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Whenever I have seen Venus described, its high surface temperature is attributed to an intense greenhouse effect. This seems to make sense, as its atmosphere is roughly 96% CO2. But on Earth, the greenhouse effect works because the atmosphere is (mostly) transparent to sunlight, but (somewhat) opaque to longer wavelength light radiated back from the surface.

The atmosphere of Venus would be very opaque to longer wavelength light from the surface, just like Earth (at least around the CO2 absorption bands). But isn't Venus also very cloudy and largely opaque to visible light? If the solar radiation that reaches the surface is limited, wouldn't this also limit the ability of the CO2 to "trap" heat?

The atmospheric pressure at the surface of Venus is much higher than it is at the surface of Earth. Isn't this a more straight-forward explanation for much of the high temperature?

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    $\begingroup$ We actually do have pictures of the surface of Venus thanks to the Soviet space program. nssdc.gsfc.nasa.gov/photo_gallery/… $\endgroup$ – sysadmin1138 Jun 25 '11 at 2:47
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There are other wavelengths of light than visible and infrared. The venusian atmosphere is transparent to some of them, which also warm the surface of the planet. As the surface is warmed by these wavelengths, the infrared that it emits is trapped by the atmosphere. Over time this causes a runaway greenhouse effect, which is why the surface of Venus is hot enough to melt lead.

Since the atmosphere and surface have different densities, and the density of a substance is what determines which wavelengths of light can interact with it, it's clear that there will always be wavelengths which can interact with the surface but pass right through the atmosphere. If electromagnetic radiation can interact with a substance, it will cause that substance to warm up. When the atmosphere is composed of a substance which is opaque to infrared light (which warm objects emit), a greenhouse effect will result.

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I think the answer is clear if you draw a distinction between scattering and reflection. The clouds will of course reflect a relatively large amount of the incident sunlight, and scatter all of the rest of it, but on Earth as on Venus, even an entire skyfull of clouds doesn't darken the day into night. The light has to take a zig-zag journey, but much does eventually reach the ground. And then the high reflection of infrared by CO2 takes over and keeps that energy near ground level.

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In addition to the composition of the atmosphere that you mentioned in your question and @Carson detailed in his answer, Venus is considerably closer to the sun (around .72 au compared with Earth's 1.0 au).

Venus is almost the same size as Earth, so from the inverse square law of radiation you can see there will be a fair amount more energy imparted to Venus.

(if anyone wants to do the maths and pop the actual answer in a comment, feel free:-)

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    $\begingroup$ 0.72 ^2 = 0.5184, so sunlight is about twice as intense on Venus. Note that Venus is hotter than even Mercury's day side, while Mercury's night side is quite cold, so intensity of sunlight is not the only or even primary factor. $\endgroup$ – Andrew Jun 24 '11 at 14:11

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