Dog days make dressing a challenge
The theme of this post and ageless style party is Dog Days of Summer, or in other words the styling challenge of putting together a comfortable, stylish and professional outfit when the heat and humidity are unbearable. For my international readers, dog days are hottest days of summer with temperatures above 100F (37.8oC) and high relative humidity.
There exist several explanations for the name. One relates to ancient belief that Sirius, the Dog Star, causes hot weather when it is close to the Sun. Another explanation relates the name to the dogs’ going crazy, barking and howling in hot, humid weather.
Interior Alaska lacks dog days in the sense of the Dog Days in the Lower 48s
Despite Interior Alaska has a larger dog than human population (Alaskans don’t have a dog, they have dogs.), the maximum temperature since onset of recording is 100F measured in 1915 in Ft. Yukon. Typically, the highest temperatures are in the 90s (32.2o). However, relative humidity remains relative low compared to the humid days during frontal passages. These summer days, however, have temperatures in the 50 to 60s (10 to 15oC).
Dog Days are sultry
The combination of high temperature and humidity is called sultriness. On these days, the temperature feels to be much higher than the actual temperature. This higher apparent temperature is due to the combination of actual (dry-bulb) temperature and the high water vapor pressure. Thus, sultriness can increase by increases in ambient temperature, humidity, and solar radiation, as well as changes in wind speed or atmospheric pressure.
The dressing challenge on Dog Days
A typical adult of 5 ft 6 (1.7 m) height and 147 lb (67 kg) has a surface area of 19.2 square feet (1.78 m2) and density of 980 kg∙m-3. An outfit of long pants and a short sleeve shirt/blouse covers about 84% of the body area. Under these considerations the clothing thickness (includes underwear plus air layers) amounts about 0.2 inches (5 mm). According medical literature, the core vapor pressure of a human body is that of a saline solution with an effective relative humidity of about 90%. At 98.6F (37oC), which is the human blood temperature, the water vapor pressure at saturation amounts 0.628 hPa (mb). Given the 90% relative humidity of the human body, our vapor pressure is about 0.56 hPa (mb). When walking outdoors at 1.4 m/s, the skin surface puts out about 180 J s-1m-2 of heat by transpiration. We also loose latent heat (water vapor) from the lungs when breezing.
Moisture transfer thru clothes
Diffusion thru the fabric pores makes up almost all moisture transfer from the skin to the ambient air or vice versa depending on the direction of the moisture gradient. Keep in mind that in fabrics, pores are not straight paths like the pores of a mosquito screen. According to the literature, their path lengths is about twice the thickness of the fabric for summer clothes. This means that the fabric has a permeability of 2. The average distance between the skin and summer clothes has a path factor of about 1.5. Body movement increases the clothes’ vapor conduction.
Why sweat cools or not
Not all heat consumed by evaporation of sweat contributes to cooling of the body. Some of the cooling effect is lost to the environment. Thus, the efficiency of sweating amounts only 59% to 71%. Dripping sweat may further reduce the efficiency of the cooling process as the dripped sweat doesn’t evaporate on the skin.
We can increase sweating for increased cooling only so much
Activity, for instance, can increase sweating. The impact of an increase in humidity is less than that of an adiabatic change in humidity. Thus, evaporative cooling always reduces the apparent temperature. However, an active person can only achieve comfort this way at apparent temperatures below 77.5F (25.3oC). At this threshold, a change in water vapor by 10 hPa has the same effect as a temperature change of 3.4F (1.9oC). In cities, adiabatic cooling often serves to modify the local urban climate.
Sweat can’t evaporate on Dog Days
At high relative humidity under hot conditions, sweat accumulates on the skin, i.e., it fails to cool the body by evaporation. Instead, the sweat forms an insulating water layer that hinders the heat transfer from the body to the ambient air. 🙁
Note to my Alaskan readers: At high relative humidity under cold conditions, sweat may re-condense in the outer layers of the clothes which leads to latent heat release in the clothes. This process is a problem when working out outside in winter.
Dog Days of Summer mean heat stress
Heat stress varies among regions depending on whether extreme moisture or extreme temperature dominate.
The theoretical limit at which humans would die due to heat stress after 6 h of exposure is a wet-bulb temperature of 95F (35oC). Here wet-bulb temperature means that relative humidity is 100% and an the air temperature is 95F at the same time. Fortunately, such conditions rarely occur for such long time.
What are your summer favorites for the Dog Days? What do you wear to work on Dog Days? What is your Dog Days weekend style? Let me know by commenting here I am curious.
Ageless Style linkup
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Photos of me: G. Kramm
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