Understanding the hot shallow layer above the earth surface

  1. In Wikipedia page of Diurnal Temperature variation, there is a statement as following: "As solar energy strikes the earth’s surface each morning, a shallow (1–3 cm) layer of air directly above the ground is heated by conduction. Heat exchange between this shallow layer of warm air and the cooler air above is very inefficient. On a warm summer’s day, for example, air temperatures may vary by 30°F from just above the ground to waist height."
    First of all, do you believe it? And if you do, how comes that there is such a discrete temperature separation (a difference of up to 30°F) of these two layers (the 3 cm one and the one above it)?
  2. jcsd
  3. russ_watters

    Staff: Mentor

    It is definitely true. Haven't you ever seen reporters put a thermometer near the ground at a sporting event on a hot day?

    The reason for the difference is that the sun is very efficiently heating the ground and the ground gets very hot.

    I'm not sure I would agree that the steep gradient near the ground is a sign of inefficient heat transfer though: steeper gradients generally means faster and/or more efficient heat transfer.
    Last edited: Nov 22, 2012
  4. The statement is indeed in the reference wiki gives, however there is no further reference.

    Many years ago, I was involved in a small local experiment on our air base because we had concerns about temperature difference between station temperature and runway temperature during scorching hot days, as engine performance is a direct function of the air temperature. I remember that we had three temperature series, runway surface, at 1.5 meters and reported station temeprature. There was a difference of a few degrees between surface and station with the 1.5 meters some where in between, so not nearly as much as the syllabus suggests. But it may not have been hot enough.
  5. russ_watters

    Staff: Mentor

    How did you measure the runway temp?
  6. russ_watters

    Staff: Mentor

    I checked the surface temperatures of some hard surfaces outside, on what is a very clear day, with light wind:

    White Concrete: 76F
    Black Pavement: 91F

    Air temperature under shade, 7' off the ground: 51F

    Since it is late November, the sun is relatively low, even for 1PM, so this differential is a lot lower than it would be in the summer.
  7. I did not, I was merely observing the process. It was done however by the NLR way back in the 80's. But I realize it may not have been surface temperature but air temperature very close to the ground. I remember that there was a discussion that the difference was lower than expected.
    Last edited: Nov 22, 2012
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