By Brian R. Johnson, Professor of Criminal Justice, University of North Alabama, Florence, Alabama; and Greg L. Warchol, Associate Professor of Criminal Justice, Northern Michigan University, Marquette, Michigan
What Is Cold Weather?
The thought of cold weather often generates the image of an icy, wind-blown tundra laden with snowdrifts and ice. Although this vision definitely resembles some cold environments, it is not the only type. In fact, “cold” is both a subjective and relative concept. The U.S. Marines identify four categories of cold weather. “Wet cold” can be characterized as temperatures around the freezing point of water; “dry cold” is from 14 degrees down to –4 degrees; “intense cold” ranges from –5 degrees down to –24 degrees; and “extreme cold” is less than –25 degrees Fahrenheit.1 Of primary concern in this article is wet cold, which is defined as temperatures between 14 and 45 degrees Fahrenheit,2 in which range most cold-weather training will likely take place. Each different category of cold provides unique challenges for police officers and training staff.
Temperature is the usually the most common measure of cold. However, other variables are involved in the perception of an environment as cold, including air velocity or speed, radiant temperature, and the relative humidity.3
Air velocity, or wind speed, has a great effect on the temperature. In fact, air temperature and air movement are interrelated. When the speed or velocity of air moving across the human body increases, a greater amount of heat is lost. This issue will be reviewed in greater detail later in this article.
Radiant heat loss or exchange occurs between a person and the surrounding environment.4 Depending on the difference between body temperature and the temperature of the environment, the body gains or loses heat through radiation. The sun, for example, generates radiant heat. When a person is directly exposed to the sun, the body warms; however, if the same person should enter a cool, dark building, the person’s body will lose heat to its environment, causing the person to perhaps feel cold.
Humidity is basically the amount of water vapor in the air. Humidity makes the air feel warmer. Humidity has little influence in cold weather because, as a rule of thumb, the colder the air, the less moisture it can hold. When air is dry and warm, for example, there is a greater rate of heat flow from the skin in the form of perspiration evaporating into the air. In high-humidity situations, meanwhile, the body’s heat loss through evaporation is retarded, because the air contains so much moisture that it cannot readily absorb more.5
Human Response to Thermal Discomfort
Humans have no innate sense of temperature. For example, people cannot tell if it is 50 or 60 degrees outside; they can only tell that an environment has become colder or warmer. Through any of the four components of heat loss, humans may experience cold or thermal discomfort. Discomfort experienced due to cold is simply the result of heat loss as detected by the cold-receptive nerve endings in the skin.6 Several variables can affect discomfort, including the following:
- The actual temperature
- Wind conditions
- Clothing/protective gear
- The material of the cold object (i.e., a metal-framed pistol will conduct more heat than a polymer-framed weapon and therefore can feel colder).
- The skin region exposed (i.e., the hands and face)
- Speed of temperature change
- Body core and skin temperatures
What makes the concept of thermal comfort or discomfort even more difficult to assess is the simple fact that human beings have different levels of thermal comfort. In fact, research has found that thermal comfort ratings vary among groups of people and from nation to nation,7 while others argue that it is a state of mind.8 That is, some individuals may not be comfortable in a setting that is 40 degrees Fahrenheit, whereas others may be very comfortable in the same setting.
Individuals’ response to perceived cold depends not only on physical (climatic) conditions and their physiological state but also on individual factors, such as past experiences, how they perceive the environment, and how weather conditions differ from the norm.9 The response to cold can also be subjective in nature and not based on body temperature, exposure time, or any physiological changes. For example, officers who engage in a variety of outdoor activities in the colder months may be less averse to the cold compared with individuals who avoid outdoor activities in cold weather. In these settings, cold may be a pleasurable sensation, making a person feel recharged and invigorated. In short, thermal comfort can be “an abstract concept relating to contentment and well being.”10
The Components of Heat Loss
The key to heat loss is that it always flows to cooler places. In effect, the human body will transfer its heat to cooler places.11 This loss or transfer of heat occurs through conduction, convection, radiation, which allow human bodies to cool through evaporation of perspiration.
Conduction can be best described as the flow of heat from a warmer object to a colder one. These objects must be in direct contact with one another. Thermoreceptors in human skin detect or measure heat flow, not temperature. As a consequence, sensations of touch and cold are influenced by how fast objects conduct heat.12
An individual can get a good sense of the importance of conduction by holding onto a piece of steel or plastic cooled to the same temperature. The cold, dense steel will conduct the heat away from the hand at a faster rate than the plastic, making it seem colder to the touch. As a general rule of thumb, the denser the object, the greater the conductive properties it has. Water, for example, is a very effective conductor and absorber of heat. The conductivity of water is approximately 25 times greater than that of air because of its density.13
To prevent or reduce conductive heat loss from a human body, there must be a barrier between the body and the objects with which it comes into contact.14 When feet are in contact (through footwear) with the cold ground and hands are in contact with a cold firearm, these areas of the body are a particular concern. To control conductive heat loss, boots and wool socks serve as a barrier between warm feet and a cold surface, slowing the flow of heat from the feet into the cold ground. Gloves, meanwhile, will serve as a barrier between the hands and the cold firearm.
The main form of heat loss in cold weather is due to convection.15 Wind, for example, in effect is a form of forced air convection that rapidly moves or transfers heat away from the human body. Convection can also be considered a form of heat conduction, which is the transfer of heat between two objects (considering the air as an object). Understanding convection when training in the cold is important; the higher the wind speed, the greater the probability of heat loss.16 That is, as wind speed increases, heat is taken off the skin/body at a faster rate, subsequently cooling the surface of the body faster (this phenomenon is commonly known as “wind chill”). Wind, in combination with low temperatures, can readily freeze the surface of the skin because the body cannot distribute enough heat to properly maintain the warmth of the body’s surface through blood flow. The body’s core temperature, meanwhile, also lowers due to the high thermal conductivity of the blood.17 The intensity of the cooling properties of wind in combination with cold temperatures is shown in the wind chill table in figure 1, which is also available from the U.S. National Weather Service (www.nws.noaa.gov).
|Figure 1. The intensity of the cooling properties of the wind in combination with cold temperatures is shown in this wind chill chart.|
U.S. National Weather Service image
Personal comfort is affected by the radiant exchange of heat.18 The body gains or loses heat by radiation according to the difference between the body’s surface temperature and the solid surfaces that the person is exposed to. The air, for example, is a poor absorber of radiant heat and cold. A cold concrete floor, meanwhile, draws heat away from a person, making the feet and body feel colder.19 Radiation can be put to good use by wearing a coat. A coat will enable the radiant heat flowing from the body to reflect back to a certain degree, depending on the insulating factors and/or materials the coat possesses.
In cold-weather training, there is also concern over radiant heat loss. Exposed areas of the body (i.e., the hands and head) will lose heat to the cooler external environment at a faster rate than areas of the body protected by clothing that serves as a heat barrier. At the same time, radiant heat can be gained. Radiant heat gain is the result of wearing too many clothes or engaging in physical activity that warms the body’s core too much. This will cause excessive perspiration in an effort to cool the body.20 If extreme enough, it could also cause heat stress and exhaustion.
A great regulator for the loss or control of radiant heat is a hat. Due to the amount of blood that flows to the head relative to its small surface area, a great deal of the body’s heat is lost there. A hat can serve as an insulting barrier to hold and reflect radiant heat back to the body. The neck is also a heat loss region because of the large amount of surface area often exposed to the cold and because of the amount of blood that flows there. Wearing a turtleneck shirt or sweater will reduce heat loss from this area of the body.
Evaporation of Perspiration
Evaporation is the process where a liquid is converted into a gas state. When the human body gains heat from the external environment or generates heat through metabolism, the only way it can cool itself is through the perspiration process: heat is absorbed into the sweat on the surface of the skin, which then evaporates off the body, cooling it in the process. In cold weather, evaporation is not a serious issue unless individuals have overdressed themselves, making the body too hot and forcing it to cool itself through a great deal of perspiration (see the previous section).
Staying dry is the key to cold-weather comfort. In the cold, evaporation will speed the cooling of the body. As pointed out earlier, water has a high thermal conductivity and will draw heat away from the body. For example, when clothing becomes wet, it can lose heat 24 times faster than dry clothing.21 In a cold setting, excess moisture must be eliminated from the surface of the skin through clothing that has wicking properties. At the same time, moisture from the external environment needs to be prevented from coming into contact with the skin. The easiest way to prevent this from occurring is to wear an outer layer of clothing that is resistant or impervious to moisture.
There are some ways to control heat loss through evaporation. Dressing in layers will allow officers to better regulate their body temperature. Selecting clothes that wick moisture away from the skin will also reduce heat loss. Officers should also recognize that evaporation is the result of respiration: exhaling results in the loss of heat and moisture through the breath. The accompanying loss of water can lead to dehydration, but this can be readily prevented by simply making sure that officers stay properly hydrated when training in cold weather.
Responses to Cold Weather
If the four factors affecting body heat that were discussed in the previous section, combined or independent of one another, are not properly controlled, a host of physiological, behavioral, and psychological responses can result.
There are involuntary physiological responses to changes in temperature.22 The human body normally maintains its core temperature of 98.6 degrees Fahrenheit (37 degrees Celsius) through metabolism. To control the loss of heat through the skin, the brain (particularly the hypothalamus), via the autonomic nervous system, controls heat loss by regulating blood flow to the body’s surface.23 In effect, the brain is the switch for the furnace. The furnace is told to turn on when cold receptors located in the skin “notify” the brain that they are cold. In order prevent core heat loss, blood flow is decreased through vasoconstriction (the narrowing of the blood vessels) to the extremities and the skin’s outer layers to maintain the body’s core temperature.24 This is done to protect the body’s core temperature from the resulting loss of heat in the blood as it is exposed to cold.
Some parts of the body are more affected by the cold than others. Essentially, the extremities are more susceptible to thermal discomfort than the body’s core. Some of the areas that are the most susceptible to cold-induced problems include the ears, cheeks, nose, toes, feet, and palms.25 The temperature in these parts of the body can drop several degrees per minute due to vasoconstriction of the blood vessels in combination with improper protection. Compared with other parts of the body, the hands and fingers lose heat at a faster rate because of their relatively large surface area and small mass.26 Due to the effects of vasoconstriction in combination with the cold, the following changes could take place:
- Manual (finger and hand) dexterity decreases. This means that fine finger movements may “deteriorate when the temperature drops.”27
- Grip strength is reduced. Some research has concluded that exposure to the cold decreases the length of time that a submaximal muscle contraction can be maintained.28
- Motor speed decreases. This has a compounding effect on manual dexterity.
- Reaction time is increased, not because of the change in temperature but because of increased distractibility and discomfort.29
- The most common noninjurious effect of exposure to cold has been reported as the numbing of the extremities, in which case officers may lose their ability to feel pressure and contact in the fingertips.30
- Studies have found that when the body’s core temperature is maintained while the hands are cooled, manual performance decreases for the first 40 minutes and then levels off.31
Heating of the body is accomplished through increased metabolism, which is dependent upon muscular activity and digestion.32 The heat produced by metabolism equals the heat lost through evaporation, radiation, and convection and conductivity. For example, the body’s metabolic rate can be increased through shivering. But not only physical and mental exertion increase metabolic rate; in the case of exposure to cold, the body metabolizes energy simply to stay warm.33
Even though physiological changes due to cold can be significant, the effect on actual performance in the cold may be limited. One study conducted by the U.S. Army on soldiers immersed in cold water found no effect on performance, even though the soldiers experienced normal physiological changes associated with exposure to cold. The study also found that body temperatures not lower than 36.4 degrees Celsius (97.5 degrees Fahrenheit), with hand temperatures less than 19 degrees Celsius (66.2 degrees Fahrenheit), do not negatively affect rifle marksmanship.34
When individuals experience some level of thermal discomfort from the cold, they also attempt to control this thermal discomfort through a variety of behaviors. First, they can simply avoid the cold by staying inside a warm vehicle or room. If they cannot avoid the cold, they can maintain their thermal resistance or protection by wearing proper clothing to insulate and protect their body against heat loss.35 Another simple behavioral response to the cold is to increasing the level of physical exertion, which can increase metabolic heat production tenfold.36
As result of thermal discomfort, humans also engage in a variety of psychological responses. These responses often result in behavioral changes. For example, officers may become verbally aggressive, complaining about their situation. These complaints, meanwhile, could result in a series of cascading events. They could, for example, affect the actions of others who are also being exposed to the cold to the point that officers could become demoralized, not because of the physiological and behavioral changes but because of their perceptions and attitudes associated with the cold environment.
1Philip C. Rudder, “Fire and Ice: Preparation and Employment of Marine Artillery in Cold Weather,” 1990, http://www.globalsecurity.org/military/library/report/1990/RPC.htm (accessed October 2, 2007).
2Arthur Montague, “Canada’s Improved Environmental Clothing System,” Law & Order 50, no. 10 (October 2002): 59.
3Kenneth C. Parsons, Human Thermal Environments (London: Taylor & Francis, 1993).
4Taeyoung Han and Linjie Huang, “A Model for Relating a Thermal Comfort Scale to EHT Comfort Index,” SAE Technical Paper Series document number 2004-01-0919, http://delphi.com/pdf/techpapers/2004-01-0919.pdf (accessed October 10, 2007), 3.
6H. Hensel, “Thermoreception and Human Comfort,” in Indoor Climate, ed. P. O. Fanger and O. Valbjørn (Copenhagen: Danish Building Research Institute, 1979), 425–440.
7Michael A. Humphreys, “Field Studies of Thermal Comfort Compared and Applied,” Building Services Engineer 44 (1976), quoted in H. O. Nilsson and I. Holmér, “Comfort Climate Evaluation with Thermal Manikin Methods and Computer Simulation Models,” Indoor Air 13, no. 1 (March 2003): 28–37.
8Han and Huang, “Model for Relating a Thermal Comfort Scale,” 2.
9William C. Kaufman, W. G. Laatsch, and Charles R. Rhyner, “A Different Approach to Wind Chill,” Aviation, Space, and Environmental Medicine 58 (1987): 1189.
10John L. Stoops, Indoor Thermal Comfort: An Evolutionary Biology Perspective (Berkeley, Calif.: Lawrence Berkeley National Laboratory, 2001), 4.
11Han and Huang, “Model for Relating a Thermal Comfort Scale,” 2.
13Rick Curtis, “Outdoor Action Guide to Hypothermia and Cold Weather Injuries,” Outdoor Action Web site, http://www.princeton.edu/~oa/safety/hypocold.shtml (accessed October 2, 2007).
14Timothy P. Gavin, “Clothing and Thermoregulation during Exercise,” Sports Medicine 33, no. 13 (2003): 944.
15Jodie M. Stocks et al., “Human Physiological Responses to Cold Exposure,” Aviation, Space, and Environmental Medicine 75, no. 5 (May 2004): 448.
16Han and Huang, “Model for Relating a Thermal Comfort Scale.”
20Hannu Rintamäki and Sirka Rissanen, “Heat Strain in Cold,” Industrial Health 44, no. 3 (July 2006): 428.
21William Atkinson, “Working in the Cold,” Occupational Hazards 63, no. 12 (December 2001): 42.
22Jan A. Stolwijk, “Responses to the Thermal Environment,” Federation Proceedings 35, no. 5 (April 1977): 1656.
23Nilsson and Holmér, “Comfort Climate Evaluation,” 32–33.
24U.S. Department of Labor, Occupational Safety and Health Administration, “Protecting Workers in Cold Environments,” OSHA Fact Sheet No. 98-55, http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=FACT_SHEETS&p_id=186 (accessed October 3, 2007).
25Juhani Hassi et al., “Impacts of Cold Climate on Human Heart Balance, Performance and Health in Circumpolar Areas,” International Journal of Circumpolar Health 64, no. 5 (December 2005): 462.
26Ingvar Holmér, Per-Ola Granberg, and Goran Dahlstrom, “Cold Environments and Cold Work,” Encyclopaedia of Occupational Health and Safety, ed. Jeanne Mager Stellman, 4th ed. (Geneva: International Labor Organization, 1998), vol. 2, 42–48. Also available as a CD-ROM.
28Richard G. Hoffman, “Human Psychological Performance in Cold Environments,” in Textbook of Military Medicine, ed. Russ Zajtchuk and Ronald F. Bellamy (Washington, D.C.: Department of the Army, Office of the Surgeon General, 2001), 393.
30A. W. Mills, “Finger Numbness and Skin Temperature,” Journal of Applied Physiology 9 (1956): 447–450.
31Hoffman, “Performance in Cold Environments,” 383.
32Nilsson and Holmér, “Comfort Climate Evaluation,” 33
33Han and Huang, “Model for Relating a Thermal Comfort Scale,” 3.
34Peter Tikuisis et al., “Investigation of Rifle Marksmanship on Simulated Targets during Thermal Discomfort,” Aviation, Space, and Environmental Medicine 73, no. 12 (December 2002): 1176–1183.
35Nilsson and Holmér, “Comfort Climate Evaluation,” 34
36Rintamäki and Rissanen, “Heat Strain in Cold,” 430.