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JESMKS
2006-Apr-02, 12:16 AM
I read somewhere that there is little difference between the day and night surface temperature on Venus. With night lasting the equivalent of 121 earth days, why wouldnt the cooling of the upper atmosphere create a denser gas with a resulting downdraft of colder gases? If there is such a downdraft, why wouldn't there be a cooling of the Venus surface?

grant hutchison
2006-Apr-02, 10:47 AM
The night is actually rather shorter than that. The combined motions of Venus' rotation and revolution around the sun means that the time from one sunrise to the next is "only" 117 days, so the night averages 58.5 days long.
Venus' atmosphere is so massive, and so opaque to the infrared radiation that the planet surface is "trying" to radiate to space, it retains heat very well, and the temperature falls only slightly overnight. It's as if the whole planet were wrapped in a very thick layer of insulation.
The thin stuff high up does cool, but it can't form a downdraught into the very densely compressed deeper layers.

Grant Hutchison

JESMKS
2006-Apr-02, 11:38 PM
Thanks for the information. I was wondering with rotation of Venus taking 243 Earth days, how can the night last only 58.5 Days? It can't have two days and two nights per rotation can it? Also, what would be the temperature at the top of Venus's highest Mountain? I can't find a thermal gradient for Venus. In regard to the thermal blanket, why doesn't it keep the solar heat out?
Jack

Damburger
2006-Apr-02, 11:44 PM
The reason the cytherian day is shorter than the rotation of venus is because the movement of the planet around the sun moves the position of the sun in the sky, if that makes sense.

grant hutchison
2006-Apr-03, 12:23 AM
I was wondering with rotation of Venus taking 243 Earth days, how can the night last only 58.5 Days? It can't have two days and two nights per rotation can it?It does. Imagine, for the moment, that Venus rotates once on its axis in exactly the same time as it takes to go once around the sun, and remember that Venus rotates in the opposite direction to the direction it orbits (looking down from the north, it rotates clockwise but goes around the sun anticlockwise). Start with the sun overhead at some point on Venus' surface. Now move forward half a year. The sun will now be shining on the opposite side of Venus, but Venus will also have turned to meet it by half a rotation, bringing the sun back overhead in the same spot we started at. Half a rotation, one full day and night.
In fact, the rotation period and year of Venus are only approximately equal, but the same reasoning applies.


Also, what would be the temperature at the top of Venus's highest Mountain? I can't find a thermal gradient for Venus.A little cooler than it is in the lowlands. Temperature does fall with height in Venus' troposphere (the lowest level of the atmosphere), but not as quickly as it does on Earth.


In regard to the thermal blanket, why doesn't it keep the solar heat out?Greenhouse effect. The sun radiates its energy at much shorter wavelengths than Venus does, because the sun is very much hotter. Venus' atmosphere is (grudgingly) transparent to these shorter wavelengths coming from the sun, so some of their energy can penetrate to the surface and heat the planet. But the long infrared wavelengths radiated by Venus' surface have trouble getting back out into space again, because they're strongly absorbed by the atmosphere.

Grant Hutchison