Wednesday, April 5, 2017

Ode to Physics on a Manure Heap

When I was an undergraduate at Penn State, Craig Bohren was my physical meteorology professor and my advisor.  He had two major influences on my career.  First, sometime perhaps late in my junior year, he told me that I had pretty good grades in math and physics and that I should think about going to graduate school.  Up until that time, I hadn't considered it.  In hindsight, it was only a brief comment, but one that completely changed my life path.

Second, Craig taught in a very unconventional way.  The usual approach in physical meteorology is to subject students to long derivations and heinous interpretations of mathematical relations for cloud and radiative processes.  The approach can be very dry and unappealing.  In contrast, Craig almost always began with an example, typically a photo taken on his walks to, from, and around campus, and then insisted that we explain the subject.

One day he came in with photos of manure heaps, and asked us to explain the processes producing the steam (an example of what is known a mixing cloud) and its optical characteristics.   The topic forms a chapter in his classic science-geek book Clouds in a Glass of Beer.  He first taught us how to think and explain observations of the natural world, and then brought in more quantitative, mathematical explanations, which is an approach that I favor and try to employ as an instructor today.

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I often think of Craig when I am out walking (I have mentioned him in previous posts) and that was the case this morning.  Back in the day, he had to wait for his film to be developed overnight (or eventually in 1 hour), but we get instant gratification with smart phones.  I find days with spotty frost to be especially interesting as I try to surmise why the front forms in some areas and not in others.  What caught my attention initially this morning was simply the existence of front in patches on my lawn.

Why was there frost at all when temperatures at the airport did not fall to 32ºF overnight (the low was 33ºF)?  One possibility is that it was colder at my house than at the airport, but I didn't have a thermometer to test this hypothesis.  A nearby observing site above 18th Avenue reported a minimum temperature of 30ºF, as did sites near the mouth of Red Butte Canyon, so this is a possibility.  Another possible contributor is that the temperature at or near the ground is actually colder than the "air" temperature measured at the airport or surrounding observing sites.  Air temperatures are typically measured at 1.5 to 2 meters above ground level or even on buildings.  At night, when a nocturnal inversion is present, the temperature is typically even colder near the ground.  Thus, frost can form at times that the temperature at 1.5 to 2 meters above ground level is just above freezing.

But then we can dig in a bit further and try to explain the patterns of frost on plants and grass.  This is where things start to get really wild.  Here are two interesting patterns that I found.  The first pattern features frost in the middle and at the top of bushy low plants.

In this instance, I suspect that the entire bush has cooled to below freezing, but frost is forming only where there is both a sufficient source of water vapor (the ground below the plant which has been protected by the sun and experienced less drying yesterday) and weak flow (due to protection from the surrounding leaves) to allow the humidity to rise high enough.  This is, however, pure speculation subject to debate by Wasatch Weather Weenies readers.

The second pattern is a bit more perplexing and features a line of frost parallel to and a few centimeters from the sidewalk.  Does the existence of the deep edge along the grass provide a conduit for deeper soil moisture to escape, but frost only forms when it gets to the grass that is a bit removed from the concrete and thus a bit colder?  I the grass just off the curb slightly higher and thus elevated just enough off the ground to cool sufficiently for frost formation?

A high-resolution infrared camera would be great in circumstances like this as it would shed light on the temperature part of the problem.  More difficult are issues related to sources and variations in water vapor, which are also very important.

Feel free to share your thoughts and hypotheses.  If this sort of thing interests you, you are a true Wasatch Weather Weenie.


  1. Could it be that there is more moisture near the cement because of the rain draining off of it and the cement also keeps the temperature higher nearby?

  2. I noticed similar random frost patches in my yard on 2nd Ave. this morning. Since I'm considerably lower than you, closer to the airport elevation, I would tend to think it is not the sheer difference in elevation between you, me and the airport that causes frost here when the airport did not get below freezing. Your hypothesis that temps at the ground are lower than temps a couple meters off the ground is more likely.
    I didn't notice exactly how my frost patterns laid out in relation to features in my yard. I just remember thinking (that's odd), and hurrying to get my kids to school. I guess I'm an inferior weenie (or someone with younger kids and thus less time).

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  4. Plants respire continuously and produce metabolic heat, whether that has impact on how frost forms on the micro level I'm not sure. I have observed that large woody plants, like trees, melt-out faster in snow that in areas where there are no trees, but I think this is more of a blackbody phenomenon.

  5. My theory is that the source of the frost is moisture from plant respiration. If the frost were from near ground saturated air, one would expect it to deposit uniformly on the blades of grass and plants. I am aware that plant respiration is dependent on a number of factors - soil moisture, amount of sunlight the plant receives, salinity of the moisture of the soil, etc... It is not inconceivable that the frosty patches we see in these photos is from "differential plant respiration".

  6. I would think that maybe these situations are not primarily related to near air temperature or are moisture limited, but that variations in longwave radiative losses and skin temperature can explain most of them. Most surfaces are colder than the air just above them at night due to longwave emission (notable exceptions are materials with high thermal inertia like concrete). This might explain the center of the shrub as the plant would probably be the surface with the highest emissivity nearby, ~0.96. Radiative cooling would be maximized at the shrubs center. Emission doesn't explain the patchy grass frost distribution well, but with dew points in the mid 20s I would think surfaces with particularly good radiative cooling, in relation to emission by other objects nearby, could cool to those temperatures and explain most of the interesting patterns.

    Also, I'm a very big fan of Bohren's texts, particularly Atmospheric Thermodynamics.