How does morning dew appear on the grass every morning?

Question

Every morning when I wake up, I go outside to see if it is hot or cold and I always see the glossy dew on the grass and occasionaly on the mailbox. I know it isn't from the rain because, it doesn't rain every single day in the spring and summer.

What causes the dew to come on the grass?

Answer 1

It has to do with the lowering temperature and the dewpoint of the air, but there's more...  First of all, if the air temperature goes below the dewpoint temperature, then water condenses out and you get fog.  That makes sense, fog is just lots of little water droplets and these fall and condense on your grass.  But you get dew even without fog.  What's the deal?

The answer is that your grass (or car window) actually can get colder than the air temperature, so that the air temp is above the dewpoint but the grass temperature is below the dewpoint.  How?  If you have clear skies, the radiation temperature of the sky is about 3 kelvin.  All opaque solids radiate proportional to their temperature.  It's called "blackbody radiation".  Some forms of mass, however, radiate very little or not at all, for example air is transparent and thus does not significantly radiate infrared energy.  So at nighttime, the ground is radiating, loosing its heat to space, and air is not radiating.  Space, near absolute zero, effectively does not radiate back. The air cools off due to convection and conduction with the ground (and presumably some radiation from contained dust).  But the ground at night is usually cooler than the air around it.  The very thin film of air in contact with the ground can reach its dewpoint temperature and deposit dew on the ground (grass or car), but the rest of the air is above the dewpoint and it does not get foggy outside, so the dew seems to arise from nowhere.    This is the same reason you can get frost when the air temperature is in the mid 30s.  If it's cloudy outside, then the clouds bounce some of that lost radiation back to the ground like a big blanket and make it much harder for dew to form.  

The opposite is also true when the sun is out, the sun's radiation heats the ground first and the ground transfers that heat to the atmosphere.

Answer 2

it has to do with the moisture in the air. hot air will hold a lot more water vapor than cold air. So, when the air gets cooler, the moisture is too much, and has to get out of the air... thus, dew.