ROCKY's
Science Fun
THE FORMATION 
OF FROST

Science Standards by State

 

 
 

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Any object that is above absolute zero (above -273°c) radiates energy.  Soil, rocks, buildings, fences, blades of grass, lakes, and animals – all send (radiate) energy out to space.  This goes on both day and night – for everything!  But by day, our sun sends energy to Earth (and everything on it).  More in summer, when the days are long, and less in winter when the days are short.  The sun sends a huge amount of energy in all directions outward into space, and we on Earth absorb a tiny bit of it.   But since we are only a tiny speck, compared to the sun, and are so far away, most of the sun’s energy goes right past us and out into space.  We never ever feel it!  Of all that makes up the surface of Earth, oceans are the largest, and they absorb larger amounts of the sun’s energy than you can possibly imagine.  Sidewalks, bicycles, and puddles absorb a little, too, but hardly enough to notice.  Even blades of grass, spiders, and leaves absorb a tiny bit.  By day, almost everything you can think of on Earth absorbs some solar energy!

Then, at night, when we are facing away from the sun, everything that got energy from the sun loses it to space --  unless it’s a cloudy night!  Then the clouds, like a huge blanket, prevent much of the energy loss.  Cloudy days tend to be cooler than clear days,  and cloudy nights tend to be warmer.  Some solar energy gets reflected back into space, and some of it gets absorbed.  In summer, a white car reflects sunlight, but a dark car absorbs it.  On a clear summer day, test with your hand cars of different shades and see which are warmest to the touch.  Which are coolest long after dark?   The way things warm or cool by day and by night depends on what they are made of, their color, and what’s between them and the sun.  It has been this way for millions of years. 

You may not think that seas, lakes, ponds, the rocks in cliffs, the sand along shores, roads, sidewalks, and houses – everything on Earth – gets warmed by the sun.  But everything does!  When Earth rotates, it appears that the sun “comes up” in the morning and “goes down” in late afternoon.  But it’s Earth that rotates!  All those things that the sun warmed by day lose energy at night – all because Earth rotates.  If it’s a clear night, some of the energy Earth received in the daytime gets lost to space.  If the things that lose energy get cool enough, some moisture in the air around them may condense on them.  People often say that “dew falls on them.”  But what happens is that the objects lose enough daytime energy to cool below the temperature at which water vapor in the surrounding air condenses on them.  Dew doesn’t “fall.”   Water vapor in the air condenses on objects that are cold enough. Things such as car tops, bicycle seats, park benches, and grass often get wet on a clear night – not because it rained, but because they radiated enough energy to cool below the dewpoint.  Those objects and many others get wet from dew, not by rain.  But if clouds are present, it’s like a blanket being “floated” in the air above them, and that blanket blocks outgoing radiation, and objects beneath the clouds don’t cool so much.

This all might seem simple, but it isn’t.  There are different kinds of clouds. Sometimes the air is full of dust and other stuff that hinders the incoming sunshine – or out-going radiation and cooling.  At some seasons, the incoming sunlight is more direct, more intense, than at others.  But clouds tend to insulate Earth and objects on it from the solar energy that comes our way by day, and that we lose after the sun “goes down.” 

In addition, Earth rotates slowly but surely so that at some seasons you might be directly in the path of sunlight.  At other seasons, you receive only a “glancing blow” from the sun’s energy.  At night, almost no sunlight shines on you.  Then what is the source of moonlight?  It’s the reflection of solar energy.  The moon is “in the way.”

When a fallen leaf radiates daytime energy back to space at night, it can get colder than the air around it.  If it cools below freezing, water vapor in the air around it begins to condense on the leaf as frost.   If the temperature of the leaf remains above freezing, the water vapor will condense as dew and the leaf will get wet.  But if the temperature of the leaf, and the air around it, drop below freezing, the water vapor may condense as frost.  It all depends on what the temperature of the leaf does as the leaf loses energy and cools.  Something to remember:  frost is not frozen dew.  It forms from water vapor in the air turning directly to frost. 

The shape of an object affects how frost forms on it.  For example, suppose a leaf lies upside-down on the ground.  The veins present a large surface area for radiating heat out to space.  So frost tends to form more readily on the veins of an upside-down leaf, but may not form on the part between veins on the upper side.  The micro-climate on different parts of a leaf depends on whether the leaf is right side up, or upside-down on the ground.

Blades of grass, too, have their top-side and under-side shapes, and this has a lot to do with how frost will form on them.  On a frosty night, or early on a frosty morning, examine places where frost has formed.  Remember that it is the temperature of the surface that determines where frost will form.  Remember, too, that the surface of a leaf or of a blade of grass has its little hills and valleys.  These make all the difference!   From looking at how and where frost forms on leaves and grass, you can learn a lot about the weather in larger places such as hills and valleys – and even the roofs of houses.  Most surprising is that frost can even form on snow!  See if your eyes and your mind are sharp enough to spot it!
 

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