THE TEMPERATURES YOU ACTUALLY FEEL At any given temperature, you can feel comfortable, you can feel hot, or you can feel cold. Which one of these you actually feel depends not only upon the air temperature, but also upon the humidity, whether you are in the sun or the shade, how hard the wind is blowing, when you last ate, the state of your health, what drugs you ve been taking (including alcohol and nicotine), how much clothing you are wearing, what you did recently, and what you happen to be doing at the time. In other words, the temperature that was comfortable before you ate might seem a bit warm after eating. A temperature fine for swimming might feel uncomfortably hot during a vigorous round of tennis or feel quite chilly when the wind picks up. The same temperature that was fine for golf in the sunshine, feels a bit cool for napping in that lounge chair in the shade. And so on. Moreover, not all of us react the same way to these things. In any group of people, some are going to feel hot at any given time and level of activity, some will be quite comfortable, and some will feel cold. Now, none of these observations are new to you, of course. You ve learned them all just by living. Over the years, you ve probably developed a pretty good idea of how air temperatures affect you compared to most other folks. You know whether you are the type who opens the bedroom window to the Arctic breezes, or who slams it shut and snuggles deep under a down comforter. Even so, you still wonder how the temperatures in the table in Sunshine Guides are actually going to feel when you get to that new destination and take up the activities that you went there to do. What do these table temperatures mean for what you have in mind. What you need to know is a rough idea of how much all of these various factors (activities, eating, sunshine, and wind) affect you and your response to air temperatures. That is what I shall attempt to provide in this discussion. I shall try to give you a rough idea of how much warmer various activities make you feel (compared to simple sitting and chatting), how much warmer eating makes you feel and for how long, how much warmer the sun is than the shade, how the humidity can make you feel either warmer or cooler, and how much cooler the wind is compared to the calm.
What you are going to get is a VERY rough idea. There is simply too much variation in body type, in age, in general health, in how vigorously people do certain activities, and in normal clothing habits, for me (or anybody else) to be precise. You are going to have to be your own best judge as to whether the temperatures at any particular destination at any particular time are suitable for whatever activity you have in mind. I can offer some help, but I can t do it for you. ENVIRONMENTAL FACTORS - The three environmental conditions that we will consider are relative humidity, sunshine, and wind velocity. Each of these is capable of both turning comfortable temperatures into uncomfortable ones, and uncomfortable ones into comfortable ones. RELATIVE HUMIDITY - Relative humidities in the Southwest are generally low, except in the higher mountains and during actual rainstorms. The overall effect is that under these conditions of dryness, temperatures that would be uncomfortably hot in more humid parts of the world are quite comfortable under conditions of low humidity. An air temperature of 105 at 15% relative humidity will feel just about the same as one of 90 at 65 % relative humidity. With no direct sunshine, no wind blowing, and relative humidity around 25% (common throughout the Southwest), four out of five people will describe themselves as comfortable when the temperature is between 74 and 80 and they are simply sitting around talking or reading. That means that about one in ten will feel slightly cool in that temperature range, and one in ten will feel slightly warm at those same temperatures. Raise the relative humidity, and an increasing number will begin to feel warm. Drop the relative humidity, and a larger percentage will begin to feel cool. At a very dry 5% relative humidity, you would have to raise the temperature 3 to get the same comfort level. At a very humid 80% relative humidity, you would have drop the temperature 3 to get the same comfort level. Many homes feel chilly in the winter months despite the thermostat setting because the inside air is so dry. Your home heating plant not only heats the air, it reduces its relative humidity tremendously. The table below shows how the effective temperature (the temperature you actually feel) varies with the actual air temperature and the relative humidity. In all
cases, we are assuming minimal activity, no direct or diffuse sunshine, and no wind. EFFECTIVE TEMPERATURE ( F) RH/AT 70 75 80 85 90 95 100 105 110 115 120 125 05% 64 69 74 79 84 88 93 97 102 107 111 116 10% 65 70 75 80 85 90 95 100 105 111 116 123 15% 65 71 76 81 86 91 97 102 108 115 123 131 20% 66 72 77 82 87 93 99 105 112 120 130 XX 25% 66 72 77 83 88 94 101 109 117 127 XX XX 30% 67 73 78 84 90 96 104 113 123 XX XX XX 35% 67 73 79 85 91 98 107 118 130 XX XX XX 40% 68 74 79 86 93 101 110 123 XX XX XX XX 45% 68 74 80 87 95 104 115 129 XX XX XX XX 50% 69 75 81 88 96 107 120 XX XX XX XX XX 55% 69 75 81 89 98 110 126 XX XX XX XX XX 60% 70 76 82 90 100 114 XX XX XX XX XX XX 65% 70 76 83 91 102 119 XX XX XX XX XX XX 70% 70 77 85 93 106 124 XX XX XX XX XX XX
75% 70 77 86 95 109 130 XX XX XX XX XX XX 80% 71 78 86 97 113 XX XX XX XX XX XX XX 85% 71 78 87 99 117 XX XX XX XX XX XX XX 90% 71 79 88 102 122 XX XX XX XX XX XX XX 95% 71 79 89 105 XX XX XX XX XX XX XX XX The table shows the effective temperature (the temperature you actually feel) at selected combinations of air temperature (across) and relative humidity (down). XX indicates conditions where the danger of heatstroke is imminent with even light physical activity. At air temperatures of less than 70 f, moderate to high relative humidities (over 50%) have little effect on the effective temperature. At air temperatures of less than 70 f, lower relative humidities will drop the effective temperature about 1 for each 10% decrease in humidity. SUNSHINE - As we all know from actual experience, stepping from the shade into the sunshine will make you feel warmer almost immediately. Similarly, stepping from the sunshine into the shade brings about an immediate sensation of relative coolness. How much warmer or cooler? That depends upon how intense the sunshine is. The sunshine in the Southwest is very intense indeed. Being in the sunshine increases the effective temperature by as much as 25 F at noon on a June day under clear skies. Or, it can have a negligible effect during a December sunrise. It all depends on how high the sun is in the sky, and how clear the sky is at the moment. A good general rule is that the temperature increase is greatest when your shadow is smallest on the ground. Obviously, it has no effect at all during the night, and only a minor effect under heavy cloud cover. The following table gives a very rough estimate of the increase in effective temperature under varying intensities of sunshine and solar angle.
INCREASES IN EFFECTIVE TEMPERATURE DUE TO SUNSHINE (ALL INCREASES IN DEGREES FAHRENHEIT) Low Sunshine Intensity Moderate Sunshine Intensity High Sunshine Intensity Low Sun Angle 1 3 5 Moderate Sun Angle 5 10 14 High Sun Angle 9 13 20 WIND VELOCITY Wind removes heat from the human body in two ways: Firstly, it strips away the layer of still, insulating air that our body hair or clothing has warmed to a comfortable temperature usually replacing it with much cooler air. This increases the body s heat loss via conduction. Secondly, it increases evaporation of perspiration by removing the saturated air and replacing it with air that is unsaturated. These two processes are usually combined into the term wind chill. The following table shows you the stress temperature for unclothed skin. This is the temperature of still air that would produce the same rate of cooling as the moving air at the higher temperature. STRESS TEMPERATURES - WIND Calm 5 10 15 20 25 30 35 40 35 35 30 27 24 20 17 15 13
30 30 25 21 18 15 12 10 7 25 25 20 15 11 8 4 1-2 20 20 14 9 5 1-2 -6-9 15 15 9 3-2 -6-10 -14-19 10 10 4-2 -8-13 -18-23 -28 5 5-2 -9-15 -21-26 -32-38 0 0-7 -15-22 -28-35 -42-49 -5-5 -13-21 -29-37 -44-52 XX -10-10 -19-28 -36-45 -54 XX XX -15-15 -25-35 -44-53 XX XX XX -20-20 -30-41 -52 XX XX XX XX -25-25 -36-48 -60 XX XX XX XX -30-30 -42-55 XX XX XX XX XX -35-35 -48 XX XX XX XX XX XX -40-40 -54 XX XX XX XX XX XX This table shows the stress temperature (the temperature you actually feel) at selected combinations of air temperature (down) and wind velocity (across). At stress temperatures of -20 F, frostbite will usually occur on exposed,
susceptible flesh (earlobes, forehead, cheeks, nose-tip, fingers, etc.) with exposures of one hour or more. At stress temperatures of lower than -60 F, frostbite may occur within minutes of exposure of susceptible flesh. Such temperatures are replaced by the symbol XX. THE EFFECTS OF ACTIVITY ON TEMPERATURE Let s say that you are one of those people who are sitting around chatting or reading. You are in the open shade, there is no wind to speak of, and you are dressed in the standard American vacation costume of short-sleeved shirt or blouse, cotton underwear and socks, and cotton slacks or jeans. You feel quite comfortable, and looking at your handy-dandy pocket thermometer you see that the air temperature is 74 F. The question now becomes, if you change your activity, what temperature should it be for you to feel comfortable again? The following table will give you a very rough idea of the effect of various activities on apparent temperatures. A negative number means that you must subtract that many degrees to get a comfortable temperature, a positive number means that you must add that many degrees. TEMPERATURE CHANGE FOR COMFORT Sleeping +6 F Lying awake 4 F Sitting and reading, writing, eating or talking quietly 3 F Standing around idly, looking at the sights 2 F Driving a car in a relaxed mood 1 F Horseback riding at a relaxed walk 0 F
Light chores such as building a fire, cleaning up, etcetera Second hour after eating a moderate meal Driving a car while mad at the world Hiking on a moderately rough trail First hour after eating a moderate meal Bicycling at moderate speed Backpacking on a moderate trail (subtract another degree for each 16 pounds of pack weight) Moving into direct sunshine Route march (4 ) Jogging or running to stress point Rowing away from a crocodile (about 6 ) -1 F -3 F *4 F -6 F -7 F -8 F -8 F -13 F -14 F -35 F -86 F There you have it. While neither as accurate nor as precise as articles on physiological climatology in the various professional journals, it is I hope a great deal more intelligible. Use it freely, but use it tempered with common sense. Copyright 2007 by Patrick J. Tyson Last edited in January of 2010 www.climates.com