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Physics of radiation

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  • SIMA
- Posted: October 5, 2016
By George Melchior, ASM

Radiation is the most drastic and influential mode of heat transfer for ice management. It’s also the most difficult to understand. Therefore, in this article - the last in a three-part series on the modes of heat transfer and their implications on ice management - let’s focus on the scope of radiation specifically to the formation of ice on walking and driving surfaces.

Recall that the other two modes transfer heat through a medium, whether a solid like concrete or asphalt (conduction) or a fluid like air (convection). In contrast, radiation does not require a medium through which to act. Instead, radiation is the transfer of heat through electromagnetic waves. However, the basic premise of heat transfer still applies: heat travels from hot to cold, regardless of direction.

How radiation works
There are two types of radiation: Ionizing and non-ionizing. Ionizing radiation is energy that transforms matter at the atomic level (think nuclear reactors, atomic bomb, Incredible Hulk, etc.). Non-ionizing radiation will excite matter at the atomic level but does not have enough energy intensity to alter the atomic structure.

When dealing with ice management, we are most concerned with ultraviolet (UV) radiation, which comes from the sun, and has a range of energy intensity that bridges the spectrum be-tween ionizing and non-ionizing radiation. All ionizing UV radiation is blocked by our upper atmosphere, and the remaining non-ionizing, but very intense, radiation makes it down to the Earth’s surface.

As discussed with the other two modes of heat transfer, the rate of transfer is important to understand because your ice management means and methods will be greatly influenced by the time it takes for a surface to warm or cool. The same holds true for radiation. However, due to the intensity of the UV energy, the rate of heat transfer is far faster than with conduction or convection.

The transfer of heat through radiation is described by the Stefan-Boltzmann law, which states that the rate of heat transfer is directly proportional to the fourth power of the temperature differential. Too complicated? Here’s a simple monetary example to demonstrate the power of radiation: If you had $100 and you multiplied it by itself four times (fourth power), you’d end up with $100 million. That’s intense! That’s radiation.

Transfer rates
As with the other modes of heat transfer, the rate of radiation heat transfer depends on material properties: absorptivity and emissivity. Absorptivity is the material’s ability to absorb radiation, and emissivity is a material’s ability to emit, or release radiation. In ice management, we deal predominantly with concrete and asphalt. Concrete has an absorptivity of 0.60; that is, it can absorb 60% of the radiant energy that is emitted to it. Conversely, it has an emissivity of 0.85, which means it releases its energy faster than it absorbs it. Asphalt, on the other hand, has both an absorptivity and emissivity of 0.91. Asphalt gets hot fast, and cools off just as fast.

A clear day can be a double-edged sword for the ice fighter. During the day, direct sunlight will heat the concrete and asphalt quickly to temperatures much higher than the air temperature.

It’s common for asphalt, exposed to sun throughout the day, to register temperatures in excess of 70˚ F when the air temperature is at or below 30˚ F. That same UV radiation will also melt ice and snowpiles, producing water that may drain across walking and driving surfaces.

While that’s nice during the day, it can quickly become a hazard after the sun sets, when the energy that was absorbed during the day will reverse radiate back into the cooler atmosphere.

Clear skies change the game
Under clear skies, this reverse radiation, called radiative cooling, is very fast (to the fourth power) and could cause the surface temperature of asphalt and concrete to plummet to temperatures below the reported air temperature within an hour or less. That is because the upper atmosphere is much cooler than the surface, and heat moves from hot to cold.

As the asphalt or concrete emit their heat into the atmosphere, their temperatures quickly cool until they equilibrate with the ground temperatures. If the ground temperature is below freezing, then the snow and ice melt from earlier in the day will refreeze on the surface. In fact, radiative cooling under clear skies is a leading cause of black ice development on our roads, parking lots  and walkways because it happens so fast. 
Case study / radiative heating & cooling
A recent premises liability case that I consulted on demonstrates the effects of radiation on ice formation. It was February in New Hampshire, and the average air temperature in the preceding two weeks was 9° F. On the day prior to the incident, the region experienced 6 inches of snow-fall, and on the day of the incident, the region experienced clear, sunny skies, and the air temperature reached 30° F. At approximately 4:30 p.m. on the date of incident, a man parked his car in the bank parking lot and went into the bank for approximately one hour. When he left the bank and was walking across the parking lot to his vehicle, he slipped and fell on black ice.

The evening prior to the incident, the snow management contractor plowed and stockpiled the snow around the edges of the lot, and then spread sand. On the date of incident, the sun warmed the snow banks and the parking lot surface, which sent significant amounts of water across the lot toward drainage structures.

The sun set at 4:20 p.m. under clear skies, and the air temperature was approximately 29° F, and the parking lot surface temperature was calculated to be approximately 36° F. By 5:30 p.m., about the time the man departed the bank, the air temperature fell to approximately 14° F, and the surface of the parking lot was calculated to be approximately 20° F.

In the hour that the man was in the bank, the temperature differential between the parking lot surface and the upper atmosphere was very high, which caused the parking lot surface to re-verse radiate and cool at a high rate. As a result, all residual water from the snowmelt froze on the parking lot surface, over the sand placed the previous evening. As one witness statement recounted, “The entire parking lot suddenly became a giant, black sheet of ice.”

The snow management contractor worked on call, and typically received calls for snow management only. Both  the bank and the contractor testified that they were  unaware of the effects of UV radiation on the parking lot surface, and neither anticipated any moisture because  the air temperatures never went above freezing.

Additionally, both parties testified that they were unaware of the radiative cooling effect of clear skies at night. Lastly, neither the bank nor the contractor inspected the grounds or surfaces on the date of incident.

The case settled for approximately $65,000. Had the bank and contractor been aware of the effects of radiation and radiative cooling, they would have treated the lot appropriately after sun-set, at a cost of a few hundred dollars.

Radiation_graphics
The rate of heat transfer in radiation is far faster than with conduction or convection. Radiative cooling is a leading cause of black ice formation.

George Melchior, ASM, is a registered architect and professional engineer and owns GVM Consulting, based in Portsmouth, NH. Contact him at gwmelchior3@gmail.com.
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