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Name: Richard
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Question:
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!


Replies:
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 air temperature.
* 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.

Forensically yours,

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.

Vince Calder


Richard,

I can only hazard a couple of educated guesses for answers.

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
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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