IIAR

BY KEM RUSSELL

Sometimes, lessons need to be repeated, which can be a good thing depending on the experience you are going through for the lesson. Even tough lessons can help. As we slide into winter and the La Nina projections for the coming months, an experience I had last year was a lesson learned about “cold.”

While skiing, an Arctic storm brought in not just more snow but also very cold temperatures and wind, which really affected the skiing conditions. This particular time it was so cold that even low temperature ski wax didn’t help my skis to slide. The skis were slow and sticking to the snow. In addition, due to the -10°F temperature and the 20 to 30 mph wind, the end of my nose, and my skin below my goggles suffered the effects of wind chill known as frostbite.

So, what is this “wind chill”? We experience wind chill as a natural event and also use this effect in refrigeration.

Wind chill has been around since the dawn of time, and people experienced it, but this concept of wind chill wasn’t developed till two U.S. researchers, Paul Siple, and Charles Passel, did tests they carried out during a U.S. Antarctic Service Expedition from 1939 to 1941. Their work examined the effect of wind in speeding up how quickly water froze in plastic cylinders. The resulting data on heat loss enabled Siple and Passel to estimate how quickly exposed skin might chill down at wind speeds of various strengths.

During my recent skiing adventure, I accidentally repeated the “water frozen in a plastic” bottle experiment. After about three hours, I returned to my car to eat lunch and found that my 700 ml plastic water bottle was nearly frozen solid. I took off the cap, and what little water was left splashed out onto the center console of my car, where it almost instantly froze. It was cold! With the outside temperature and the wind, the car struggled to heat up. Even after 35 minutes, the car temperature gauge barely rose above the cold mark!

What’s happening with the cold temperatures and the wind effect?

Whether it’s your body or a product you want to cool, they both contain heat. In this case, we are looking at the conditions where the inside is warmer than the environment. There is a thin boundary layer of warmer air just outside of you or the product. If air is moved across the surface, that boundary layer is disturbed, with this warmer boundary layer moving away from you or a product in the direction of the air movement. The heat inside is continually transferred to the boundary layer as long as there is some temperature difference between the outside environment and the inside. This boundary layer heat transfer process is convective cooling. In addition, moisture is also drawn to the surface of and possibly a product that adds to the cooling effect through evaporation.

One important fact to remember is that the final temperature of an object (you or a product) will be close to the temperature of the environment, not the wind chill factor.

The speed of the wind over the surface will affect the rate at which interior heat in you or a product is removed. When Paul Siple and Charles Passel did their Antarctic research they measured the heat loss from the water container in watts per square meter. Although informative for the research data this is not a heat loss rate that most people can easily relate too.

While wind chill is a commonly reported measurement in weather forecasts in the winter, it is somewhat problematic. One of the primary issues with measuring wind chill is that there is no global standard for determining the value. Another concern is that wind chill calculations do not account for factors such as relative humidity and solar radiation (solar heating on a bright day can somewhat counter the wind chill effect). Despite these issues, wind chill is still widely used and included in weather forecasts because, historically, the purpose of a wind chill index was to provide an indicator of the potential of getting frostbite.

Canadian meteorologists and not many other people used the wind chill formula from Siple and Passel until the 1970s, when they and their fellow meteorologists began converting it to the familiar temperature equivalents that allow forecasters to say, “It’s 20 degrees this morning, but it feels like 4 out there.”

With further research the wind chill formula was modified in 2001 after data was collected from volunteers who had temperature sensors placed on their faces and inside their mouths to measure heat loss. The volunteers were exposed to various temperatures (as low as 14°F) and wind speeds (as high as 18 mph). The tests resulted in the wind chill chart that is presently used. See below.

The values in this scale tended to run higher (less extreme) for a given temperature and wind speed than the previous version of the scale. One reason is the height at which the wind is measured. The old formula used standard temperature measurements, taken at a height of around 6 feet (2 meters), together with wind measurements that are typically taken at 33 feet (10 meters). However, friction near the ground tends to reduce the wind speed to 6 feet (2 meters), which also happens to be close to the height of a typical adult’s face.

The revised formula:

• It is based on a human face model,
• Incorporates modern heat transfer theory,
• Lowers the calm wind threshold to ≤ 3 mph,
• Uses a consistent standard of skin tissue resistance and
• Assumes the worst-case scenario for solar radiation (clear night sky)

Notice on the wind chill chart that across the top temperature is “°F”, and down along the left side wind speed is in “mph”. However, the numbers within the chart are actually expressed in temperature “like units”, which nearly everyone is more familiar with. The wind chill index is not actually a real temperature but, rather, represents the feeling of cold on your skin.

The wind chill index chart could “generally” be applied to the resultant refrigerating effect on products being cooled. If using convective cooling, the temperature of the cold air exiting an evaporator and the airspeed going over the product gives a wind chill factor that results in a faster rate of heat removal than just being exposed to the environmental temperature in “still” air. Think, for example, of a blast freezer, tunnel freezer, or spiral freezer. Very low air temperature exits the evaporator, and there is high air velocity in a contained space where the air is directed to pass across all of the product in the space. Product heat is removed fairly quickly, which helps maintain product quality and increase the quantity of products being processed.

As more research is done, with improved measurement techniques and bettercomputerized calculations, the wind chill index will probably again be revised to give more “close to reality” effects. For the wind chill effect on humans, revision may take some time due to the variations in people. Large, small, muscular, or not-somuscular all have an effect on how wind chill impacts a person. The bottom line is that this winter, dress appropriately for the conditions, and if it seems too cold, stay where it’s warm. This isn’t rocket science; it’s just cold, hard fact . . . literally.