1k degrees F. multiply this as follows 51 streetlights per mile, 74 million miles in america alone. It comes to about 3,774,000,000.
Think about it over 3.5 billion heating elements burning all night long…

Reply to this comment

By John Nielsen-Gammon (Bryan, TX 77807)
on August 4th, 2011

Of some relevance here is our study (Fall et al. 2011, JGR: http://pielkeclimatesci.files.wordpress.com/2011/07/r-367.pdf) that looked at surface siting issues in the US historical climatology network.  At most stations, nights have warmed faster than days over the past century, but according to the best-sited stations, the days and nights have warmed at an equal rate.  This is evidence (needing corroboration) that most or all of the max/min record discrepancy is due to changes in station siting & instrumentation.  No dispute with the overall warming, though.

Reply to this comment

By Andrew
on August 4th, 2011

Hi John,

Thanks for your comment. Do you suspect the same siting issues are responsible for the disparity between the huge number of overnight low temp. records set this summer so far, vs. daytime highs?

Seems to me that it would be difficult for the urban heat island effect, for example, to account for all of that.


Reply to this comment

By Abraham Ben Judea (FL)
on August 4th, 2011

Can county wide poweroutages be used to measure night temperature differences? It would be interesting to compare, Now that would be a study that would make headlines.

Reply to this comment

By Carolina Martini (Treviso/Italia/31100)
on August 4th, 2011

It has to be with the water properties. So as the trees and the forest mass reduces, the capacity of retaining the temperature decreases, and so when the night comes the termic isolation made by the plants doesn´t work. So this has to do with the urgency in having more native plants species in all the Earth. And it has to do, at the same time, with the necessity of a drastic decrease of our material consumption habits, so as to say that we must stop destroying our forests and stop consumption, and also start an holistic and enthusiastic way of living without any kind of stablishment concernings. I´m part of an ONG in Brazil called Amedi. Our site is being constructed. Be in touch if you can. Thank you.
Carolina Martini

Reply to this comment

By John Nielsen-Gammon (Bryan, TX 77807)
on August 4th, 2011

My guess is they’re responsible for a large part of it, and I think the urban heat island also plays a major role in preferring record warm nights over record warm days.  There are a lot of effects to sort through, and it’s hard to be quantitative about the relative roles of different factors, including greenhouse gas increases.

Reply to this comment

By Pierre Bull (New York, NY 10011)
on August 4th, 2011

Thanks for this post.  I liked your use of the map and showing where records were broken.  The explosion of red dots in just one month alone is, forgive the pun, ‘eye popping’.

Reply to this comment

By Sven Ake Bjorke (Norway, 4838)
on August 6th, 2011

The greenhouse gases ‘trap’ the infrared radiation going from the warmed earth surface to outer space, thus warming the atmosphere.
Technically, when the infrared rays hit a GHG molecule, it starts vibrating.
When this molecule vibrates, it makes adjacent gas molecules of any kind to vibrate as well.
Vibration is a form of energy that is converted to heat energy according to the physical laws of thermodynamics.

The GHG molecule vibrations makes evenings and early nights feel warm after a sunny day.
Without the GHG, the nights would immediately turn extremely cold at once the sunlight disappears.

An increased greenhouse effect therefore can be observed as increasingly warmer nights.
The warming effect of the lamps turned on at night time will have minimal effect, and probably not be measurable

Reply to this comment

By Dale Sedler (Franklin, IN 46131)
on August 8th, 2011

I can agree with the increased cloud cover as having an effect.  However, what I’ve noticed in Indiana and elsewhere seems to be an absense of the winter severe lows setting records or even matching them.  The coldest nights in winter occur when the sky is clear and infrared radiation escapes into space.  Increased CO2 would block that and the lows wouldn’t get as severe.  Is what I’ve noticed backed up in the data?  I’d be interested to know.  Thanks.

Reply to this comment

By rene (Vancouver)
on January 24th, 2013

Hi GUys

This is not rocket science why night time temperatures are rising faster then daytime temperatures———

we need a math whiz—some one please solve this:

average amount of heat absorbed by earth using a base temperature using data below

temp stats from World Meteorological Organization

“Between 1961 and 1990, the annual average temperature for the globe was around 57.2°F (14.0°C), “

please calculate how much heat the earth absorbed during the day and then released on average back into space at night time

then calculate the same model using these numbers
In 2011, the global temperature was about 0.74°F (0.41°C) above that

please calculate how much heat the earth absorbed during the day and then released on average back into space at night time using the 2011 numbers

Who wants to bet that the earth is still releasing the same amount of heat back into space now as in 1961-1990 stats?

Why? because our nights in 1961-1990 were as they are in 2011 about 10-12 hours long but the earth cannot cool fast enough because there is too much excess heat for it too cool off hence=global warming

Reply to this comment

Name (required):
Email (required):
Enter the word "climate" in the box below:

[+] View our comment guidelines.

Please note: Comment moderation is enabled. Your comment will not appear until reviewed by Climate Central staff. Thank you for your patience.