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Thread: Desert nights

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    Desert nights

    Really some information I'm after... but it's an interesting "puzzle" too. All the "hot" desert regions I've read about are reportedly blisteringly hot in the day, but bitterly cold at night. Why is this? Is there something about a desert that can't retain heat? Does it heat up quickly, but cool down just as fast?

    Alan

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    Re: Desert nights

    Alan

    Have a look here


    Jerry
    Jerry

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    Re: Desert nights

    Thanks for that. It looks like a simple case of no insulation. I'd have thought sand would have held onto the heat of the day, but then I think of barefoot on the beach at night after a hot summer day, and the sand is pretty cool.

    Alan

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    Re: Desert nights

    I pictured it more as too much insulation.

    The top layer of sand gets hot, but it insulates the sand underneath. You get very high surface temperatures but underneath the sand can remain cool. Since only the top layer of sand is hot, when the air temperature drops, only the top "has to" cool so it drops to the air temperature quickly on the surface.

    Steve

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    Re: Desert nights

    According to What's a Desert Like?
    <hr>Everyone knows that during the day many deserts are hot, very hot. Temperatures in excess of 100 degrees fahrenheit are not uncommon. Yet at night, the same deserts can have temperatures fall into the 40s or 50s? Why?

    Other biomes are insulated by their humidity (water vapor in the air). Temperate deciduous forests, for example, may have 80 percent humidity or more during the day. This water reflects and absorbs sunlight and the energy it brings. At night the water acts like a blanket, trapping heat inside the forest.

    Since deserts usually have only between 10 and 20 percent humidity to trap temperatures and have so few trees and other vegetation to retain heat, they cool down rapidly when the sun sets, and heat up quickly after the sun rises. <hr>
    I was out in Abu Dhabi in January and believe me, it does get cold at night, even 10 miles from the coast. 100 miles in and it wasn't unusual for people to be wearing jumpers and anoraks.

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    Re: Desert nights

    I'd describe that aspect of it as the poor conductivity of the ground, common to desert and non-desert environments. The ability of the ground to change its "surface" temperature so markedly (and quickly) in the desert seems to be due to the lack of insulation above. The extreme of this might be a planet with no atmosphere. The other extreme could be the dreaded greenhouse effect. <img src=/S/2cents.gif border=0 alt=2cents width=15 height=15>

    Alan

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    Re: Desert nights

    -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    The ability of the ground to change its "surface" temperature so markedly (and quickly) in the desert seems to be due to the lack of insulation above.
    -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    It's more due to the low value of the Specific Heat Capacity of the sand/rock which means that only a small amount of absorbed energy is required to raise its temperature. Although the desert is hot, it doesn't have a lot of energy to lose.
    <font color=blue><font face="Script MT Bold"><big>Rob</big></font face=script></font color=blue>

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    Re: Desert nights

    That makes sense too as a contributing factor. I'd think that poor conductivity would also be a "requirement" for such behaviour, since there's an almost "infinite sink" of sand/rock to act as a heat bank. With low heat capacity, but good conductivity, it could still continue to absorb heat all day and deliver heat all night.

    Alan

    Edited - And to further complicate the heat transfer, I seem to recall that silica has a high emissivity for IR, so it would reradiate its stored heat quite effeciently.

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    Re: Desert nights

    You can make ice in the desert.

    Although the temperature doesn't go to freezing, for some reason a shallow tray of water pans, insulated from the ground left exposed to the clear desert night sky, will turn to ice.

    matt

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    Re: Desert nights

    Interesting. Maybe it's from evaporative cooling effects. The very dry air would certainly assist with this. In fact, it's amazing how much more effective domestic evaporative coolers are in the drier regions of Australia, compared with the areas of "average" summer humidity... and in the very humid areas they're just a waste of power.

    Alan

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