Help me understand Dew Point, I just don't get it. . .

The dew point is the temperature to which you would have to cool the air to get 100% humidity. Dew point is fairly conservative during the day. This means that the dew point does not go up and down anywhere nearly as much as the temperature during the day.

You hear people talking about the temperature is 95 and the humidity is 100%. Wrong!!!! What they meant is that at sunrise, the temp was 70 and the dew point was 70, which meant the humidity was 100% at sunrise. However, at 4pm, the temp was 95 and the dew point was 72, which meant that the humidity was waaaayyy lower than 100%.

It's a measure of actual air moisture, unlike relative humidity, which is a function of temperature. For every 18-degree Fahrenheit rise in temperature, the air can hold twice as much moisture. So 90% relative humidity at 70 degrees translates to the same amount of air moisture as 45% RH at 88 degrees.

The dew point is a much better index of human misery. It's the temperature at which water can no longer evaporate into the air and thus condenses. 70 is nasty. 75, and you might as well not even do more than jog.

It's a measure of how saturated the air is with moisture. You can use it without knowing how it's calculated but if you're interested, it's the temperature at which the humidity in the air would condense (i.e. the temperature at which the relative humidity would be 100%).

A rough scale, in degrees F:

<50 : very dry

50-60 : nice

65 : getting humid

70 : humid

75 : very humid

80+ : oh look out

Some current dew point readings:

Edmonton 43

Las Vegas: 56

Boston 70

Houston 75

St Louis 83

wx speaks the truth.

Generically speaking, dew points under 55 feel great, getting above 60 starts feeling humid, and above 70 starts getting truly brutal. It can be incredibly hot and humid with a very low RH percentage, because the atmosphere's ability to hold moisture is an exponential function of temperature (a bit warmer can mean the ability to hold way more moisture). A temperature of 95 with a 75 dew point would be brutally hot and humid, yet the RH would only be about 53%.

An experiment my classes did in science helped the students to understand and calculate the dew point. Get a metal can and fill it about half full room temperature water. As you stir the water with a thermometer, gradually add ice. the split second you see condensation (dew) on the outside of the can, read the temperature. That temperature is the dew point. You can use that temperature to calculate the relative humidity. If you know the dew point and the temperature, you can calculate the relative humidity.

When you get up for your morning run, the relative humidity is usually very high, it may even be 100% and foggy. As the temperature rises during the day, the relative humidity drops but the amount of actual humidity in the air may stay the same.

There are other little experiments like the dry bulb/wet bulb thermometers, to find dew point, relative humidity, and to help understand heat index and wind chill.

Some poster explaind it once

its like if the air was a sponge and that you can only put in a certain amount of water until it leaks. That number fluctulates as the temperture and ambient humidity change. The sponge already holds the water thats in the ambient air. In your example the sponge leaks at the point you have added 71% of the sponges capacity and the air has put in 29% or is it that with the amount of water in the sponge you could lower the tempreture to 71 and the sponge would give up all its water no maybe its that if it were 71 degrees the water in the sponge would leak out normally but the air is so wet around it the sponge holds that water to 95.4 degrees before it leaks out no ...

Err...

http://www.youtube.com/watch?v=V3FnpaWQJO0&feature=related

What about Southern Cali? Wow, I thought I came here on vacation to escape it. Not sure what the dew point was this morning when I raced but a lot warmer than I recall from last year.

I call something the Misery Index. Temp + Dew Point. I wouldn't want to run a marathon over 120. Over 130 starts to suck. Over 140 is a sweat feat. Over 150 is very tough. Over 160 is brutal, you must be in the south. Highest I ever ran in on Long Island was 158.x for an 800m race. I'm sure you Southern, midwesterners have seen mid 160s

Is that something just made up ?

Another wrote:

I call something the Misery Index. Temp + Dew Point. I wouldn't want to run a marathon over 120. Over 130 starts to suck. Over 140 is a sweat feat. Over 150 is very tough. Over 160 is brutal, you must be in the south. Highest I ever ran in on Long Island was 158.x for an 800m race. I'm sure you Southern, midwesterners have seen mid 160s

wx wrote:

The dew point is the temperature to which you would have to cool the air to get 100% humidity. Dew point is fairly conservative during the day. This means that the dew point does not go up and down anywhere nearly as much as the temperature during the day.

You hear people talking about the temperature is 95 and the humidity is 100%. Wrong!!!! What they meant is that at sunrise, the temp was 70 and the dew point was 70, which meant the humidity was 100% at sunrise. However, at 4pm, the temp was 95 and the dew point was 72, which meant that the humidity was waaaayyy lower than 100%.

You do not have to go much further than this very good answer. Included here is even the misconception people have about high temps, like 95 degree and 100% humidity. This would give a dew point of 95 degrees. I think that 95 degrees is the record dew point temperature EVER RECORDED ANYWHERE (well, not in man-made conditions like a sauna or oven). BTW, the Relative Humidity for 95F temp/72 dew pt is 47.57%.

There is also a wet-bulb temperature measure that captures part of this effect. The wet bulb temperature has a wet wicking material; when it is 100% RH, the wet bulb temperature equals the dry bulb temperature (the temperature measurement that we all 'know'). However, the lower the dew point and relative humidity, the larger the gap between the wet and dry bulb temperatures for any given dry bulb temperature.

There is a detail that I do not understand completely right at/near the dew point. Suppose that you are running and the dew point is 75 degrees. Would you actually be better off if the temperature was 76 or 77 than 75 so that sweat would evaporate and cool you? The heat index is higher for 77/75 than for 76/75 than for 75/75. The values are

80.71, 81.91, and 82.99; not only are this going up, but they are going up by more than the one-degree rise in temperature.

See also:

The Heat Index "Equation"

(or, More Than You Ever Wanted to Know About Heat Index)

Formula (only given with RH, so you cannot take the right derivative here)

THI =

16.923 + 1.85212·10-1·T + 5.37941·RH - 1.00254·10-1·T·RH + 9.41695·10-3·T2 + 7.28898·10-3·RH2 + 3.45372·10-4·T2·RH - 8.14971·10-4·T·RH2 + 1.02102·10-5·T2·RH2 - 3.8646·10-5·T3 + 2.91583·10-5·RH3 + 1.42721·10-6·T3·RH + 1.97483·10-7·T·RH3 - 2.18429·10-8·T3·RH2 + 8.43296·10-10·T2·RH3 - 4.81975·10-11·T3·RH3

The derivative from this formula indicates that yes it would be effectively hotter as measured by the Heat Index, but that is just an index that does not necessarily answer my question.

Here is a chart if it parses it correctly:

90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105

65 94 95 96 97 98 100 101 102 103 104 106 107 108 109 110 112

66 94 95 97 98 99 100 101 103 104 105 106 108 109 110 111 112

67 95 96 97 98 100 101 102 103 105 106 107 108 110 111 112 113

68 95 97 98 99 100 102 103 104 105 107 108 109 110 112 113 114

69 96 97 99 100 101 103 104 105 106 108 109 110 111 113 114 115

70 97 98 99 101 102 103 105 106 107 109 110 111 112 114 115 116

71 98 99 100 102 103 104 106 107 108 109 111 112 113 115 116 117

72 98 100 101 103 104 105 107 108 109 111 112 113 114 116 117 118

73 99 101 102 103 105 106 108 109 110 112 113 114 116 117 118 119

74 100 102 103 104 106 107 109 110 111 113 114 115 117 118 119 121

75 101 103 104 106 107 108 110 111 113 114 115 117 118 119 121 122

76 102 104 105 107 108 110 111 112 114 115 117 118 119 121 122 123

77 103 105 106 108 109 111 112 114 115 117 118 119 121 122 124 125

78 105 106 108 109 111 112 114 115 117 118 119 121 122 124 125 126

79 106 107 109 111 112 114 115 117 118 120 121 122 124 125 127 128

80 107 109 110 112 114 115 117 118 120 121 123 124 126 127 128 130

81 109 110 112 114 115 117 118 120 121 123 124 126 127 129 130 132

82 110 112 114 115 117 118 120 122 123 125 126 128 129 131 132 133

Wow - some excellent info and explainations!

I run in the afternoon year around - so i've pretty much experienced it all and from this have a few conclusions:

1) the heat index charts are a little off (in terms of true misery - they do not fully account for the high dew point)

Example - If it 100 degrees and the dew point is 65 = heat index will come out to about 105...and i agree with this, but if the temp is 100 and the dew climbs to 70 = this is way worse

here is the formula i use for the dew point:

60 = minor

65 = moderate

70 = major

75 = danger

this simple methosd really seems to capture the essence of the dew point, becuase no matter what the heat - i find these dew point guidelines will tell you how it is really going to feel.

i also devised a formula since the current formula posted previously is a little atrificial (too made up).

Excercise heat index = temp + (the number of degrees the dew point is above 60 x 2)

so:

100 degrees With 60 dew point = 100 heat index

100 degrees with 65 dew point - 110 heat index (5 degree above the 60 dew X 2)

100 degrees with 70 dew point = 120 heat index (i know that sounds high, but when you've ran in those conditions you would agree = not fun)

90 degrees with 60 dew = 90 heat index

90 degrees with 65 dew = 100 heat index

90 degrees with 70 dew = 110 heat index

on days when it is 105 but the dew is at or under 60 i can run 20 miles - literal no sweat.

but on days when it may only be 90 and the dew is over 70, i can not even come close to that...even though the heat index value will be moderate. this is why i say the heat index charts are flawed.

in fact just last week:

Day 1 - temp 97 dew 73 = had to stop at 13 miles (107 heat index)

Day 2 - temp 105 dew 58 = no problem going 20 miles (105 heat index)

so both days heat index values were very close - but these two days conditions on the human body are no where near the same. Day 1 - lot of sweat, little evaporation Day 2 - little sweat.

There is a detail that I do not understand completely right at/near the dew point. Suppose that you are running and the dew point is 75 degrees. Would you actually be better off if the temperature was 76 or 77 than 75 so that sweat would evaporate and cool you? The heat index is higher for 77/75 than for 76/75 than for 75/75. The values are

80.71, 81.91, and 82.99; not only are this going up, but they are going up by more than the one-degree rise in temperature

this is very interesting question. i do not have the answer but i think 1) the fact that the heat index charts were not made by someone sitting or running in a weather controlled lab are why the heat index values are more of a simple mathematical calculation than an actual measurement of the effect on the human body.

2) i also think the heat index charts over value the temp and undervalue the dew point - so in your senario that is why the values are going up by more than 1 degree eventhough the dew point is the same.

3) this would be a great study - to really find the effects on the human body

Has anyone done research on the effects of heat and humidity in a weather controlled lab?

If so, have they ever thought to include sunshine in the lab by way of a skylight or something? and wind?

There are some interesting links; I may put a couple here, but in the long piece below you can just type in a long string and it will find the article on the web.

[Note, an interesting tidbit that I fould is the relationship between wind at 10 meters (where readings are taken) and wind at face level: increase wind speed at face level by 1.5 to get wind at 10 meters (or take two-thirs wind at 10m to get face-level speed. Since running (e.g., Boston) covers the whole body and the wind progressively slows at the boundary level -- the ground-- a factor of 1.6 runner to 10m wind, or 0.62 (10m to runner) is a better estimate. For Boston, the inland wind readings were about 14 mph. Using the 1.6 value I get wind speeds of 8.75 that they got about 90% of the time, on average, so average effect is 7.875, or 3.56 meters per second -- fast for track readings but not real fast in its effect on times because running speeds are much slower than in the sprints.

The actual research had a large set of variables and the index formula was derived from holding a number of these constant while changing the remaining key variables (temperature and relative humidity. They then ran regression analysis to find the best fit, using a general third-order function, including interaction terms totaling third order (that is what it looks like).

SR 90-23

Technical Attachment 7/1/90

The Heat Index "Equation"

(or, More Than You Ever Wanted to Know About Heat Index)

Lans P. Rothfusz

Scientific Services Division

NWS Southern Region Headquarters, Fort Worth, TX

Now that summer has spread its oppressive ridge over most of the Southern Region, NWS phones are ringing off their hooks with questions about the Heat Index. Many questions regard the actual equation used in calculating the Heat Index. Some callers are satisfied with the response that it is extremely complicated. Some are satisfied with the nomogram (see Attachment 1). But there are a few who will

settle for nothing less than the equation itself. No true equation for the Heat Index exists. Heat Index values are derived from a collection of equations that comprise a model.

This Technical Attachment presents an equation that approximates the Heat Index and, thus, should satisfy the latter group of callers.

The Heat Index (or apparent temperature) is the result of extensive biometeorological studies. The parameters involved in its calculation are shown below (from Steadman, 1979). Each of these parameters can be described by an equation but they are given assumed magnitudes (in parentheses) in order to simplify the model.

# Vapor pressure. Ambient vapor pressure of the atmosphere. (1.6 kPa)

# Dimensions of a human. Determines the skin's surface area. (5' 7" tall, 147 pounds)

# Effective radiation area of skin. A ratio that depends upon skin surface area. (0.80)

# Significant diameter of a human. Based on the body's volume and density. (15.3 cm)

# Clothing cover . Long trousers and short-sleeved shirt is assumed. (84% coverage)

# Core temperature. Internal body temperature. (98.6°F)

# Core vapor pressure. Depends upon body's core temperature and salinity. (5.65 kPa)

# Surface temperatures and vapor pressures of skin and clothing. Affects heat transfer from the skin's surface either by radiation or convection. These values are determined by an iterative process.

# Activity. Determines metabolic output. (180 W m-2

of skin area for the model person walking outdoors at a speed of 3.1 mph)

# Effective wind speed. Vector sum of the body's movement and an average wind speed. Angle between vectors influences convection from skin surface (below). (5 kts)

# Clothing resistance to heat transfer . The magnitude of this value is based on the assumption that the clothing is 20% fiber and 80% air.

# Clothing resistance to moisture transfer . Since clothing is mostly air, pure vapor diffusion is used here.

# Radiation from the surface of the skin. Actually, a radiative heat-transfer coefficient determined from previous studies.

# Convection from the surface of the skin. A convection coefficient also determined from previous studies. Influenced by kinematic viscosity of air and angle of wind.

# Sweating rate. Assumes that sweat is uniform and not dripping from the body.

[page 2]

As an aside, these assumptions are important for the forecaster to keep in mind. For example, a common perception is that wind is not taken into account in the Heat Index. In actuality it is. It is assumed to be 5 knots. This may seem trivial but a forecaster may be able to use this information creatively when writing Public Information Statements regarding heat stress, heat stroke, etc.

# Ventilation rate. The amount of heat lost via exhaling. (2-12%, depending upon humidity)

# Skin resistance to heat transfer . A function of activity, skin temperature, among others.

# Skin resistance to moisture transfer . A function of the vapor-pressure difference across the skin (and, therefore, relative humidity). It decreases with increasing activity.

# Surface resistance to heat transfer . As radiation and convection from the skin increases, this value

decreases.

# Surface resistance to moisture transfer . Similar to heat transfer resistance but also depends upon conditions in the boundary layer just above skin's surface. These last five variables are used explicitly to derive the apparent temperature. By an iterative procedure which relies on the assumptions in the first list, the model is reduced to a relationship between dry bulb temperature (at different humidities) and the skin's resistance to heat and moisture transfer. Since these resistances are directly related to skin temperature, we now have a relationship between ambient temperature and relative humidity versus skin (or apparent) temperature. As a result of this procedure, there is a base relative humidity at which an apparent temperature (e.g., 90°F) "feels" like the same air

temperature (90°F). Increasing (decreasing) humidity and temperature result in increasing (decreasing) apparent temperature, and, yes, apparent temperature can be lower than air temperature. Steadman (1979) developed a table based on this relationship and the nomogram in Attachment 1 summarizes that table.

In order to arrive at an equation which uses more conventional independent variables, a multiple regression analysis was performed on the data from Steadman's table. The resulting equation could be considered a Heat Index equation, although it is obtained in a "round-about" way. Thus, here is an ersatz version of the Heat Index equation:

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2 R2 + 1.22874x10-3T2 R + 8.5282x10-4 TR2 - 1.99x10-6 T2 R2

where T = ambient dry bulb temperature (°F) R = relative humidity (integer percentage).

Because this equation is obtained by multiple regression analysis, the heat index value (HI) has an error of ±1.3°F. Even though temperature and relative humidity are the only two variables in the equation, all the variables on the lists above are implied.

References

Steadman, R.G., 1979: The assessment of sultriness. Part I: A temperature-humidity index based on human physiology and clothing science. J. Appl. Meteor., 18, 861-873.

I have been having typing and keyboard problems; that aside under [page 2] should read:

[Note, an interesting tidbit that I found is the relationship between wind at 10 meters (where readings are taken) and wind at face level: increase wind speed at face level by 1.5 to get wind at 10 meters (or take two-thirds wind at 10m to get face-level speed. Since running (e.g., Boston) covers the whole body and the wind progressively slows at the boundary level -- the ground-- a factor of 1.6 runner to 10m wind, or 0.62 (10m to runner) is a better estimate. For Boston, the inland wind readings were about 14 mph. Using the 1.6 value I get wind speeds of 8.75mph that they got about 90% of the time, on average, so average effect is 7.875mph, or 3.56 meters per second -- fast for track readings but not real fast in its effect on times because running speeds are much slower than in the sprints.]

Square track wrote:

Is that something just made up ?

Another wrote:

I call something the Misery Index. Temp + Dew Point. I wouldn't want to run a marathon over 120. Over 130 starts to suck. Over 140 is a sweat feat. Over 150 is very tough. Over 160 is brutal, you must be in the south. Highest I ever ran in on Long Island was 158.x for an 800m race. I'm sure you Southern, midwesterners have seen mid 160s

Yes, to kind of track why my runs during the summer suck so much. Its just a good data point, instead of looking at just temp, because a run at 70F/40DP is a hell of a lot different than 70F/70DP.

Agree, today I ran 5 miles (76'F air temps/70% dew point) in 85% humidity at 11:50-12:30 or so.

I like the idea someone presented above wrt combing Dew Point and air Temp to come up with their own misery index (whereby the sum >160 is miserable).

I think you bring up a very interesting point with regard to somehow including sunshine (vs. cloud over) in the heat index (aka ‘misery index’).

This week I slogged through a couple of two hour runs, in which I tried to keep my heart rate around 75%MaxHR. Temps were in the mid 90’s, the Dew Point was in the lower 70’s, and the heat index was ~107F. Both runs were on a bit hilly trail, with most of it under a tree canopy.

In those conditions, the difference in comfort between being in shade vs. full sun exposure is HUGE. And having at least a bit of breeze can also make a big difference in comfort.

*combining

cloud *cover