We are going to return to this question again in other sections of this temperature and heat unit, because it involves many different aspects of heat transfer.  Part of the question ("Why does dew form?) will be answered in a later section.

For now, let's just focus on the role thermal conduction plays in the difference between grass and sidewalk in the dew making process.  Concrete is a pretty good thermal conductor.  You've perhaps experienced this firsthand when sitting on some concrete steps on a winter day.  The concrete pulls heat away from you quickly - that's why it feels cold: it's a good conductor of your heat.  Because it is a conductor, concrete that is attached to the ground tends to be the same temperature as the ground.  (It also touches the air, but the ground can transfer or absorb much more heat energy than the air, so the ground is the main influence.)  Ground temperature does not vary much overnight, and so neither does the temperature of the concrete.

However, air temperature can vary greatly from day to day, and things that are in contact primarily with air go through the same wide temperature changes.  For example, grass: the tip of a single blade of grass is surrounded by air.  When the temperature of the air plummets, so does that of the grass.  And as we'll learn later, cooler temperatures make water more readily condense out of the air - forming dew on the cold blades of grass.

So part of the reason that dew forms on grass but not on the concrete is due to the different thermal conductivities of air and concrete.
 

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