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Difference Between Night and Day

By: Dr. Ricky Rood, 5:20 AM GMT on June 20, 2012

Difference Between Night and Day

Somewhat to my aging surprise, I have resided in several states: North Carolina, Florida, Virginia, Maryland, Michigan and Colorado. Plus, I spent a year in Livermore, California. This is a climate blog, so I will get quickly to the subject, temperature. What I remember about the Florida air was that it was cleaner, had a remarkable pulse of sea breeze thunderstorms and lightning in the summer, and in the summer, it was not as hot as people complained about – at least during the daytime. I have spent far more time over a 100 degrees F in North Carolina, Virginia, Maryland, Colorado, and California than I have in Florida. Of course, this is the humidity not the heat.

When I was immersed in this heat, I used to think of the air as “heavy.” That’s the sort of thing that writers talk about, thick Southern air redolent with magnolia and sticky in mystery in the graveyard next to the swamp. So I used to think of this Florida air as so heavy and thick that it just couldn’t get hot. Of course, this is not a very smart way to think about heat and humidity. As I recall, water, two hydrogen atoms plus an oxygen one, is lighter than either the nitrogen or the oxygen molecules that make up most of air – not to mention that weighty molecule made of two oxygen atoms and a carbon atom.

Some time ago now I did a series of articles called Bumps and Wiggles. The sixth one in that series was called Water, Water, Everywhere and discussed the important role of water as a greenhouse gas. We have a lot of water; the oceans are full of it. Water evaporates from the ocean and lakes and the soil, and gets into the air. Warmer air holds more water, hence, if the temperature goes up, then we expect there to be more water in the air. Since water is a greenhouse gas, we would increase greenhouse warming with the addition of water vapor.

So back to Florida - there’s a lot of water in the air, but I stated above that my experience was that in the summertime during the heat of the day, it was not so hot. But nighttime is a different story in Florida. It does not cool down that much. There is a simple way to think about this. Daytime heating is strongly related to the presence of the Sun and direct heating by solar radiation. This radiation is primarily visible light. Carbon dioxide and water are not the primary absorbers of visible radiation. So this visible radiation goes through the atmosphere with relative ease. It heats the surface, and it is converted to thermal or infrared radiative energy. It is this infrared radiation that water and carbon dioxide absorbs and emits, acting like a blanket, and warming the surface. The infrared radiation is emitted day and night, and with the absence of the Sun at night, the greenhouse warming of the water vapor becomes prominent.

This relation between water vapor and daytime and nighttime temperatures is well known, and it influences decisions about where we live, and perceptions of nice and not so nice “climates.” Deserts are dry and notorious for wide swings between daytime and nighttime temperature. The lack of water vapor and clouds allows, at nighttime, the surface of the Earth to cool to space with some effectiveness. During the daytime it gets very hot. Look at the temperatures of Tucson and Phoenix, Arizona; they are frequently above 100 F. Florida has far smaller differences between daytime and nighttime temperatures. (Some day I will tell you about my July hike in the Anza Borrego Desert. Today, I will only mention that while planting this afternoon, 20120619, in Colorado the very dry dark soil was so hot I could not kneel on it. Happy plants.)

Last year, 2011, we had historical heat in large portions of the U.S., and it was notable that the nighttime lows were extraordinary and quite moist. (Rood from 2011). A couple of blogs ago, I mentioned the observation that there was a strong trend showing increase in nighttime lows – that is, it is not getting as cold at night as it used to (Heads and Tails: Still thinking about Spring 2012). This is suggestive of an increase in water vapor keeping the nighttime temperature high, but that is not the entire story. Because of the water vapor increase, there is more likelihood of clouds, and clouds have a stronger greenhouse effect than water vapor.

So I started this blog talking about Florida, and how the daytime temperature did not seem as hot as I might have expected. And, of course, I mentioned my not so bright idea of the air just being too laden with water. So I need to go back to that and think about that a bit. If there is water available to evaporate into the air, then it takes energy to convert that water from liquid to vapor. In the Florida afternoon, it gets hot, there are a lot of updrafts and downdrafts which enhance mixing, and there is a persistent flux of water from the ground and plants into the air. This evaporation consumes some of the heat, and effectively, limits how hot it gets. At night, of course, as the air cools, the water condenses, releasing some of that heat back – contributing to those warm nighttime temperatures.

Now I have a couple of threads wandering around here. I have water vapor and clouds acting as a greenhouse and keeping nighttime temperatures high. I have water vapor evaporating in the Florida afternoon and keeping daytime temperatures, in some sense, moderated. That is, it does not get as hot as one might imagine. I have introduced this idea of nighttime minimum temperatures increasing, and I have talked about these temperature increases being related to water, not carbon dioxide.

So, let’s answer the carbon dioxide question. If the climate were stable, that is, the Sun did not vary, carbon dioxide was not increasing, there were no volcanoes, etc. then water would still cycle back and forth between ice, liquid, and vapor. There would still be impacts on nighttime lows and daytime highs. Water vapor would be important to regional climate differences, like the difference between Florida and Colorado. We can reason that in this stable climate, over the course of the year, there would be a certain average amount of water vapor in the atmosphere. In this stable world, this average amount of water vapor in the atmosphere would be stable. When we add carbon dioxide, we make it a little warmer and the atmosphere can hold a little more water. We see here that a water vapor increase follows from carbon dioxide increase, greenhouse warming building on top of greenhouse warming.

That said, it is also possible to change the water regionally, for instance by irrigation. In this case there are regional impacts on the temperature that are not caused by carbon dioxide. Opposite of irrigation, there might be drying due to agriculture. From the discussion above, we might expect surface irrigation to contribute to nighttime warming and drying to contribute to daytime warming. In either case this is “climate change” caused by the activities of humans, but it is not directly caused be carbon dioxide increases. Such regional changes are very confounding to attribution studies of regional warming and cooling, and also cause of controversy amongst scientists and others who sometimes fight over one mechanism versus the other.

What I have described here has the potential for substantial complexity. If we just had carbon dioxide, no water, it would be trivial, at least in might be trivial, to understand warming. Adding water, ice, liquid, and vapor, makes the problem more complex. Then the fact that when water vapor condenses in the atmosphere it makes clouds – well, it’s a lot to manage. But, it might be reasonable to expect water vapor to have different effects on nighttime lows than daytime highs. It also means that great changes to the surface water budget, like the Dust Bowl and irrigation in the Corn Belt, would be expected to have important impacts on regional climate. I plan to address this more completely in the next couple of months.

I will end this entry with reference to a paper that in the march of climate science is getting old, Kukla and Karl, 1993, Nighttime Warming and the Greenhouse Effect (from Rood’s Class Website ) It is historically interesting to me. This paper talks about the decrease of day to night temperature variability that is occurring due to increases in the nighttime minimum. There are in 1993 and in the decade following many papers investigating this subject. The paper is written at a time when the greenhouse warming due to carbon dioxide had not so clearly emerged as it has today. It was also written when climate models were at least 2 full generations prior to today’s generation. That means that clouds, aerosols, and land-surface processes were not represented at the same level of fidelity as they are now. The paper marches through different mechanisms, setting the foundation for future investigation. A good read for readers of this blog.

As I understand it, heat wave coming to the East - Delaware, Maryland, Virginia, southeast Pennsylvania and southern New Jersey are expecting the heat index to reach from 100 to 105 degrees on Wednesday and Thursday (June 20 and 21, 2012). New York, Vermont and Massachusetts will see near record-breaking temperatures. Here in Colorado, Wednesday will be cooler, but Thursday back up to nearly 100. Keep an eye on those nighttime lows. Stay hydrated, especially if you are trying to use a fan to stay cool. Fans cool you through evaporation, and dehydrate you faster.

I am expecting this summer to offer many opportunities for such discussions. Here in Colorado, a land of scant water, I have just experienced both record daytime highs and record warm nighttime lows in the past couple of days. In the grand scheme of things, that’s relatively rare.


Figure 1: Self explanatory. Credits – several times removed, but taken from Planetsave. Isn’t the Web Wonderful?

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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.