It’s time to talk global warming again. New high temperature records are outpacing new low temperature records, but not by as much as you would think, given the recent uptick in global temperatures.
But if you look at the new record high daily lows (yeah, that sounds a little funny, but you’ll see what it means), things look a little different — a little hotter.
The June Maximum And Minimum Temperature Records In The United States
In the past year, there have been 22,586 new high daily temperature records in the United States. In the same time, there have been 17,022 new daily lows. That’s about 25% more daily highs than lows.
But in the last year there have been 34,065 new high daily minimum temperature records, more than 50% more than the number of daily new maximums.
The clear implication is that the range between high and low daily temperatures is getting smaller. Why?
Before we get on with the explanation, let’s look at this graphically, which shows what’s happening in easy-to-understand form.
If we look at the overall temperature map of the US, the latest month (June) was the second warmest on record (there are 121 years in the record). For minimum temperature, June ranked as warmest all-time. But for maximum temperature it was only thirteenth warmest.
If we look at the data state-by-state, the difference in changes in maximum and minimum temperature become stark. The departure from the long-term average of minimum temperature is greater than that of maximum temperature in every state. Both, of course, are above the long-term average.
There are a number of theoretical reasons why the difference between minimum and maximum temperature should be narrowing.
Global Warming Implies An Increase In Cloudiness
The atmosphere has an ability to contain water vapor molecules. But that capacity is not unlimited. At a certain point, the atmosphere cannot accommodate any more water vapor molecules and a further increase results in condensation.
In the air more than a few feet above the ground, we call the condensation droplets a cloud; near the ground we call them fog. The atmospheric tolerance for water vapor increases as the temperature increases. And the warmer air provides the energy to rip water molecules off the ocean surface or other bodies of water (that was a fancy way to describe evaporation).
Thus, global warming implies that there should be more water vapor in the atmosphere. Since clouds are made of water vapor, we should not be surprised if there are more clouds.
The Effect Of More Clouds On Temperature
It is obvious from the amount of light that reaches the ground on a cloudy day versus a sunny day, that clouds block sunlight, mainly by reflecting it to space. The percentage of incoming radiation reflected by a surface is called the albedo.
An increase in cloudiness would re-direct more of the sun’s radiation; the albedo of clouds varies from ten to ninety percent, depending on the type and thickness. On average, clouds have a higher albedo than the earth’s surface. The albedo effect of increased cloudiness is a negative feedback of global warming, and very likely the reason that daytime maximum temperatures are rising more slowly than nighttime minimums: Clouds only reflect sunlight during the daytime.
On the other hand, clouds trap the earth’s microwave radiation. In addition, water vapor is a greenhouse gas, so even in the absence of more cloudiness, more water vapor would increase the greenhouse effect.
It turns out that it is extremely difficult to get an objective measure of clouds. They can be thin or thick, high or low, deep or shallow. Virtually all recorded observations of cloudiness involve a subjective interpretation from a human being.
Most of the observations indicate that cloudiness is indeed increasing, but that is not certain over the whole earth — or even over the United States.
However, there is one type of cloud that is clearly increasing: the contrail. Contrail is a short way to say condensation trail, and these clouds form in the wake of airliners flying at high altitudes. We’re all familiar with them. The airliner exhaust contains water vapor plus various kinds of particles that can act as condensation nuclei.
And we have a unique measure of their effect, compliments of Osama Bin Laden
The Airline Shutdown In The Wake Of 9/11
After the destruction of the World Trade Center towers in 2001, all civilian air travel in the United States was shut down for three days. So meteorologists had a unique opportunity to measure the effects that no contrails would have.
The temperature measurements for those three days show a two degree wider spread between high and low temperatures than the recent average. This is the best evidence we have that clouds affect the temperature, and that the effect is to narrow the spread between daily maximum and nighttime minimum.
Climate Change From Global Warming Is Not Uniform
The fact that the minimum temperatures are rising more than the maximum temperatures is just one more piece of evidence that global warming can produce unexpected results.
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