Ground Temperature and Frost Formation
I was working in my garden on a cool morning in October.
There had been frost on shady parts of the grass when I started, but
it was gone. I dug a hole to transplant a plant, noticing that the
ground was not frozen. The water in the water bottle that I had left
on the steps overnight was not frozen or iced over. I ran water at a
slow rate into the hole to soften the heavy clay, noticing that
there was no ice in the garden hose. A few minutes later, I turned
off the hose, and attended to another plant while I let the water
soak in. Glancing at the hole a few minutes later, I noticed that
ice shards had formed at the mouth of the hose and in the hole. It
was not clear, it was white like snow. It was made of dry crystals,
with several inch-long shards. Was it ice? Snow? Slush? How could it
have formed so quickly, and above the freezing point? Is my water
supply contaminated? I am mystified. Help!
Aah, a thermodynamic mystery. How delightful.
Basically, your question boils down to "How did the water in the
hole get cold enough to freeze, when there was no evidence of ice
anywhere else?" Although I cannot answer the question with
certainty, as I do not know exactly what the thermal conditions were
at the time, I can hazard some suggestions.
First, I will establish some facts that will bear on what could have happened:
* Pure water freezes at a higher temperature (that is, more
easily) than water with something dissolved in it.
* Frost forms on a solid surface when the surface
temperature is below freezing and at or below the dew point of the
air. The dew point depends on how much water vapor is in the air;
the more water vapor there is, the higher the dew point. The
higher the relative humidity, the closer the dew point is to the
* Water absorbs a tremendous amount of energy when it
evaporates. For a sense of just how much energy this is, it takes
about eight times as much energy to evaporate a quantity of water
as it does to heat the same quantity of water from the melting
temperature to the boiling temperature. So, when part of a water
sample evaporates, the remaining liquid water cools substantially.
* Water readily loses energy by emitting infrared radiation.
* Gases, such as air, are more soluble in cold water than
in hot water, but they are not incorporated well into ice.
So what do I think was going on?
Disappearance of frost from the grass does not necessarily mean that
the air was above the freezing temperature. If the air was fairly
dry (low relative humidity), it could have still been quite cold
even if frost was disappearing.
Lack of frozen dirt also does not necessarily mean that the dirt was
above the freezing temperature of water. Wet dirt freezes at a
lower temperature than pure water.
My hypothesis is that some of the water in the hole evaporated,
bringing it down to or below its freezing temperature. Especially
if the hole was fairly broad and shallow, it could lose a lot of
energy by evaporation from its surface. The water in your water
bottle did not lose energy this way, because the sealed bottle
prevented escape of water vapor.
Why were your ice crystals white? Probably your water had dissolved
air in it. When it froze, the air formed small bubbles, which
scatter light. (This is why ice in an ice-cube tray tends to have
bubbles in the center.)
None of this indicates that your water is contaminated. The only
dissolved material it suggests is dissolved gas, probably air.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
"Be Ye Not Mystified!!" You are to be congratulated on your multiple
careful observations, but you also had the honor of witnessing a rare
balances of Nature. I cannot begin to explain in detail all the physics that
was happening. It was not a miracle, but an uncommon convergence of
temperature and weather conditions. The hidden variable that you had no way
to measure is the atmospheric humidity.
Some examples: The unfrozen water is because the temperature is just
around the freezing point of water. If not disturbed, it can remain fluid
several degrees below the melting point.
The shard formation is a balance of freezing and evaporation.
This is the short answer, because the confluence of conditions that you
witnessed are extraordinary.
I can only hazard a couple of educated guesses for
Since there had been frost the night before (as evidenced
by the frost on the grass), it is clear that radiational
cooling had lowered the temperature of the grass and probably
the hose to below freezing. Radiational cooling can lower
the temperature of grass, etc. below the air temperature.
Even though the air temperature is above freezing, the
temperature of objects that are cooled at night can remain
lower than the air temperature until they are warmed by the
Sun or by the air itself, just as the frost in the shade was
kept from exposure to the Sun and thus remained.
If the hose nozzle temperature was below freezing, ice crystals
may have formed as the water passed by the nozzle. Ice crystals
form from pure water as the water is being cooled to the freezing
point. Even though your hose water probably was not pure, the same
kind of effect may have occurred. Another possibility is that the
ice shards had formed next to the cold inside of the hose, in the water
sitting in the hose during the night. The shards may have come
out of the hose once you turned the water on. The ice shards in
the hole may have come off of the hose nozzle or out of the hose.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012