Diurnal Trends in Dewpoint Averaged over the U.S. Since 1973

June 4th, 2013 by Roy W. Spencer, Ph. D.

The Integrated Surface Hourly (ISH) weather data I have described before allows one to examine how various surface weather elements have changed as a function of time of day. (The ISH data volume is very large and it is not a trivial task to decode and analyze many years of it.) Three-hourly synoptic weather observations have been made at many U.S. weather stations for at least 40 years: 1973 seems to be the year when the number of stations reached a fairly large number, and so that is the year my analyses begin with.

I have previously mentioned that ISH surface data shows U.S. warming since 1973 (primarily a winter phenomenon, due to unusually cold winters in the 1970s), and a curious decrease in surface wind speed.

Here Iíd like to point out another curiosity: while the dewpoint temperature has increased in step with air temperature at 12Z (around 6 a.m.), it has increased much less so at other times of the day, and even decreased slightly at 21Z (around 3 p.m.), during the period 1973-2012:
ISH-US-diurnal-trends

Assuming that dewpoint sensor design changes over the years have not introduced a diurnally varying measurement bias, a natural question arises: what would cause afternoon dewpoints to not rise in the face of warming both day and night? (Note I have not made any adjustments for sensor changes, siting changes, or urbanization in the above plot).

The first explanation that comes to my mind is a change in daytime convective mixing of the troposphere. If there is a slight increase in the depth of convective mixing, then drier (lower dewpoint) air aloft will be mixed down toward the surface. Such a change would probably also be associated with deeper moist convection and probably an increase in heavy rain rates, evidence for which has been claimed elsewhere (e.g. here). The implication of such a change for climate feedbacks is complicated and not obvious.

A second possibility is a long-term decrease in middle and upper tropospheric humidity, and no increase in convective mixing. In this case, daytime mixing would bring down the lower humidity air to the surface from the same altitude as before. There is some radiosonde evidence for such a decrease in absolute humidities above the turbulent boundary layer (e.g. Paltridge, 2009). If real, such a decrease might well result in negative water vapor feedback, since a small decrease in mid- and upper tropospheric humidity can have a natural radiative cooling effect which outweighs the warming from a larger increase in lower tropospheric humidity (e.g. Spencer and Braswell, 1997; Miskolczi, 2010). Of course, all climate models exhibit strongly positive water vapor feedback, approximately doubling the direct warming effect of increasing CO2 alone.

I donít have a strong opinion on which of these possibilities (sensor problems, deeper convection, or a dryer mid- and upper troposphere) is more likely. Too little information, too many questions.


11 Responses to “Diurnal Trends in Dewpoint Averaged over the U.S. Since 1973”

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  1. James Mayeau says:

    Deeper convection would be noticed at the rain gauge.
    My local newspaper (a global warming cheerleader which by dent of editorial policy has never allowed a doubt, while simultaneously never asking a question about the science, for instance the names Micheal Mann and James Hansen never appeared in their paper) has increased the average annual rainfall total by averaging only the latest 30 years, there by moving the total from a little over 17 inches in the 1980′s, to over 20 inches in the current.

    So we are reduced to doubting our eyes?

    It seems to me that under any of the possiblities (sensor problems, deeper convection, or a dryer mid- and upper troposphere) strongly positive water vapor feedback in climate models is excluded.

  2. well, James, except that the climate models all have strongly positive water vapor feedback, and all increase convective rainfall intensity as well. Of course, models might not represent reality.

  3. A third possibility is lawn irrigation.

    As I have reported in my surface stations project, many stations (airport ASOS as well as traditional COOP stations)are near lawns, golf courses, etc.

    Further, lawn irrigation has greatly increased in popularity in the last few decades. When I was a kid, lawn sprinkler systems were a luxury only seen with the wealthy, cemeteries, golf courses, and municipal properties.

    Now, it is nearly ubiquitous with low cost sprinkler systems you can get anywhere and generally standard on all new homes.

    I think it may have to do with lawn watering schedules, which are mostly done in the mornings. For example, it is illegal in the area served by the north Texas Municipal Water District to water between the hours of 10AM and 6PM. Water conservation is the reason.

    More:

    The best time of day to water your lawn is at about three or four in the morning when the water pressure is the highest. This way you will accomplish two vital things:

    1. The water will have a chance to sufficiently soak into the topsoil.

    2. Whatever water is left over will be evaporated when the sun comes up, thereby not giving the water a chance to create any fungi or molds which come about when water just sits around on the surface of your lawn.

    Source: http://www.shorelineirrigation.com/the-best-time-of-day-to-water-your-lawn.html

    In California, almost everybody does it before sunrise.

    This tendency for early morning watering, with subsequent enhanced evaporation as soon as the sun comes up, combined with proximity of weather stations to such urban influences might explain what you are seeing.

    Then again, I may be all wet.

  4. Stephen Wilde says:

    “deeper convection, or a dryer mid- and upper troposphere”

    Either way it looks like a negative system response involving a circulation change.

    Why not a combination of the two processes?

    Deeper convection causing the drying out process by decreasing average humidity throughput the vertical column due to the incorporation of more higher,dryer air.

    We know from the Earthshine project that there was less cloud during the late 20th century warming period (but more now)so the deeper convection would have been induced by more solar energy getting through to fuel the system but would that deeper convection would then represent a faster flow of energy from surface upwards to negate the effect of reduced global cloudiness and albedo.

    The most interesting question for me is whether Roy sees any change in trend between the periods before and after 2000 which is when global cloudiness seems to have changed trend from reducing to increasing again.

  5. Dr. Roy

    In my research of soil (agronomy) found that our soils are clogged with their micropores hindering the penetration of rainwater.
    As the convective motions are controlled by the amount of steam and water shortage in the soil, they are dependent on the humidity of the air does not reach high altitudes. With convection, air descends from the upper layers that are by nature drier. The lack of water in the soil is not denounced by lack of rain because the warming oceans compensates by increasing the rate of oceanic evaporation.

    • CLOUDS INVISIBLE

      Clouds invisible control our climate.
      We built a temperature gauge dew point in the air that we pass on a plate (one spreads flames stove) heated to vaporize all the water contained in the air and measure its dew point.
      We repeated this experiment four times on 15/06/2013 and obtained.
      The dew point temperature 28.2 degrees centigrade which corresponds to a moisture content of 24.4 grams of vapor per kilogram of air.
      The lowest point of the dew point in the atmosphere occurred at six o’clock in the morning and reached 24.7 degrees centigrade corresponding to a quantity of steam of 19.7 grams of vapor per kilogram of air. (P=1012 mb).
      A difference of 4.7 grams of water remains suspended in the air and is invisible to our eyes. This amount of water when it evaporates into the atmosphere removes energy as latent heat (no temperature change) and a column of one square meter for tree miles high we have 14.25 liters of water with it wielding 32.165 megajoules per day and this is that maintains the temperature of our planet, nothing to do with carbon dioxide. With a IR flux of 330 to 380 w/m2.

  6. R James says:

    Something I don’t understand is why the influence of clouds receives little attention. If I stand outside on a sunny day, and a cloud suddenly comes between me and the sun, the effect is dramatic. It seems logical that only a small change in cloud density or area would have a very significant effect on temperature.

    Yet billions of dollars are spent chasing the CO2 effect, which seems to be of little significance. All the IPCC seems to say is that cloud science is poorly understood, so let’s ignore it.

    The main drivers for CO2 research seems to be : 1. Scientists love it because governments hand out research grants. 2. Governments love it because they can introduce new taxes. 3. Some manufacturers love it because they can sell “green” products that are useless (eg wind farms etc) but make lots of money. 4. The public love it because they feel good about falsely doing something environmentally friendly. Somewhere, the science got lost.

  7. Fulco says:

    Roy,

    The key factor is that the earth is a water planet, Miskolczi shows that the optical thickness is the key to understanding the reaction of the atmosphere to introduction or removal of extra greenhouse gasses. It simply lowers or rises humidity, and it can because it has plenty of it. This is why we can not compare the earth to Mars or Venus.
    Miskolczi sticks to plain physics and measurements, that’s why I like his approach; just simple physics no strings attached, no introduction of uncertain factors.

  8. Fulco says:

    And for once and for all lets ban the word feedback, you don’t need it as an input.

  9. TonyB says:

    R James:
    “Something I donít understand is why the influence of clouds receives little attention. If I stand outside on a sunny day, and a cloud suddenly comes between me and the sun, the effect is dramatic. It seems logical that only a small change in cloud density or area would have a very significant effect on temperature.”

    For the same reason H2O as a GHG is not considered to be a driver of GW. In that it is a variable over a relatively short time scale, and therefor a constant on any meaningful timescale globally. It is merely the visible aspect of the hydrological cycle. I can see an effect where the like of the ENSO and PDO affect cloud formation over a longer time scale, but other than that I see it as noise in the background.

  10. I agree with every word you have written.