Heat Index

[packsaddle]

One point of clarification, small point...I actually have a globe thermometer. It is used to measure the resultant temperature due to radiant energy from ALL sources: sun, sky, ground, trees, buildings, water, etc. That's why it is spherical, it presents the same absorbing surface in all directions (sometimes they are referred to as a 4 pi collector) and a thermometer is mounted at the precise center of the interior of the sphere. They are usually painted black with a special paint that absorbs radiant energy very well. I saw one that had to be coated with carbon black. The idea is to make it close to a black body absorber. The resultant temperature integrates the net result of all radiant exchanges, even the losses from the sphere to the environment.

It is an easier measurement than trying to measure or calculate all of those radiant sources and then integrate them mathematically. But for some wavelength ranges (i.e. visible or PAR) a similar approach can use a light detector in an integrating sphere. I'm guessing this is more than anyone wanted so I'll stop here.

[rdclements]

The common threshold for the military is Black Flag at 90 WBGT. They use the flag system to standardize and simplify:

- Green Flag - WBGT Index 80-84.9 degrees. Discretion for heavy exercise; marginal heat stree limit for all personnel.

- Yellow Flag - WGBT Index 85-87.9 degrees. Strenuous activities curtailed for unacclimatized personnel; avoid outdoor classes in the sun.

- Red Flag - WGBT Index 88-89.9 degrees. Strenuous activities curtailed; acclimatized limited to 6 hours.

- Black Flag - WGBT Index 90+ degrees. Non-essential outdoor activities suspended.

 

With the standard of Black Flag at WGBT 90 degrees; we're talking about 95 F at around 60% humidity, yielding a Steadman Heat Index of around 114F. This is fairly typical for Virginia in the summer.

Whatever scale you are using, the point of course is to teach and practice safety. Many of your scouts are athletes and the same cautions apply to their sports activities.

The science behind all of these numbers is fun to play with. Download the instructions for calculating WGBT and have your guys take some readings for themselves while hanging out in the shade.

[Twocubdad]

Thanks! That's the point of reference I'm looking for.

[Eagle92]

I remember being red flagged on a JROTC exercise at NAB Little Creek. So instead of doing the Confidence Course, we played football instead. Corpsman was freaking out.

 

As others have said, the keep is to keep hydrated. 4 gallons of water per day per person is not uncommon.

[Gags]

Sorry - couldn't resist, but:

"That's why it is spherical...and a thermometer is mounted at the precise center of the interior of the sphere. They are usually painted black with a special paint that absorbs radiant energy very well. I saw one that had to be coated with carbon black."

 

So if the thermometer is in the middle of a sphere painted black - how do you read the temperature? Or is this one of those philosophy / quantum physics tricks - theoretically it is both hot AND cold out? (IIRC, the actual "experiment" was that the cat was both alive AND dead inside a closed box). Is this another one of those "global warming" propaganda tools?

 

Currently enjoying the 106 F Heat Index right now in sunny Northern VA, and that's only with a 35% humidity. (Glad I'm not still home - 15 miles away it's a Heat Index of 111.) Numerous sightings of large marble and bronze statues cooling off in the Tidal Basin have been reported by tourists, but National Park Service personnel insist it's hallucinations brought on by sun exposure and their failure to stand on the right when using Metro's escalators.

[Engineer61]

Globe Temp is a considerable factor in places like the desert SW where your temp is 110 but your RH is 10%...solar heating is a big factor.

[packsaddle]

Gags, I suspect you know this already but....the bulb of the thermometer is in the center. The stem extends out from the 'immersion' point. Actually mine has a thermistor in the center, connected to a electronic readout. It's pretty old, I think it originally had a thermocouple. You can make a pretty good approximation of one if you seal a stick thermometer with a 76 mm immersion length into an old copper or brass toilet float. You still need to 'smoke' it with carbon black. I just love antique instruments. Makes one appreciate the new ones as well as the ingenuity it took to do the work way back when.

[SctDad]

Shortridge. The issue is that the military has THOUSANDS of these Wet Bulb measuring devices and humdreds of people trained to implement, read and interpret the information. If you have the resources use them.

 

I guess you are the kind of person that if a National Guard unit wanted to give you 100 free sleeping bags you would turn them down because they are OD green.

 

It is not a matter of scouts being military, it is a matter of resources, and I think that the military has had enough experience with different climates do be able to determine what activity levels should be done at certain levels.

 

So quit jumping over me like I just tried to recruit the entire staff and attendants of the Jamboree into the Military.

[moxieman]

I'm glad the original poster has received the answer they're looking for.

 

Tokala wrote: "It's not Florida! I'm used to the Florida heat & humidity."

 

I spent two summers in DC. I almost grew gills that first summer when the air temp topped 100F and the dewpoint was near 90F a few days. Set some sort of record if I recall.

 

I also spent 2.5 years (3 summers in there) in West Texas "dry heat". I got news for you, 115F dry heat is no more bearable than that humid soup of DC (never mind it didn't drop below mid-90F day or night for nearly three months straight in Lubbock).

 

If Florida is hotter than either of those, then it's just another good reason for me to avoid it.

[rdclements]

Heat and humidity are the main factors, but don't forget that an active body generates its own heat. This can be significant in heat stress cases. With a lot of emphasis being placed on drinking plenty of water, you also have to remember nutrition. Hyponatremia can be a serious risk as well.

 

Previous jambos have clearly demonstrated the need for abundant caution regarding heat-related illness. The jambo is being held at a US Army base. A scout is obedient - if you go to jambo, you follow the advice and rules of the host. This is about the health and safety of your scout family, not about attitudes towards the military.

[Tokala]

Re:moxieman

"Tokala wrote: "It's not Florida! I'm used to the Florida heat & humidity."

 

I spent two summers in DC. I almost grew gills that first summer when the air temp topped 100F and the dewpoint was near 90F a few days. Set some sort of record if I recall."

 

Florida weather gets hot in May and lasts through October. Humidity is always high and the temperature is usually above 90. I'm used to sweating buckets and working in that environment. We just held a week long Order of the Arrow service project where temps were 97 and heat indices were 105-110. We had 75 Scouts and leaders working in the sun from 8:30a-4:00p every day. Planning enough water and managing the crews kept us from having any heat-related issues.

 

On a side note, I do believe that with kids spending more time indoors than kids did 20+ years ago that they aren't acclimated as much to heat especially during any strenuous activity. I spent my time outdoors (after my homework was done) when I was a kid and now they generally seem to stay indoors attached to some media device.

[shortridge]

SctDad - Methinks you doth protest too much. Nowhere did I jump over anyone.

 

My concern is not with the military connection - if i were anti-military, I'd be protesting the location at A.P. Hill. Give me a break!

 

My questions deal with the general public understanding of how this system will work. Colored flags to govern activity levels are good, but I'm still failing to understand why the standard heat index isn't good enough. It's understandable by everyone and doesn't take two forum pages to explain.

 

My worry is that someone's going to look at the WBGT readings, say "Oh, that's not as hot as it gets back home in _______," without understanding the methodology and differences, and proceed to ignore the warnings, with dangerous results.

 

Yes, we have to use our resources. But that doesn't mean we have to completely convert to a system that only a small fraction of the population understands. Does the military not have "civilian" thermometers around? Does National not have standard thermometers in a supply crate somewhere? Can the military not convert its temperatures into the standard system?

 

Either way, I'm not going to Jambo, so it's not really any of my business. I'm simply confused as to why National wouldn't use the standard U.S. temperature system.

[Twocubdad]

My hunch, and it's just a hunch, is that we will hear "code red, blue, black, yellow,...." whatever and adjust activities accordingly. The criteria for that decision is background. When the man says the 5k run is cancelled due to the heat, it's cancelled.

 

Only because I have a copy of the medical guides and took the time to read it did I get into a deeper level of detail. When I read that anything above 90 degrees WGBT is a black flag day, my ears went up. Since June 1 we only had four sub-90 degree days. I simply wanted to know if that is an apples-to-apples comparison (apparently it's not.)

 

 

[SctDad]

The other thing that I am saying about the Heat Index in relation to the WBGI (Wet Bulb) as the military kows it, it is a lot easier for someone trained in the set-up, reading and interpretation of this device, (that would be preventative medicine branch of the Army) and then they can make it simple by displaying a specific colored flag, (Green, Yellow, Red, or Black) and eeryone around can look and see, then refer to their Jambo SM or ASM and they will know without any question as to what can and cannot be done. It will also give those that are running events a place to look and easily see if their event can keep going or should they shut down. Bummer? Yes! Safer?Yes

 

A flag high enough for everyone to see or multiple flags around the jambo is a simple form of communications and does not require special technology. Just a pole and 4 flags.