Tuesday, January 25, 2011
iCandy: What-frost?

Snowflakes and icicles are beautiful and well-known sights of winter. Less familiar, but equally beautiful, is what you see above: Hoarfrost.


Hoarfrost is essentially frozen dew. Droplets of dew form on plants when relative humidity in the air is above 100 percent. But when plants and outdoor objects are cooled to well below freezing, the liquid dew that forms on them turns to frost.


Hoarfrost can form intricate, feathery patterns, or it can cover enough of a plant or object to look like snow.


"It looked like snow, but more fuzzy," said Harry Ewart, who helped his wife shoot the above photo in Memphis, Tennessee. "When she stepped into the back yard, the crystal formations just stood out."


And, unlike the solid glaze of ice that can sometimes form on branches and leaves, the spines of hoarfrost can be very delicate.


"Even a slight breeze would cause them to break," said Ewart.


Is the winter weather creating beautiful scenery where you are? Share your photos with CNN iReport.

January 25, 2011
Click to view LeAria's profile

Down in Lancaster, V.A. it's just freezing not much of a beautiful scenery.

January 26, 2011
Click to view tedlas's profile

um.... it's impossible for anything to have more than 100% humidity...

January 26, 2011
Click to view aquariuss310's profile

I've heard of hoarfrost but never seen a close photo of it. When hoarfrost forms on the surface of snow and then is buried under more snow it creates avalanche conditions. The new snow slides easily over the hoarfrost. 

January 26, 2011
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I'm not sure that's correct. I suspect that when the moisture content of the air goes over 100%, then the moisture must "seek life elsewhere". When the temp is below freezing, the moisture shows up as frost. When it's above freezing, it shows up as dew. Yet the dewpoint changes with temperature as well and dew can appear when the humidity is below 100%. The term supersaturation comes to mind.

January 26, 2011
Click to view RedDragon11's profile

They used the term "relative humidity".

January 26, 2011
Click to view Beagle812's profile

The dewpoint doesn't change "with temperature."  The dew point is a temperature.  It is the temperature at which the relative humidity equals 100%.  Relative humidity is defined as the amount of water actually in the air divided by the amount of water the air can actually hold, AT A GIVEN TEMPERATURE.  Relative humidity changes with temperature.


Take a hot, humid day.  Relative humidity maybe 80%.  Evening comes and the air temp begins to drop.  Relative humidity begins to increase.  The amount of water actually in the air hasn't increased, but the amount of water the air can hold has decreased.  When the temp drops to where the RH is 100%, that is the dewpoint and that is where condensation begins.  The statement that the RH can be above 100% is erroneous because water will continue to condense out of the air as dew, rain, frost, snow, whatever forms, such that the RH never exceeds 100%.  100% is defined as the maximum moisture the air can hold.  You can't force it to hold any more.

January 26, 2011
Click to view 10001NYC's profile

I always thought hoarfrost was the natural result of johndew.

January 26, 2011
Click to view DasLeezard's profile

Hoarfrost: A condition among prostitutes in Alaska.

January 26, 2011
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It's what ho's get when their skirts too short in the winter

January 26, 2011
Click to view Odin63's profile

RH can indeed exceed 100% 

January 26, 2011
Click to view HowardCord's profile

RH can go above 100% as easily as someone can eat over 100% of a pie.  In other words, it's impossible.  If the temp is at 80 and the dew point is 80, the RH is 100%, it is scientifically impossible for the dew point to be above the ambient air temperature.   

January 26, 2011
Click to view HowardCord's profile

Note: In nature, RH can not go above 100%.  Supersaturation can be obtained using man made machines.  We are discussing natural weather phenomenon here though, so supersaturation will not be a factor.

January 26, 2011
Click to view CarolinaJack's profile

Beagle 812! Well said!  Someone who understands this reasonably simple but universally misunderstood concept has my admiration


Chemistry teacher

January 26, 2011
Click to view MDMick's profile

Around Baltimore we had snow, then rain, and now it's beginning to freeze and snow again and, looking outside my window right now, I can see the same beautiful crystals forming where my lawn meets my sidewalk.

January 26, 2011
Click to view Burbank's profile

I grew up in southern California and even I know what hoar frost is, people need to read more instead of tweeting their lives away. I've only seen pictures. I hope I actually get the chance to see it for real one day.

January 26, 2011
Click to view hewart's profile

I found this definition while surfing the web.


Hoarfrost A deposit of interlocking ice crystals (hoar crystals) formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc., which surface is sufficiently cooled, mostly by nocturnal radiation, to cause the direct sublimation of the water vapor contained in the ambient air.


January 26, 2011
Click to view cpx32's profile

Thanks Beagle812. Excellent information.

January 26, 2011
Click to view hikerfisher's profile

Relative humidity exceeding 100% is entirely possible.  If you continue to cool air after it has reached the relative humidity of 100% and there is not enough condensation nuclei for the satuated water vapor to condense upon, the water vapor will then exist in a state of super-saturation.  Google it.

January 26, 2011
Click to view wwkayakit's profile

What's hoarfrost? A hooker that says no? Bada Bump. I'll be here all week folks. Don't forget to tip the waitress.

January 26, 2011
Click to view frontgate's profile

hoar frost,common here in western montana

happens two-three times a week from dec. to mar.

January 26, 2011
Click to view Wusssssa's profile

I thought it had something to do with a cold hooker.


January 27, 2011
Click to view Liberty1955's profile

Beautiful..and I learned a new word..LOL..thanks for sharing this

January 27, 2011
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tedlas said: "um.... it's impossible for anything to have more than 100% humidity..."


You have obviously never lived in Guam, my friend.  It may not be over 100% humidity after a quick rainy season rain storm....but it sures feels like it!!

January 27, 2011
Click to view skeehee's profile

Just as interesting as hoar frost are frost flowers. These are formed when saturated ground has the water squeezed out tiny cracks by cold weather and then the water freezes into flowery looking petals as it grows.

They happen a lot here in Arkansas when cold temps follow a heavy rain. They look like delicate white petals or curled ribbons laying on the ground.

January 27, 2011
Click to view mjporter's profile

@hikerfisher is pretty much right.


I'm a meteorologist and here's annother way to envision the whole 100% thing...


whenever the RH is <100, evaporation rate > condensation rate, when RH = 100 the two are equal, and when RH > 100, condensation > evaporation.


So effectively if RH goes over that 100, then condensation increases, pulling vapor into the liquid phase.  This is not instantaneous, and it also is much more effective when there are things (aerosols) for the droplets to grow on... this is why cloud seeding works.


in any case, pretty picture! :)

January 31, 2011
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February 4, 2011
Click to view manda009's profile

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