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  • Heat Index

    Can someone explain Wet Bulb Globe Temperature? According to the info we've received from Region, this is the criteria Jamboree will be using to monitor and restrict activities due to heat. What is this? Is it different from the Heat Index you see on TV weather?

    According to the info from region a WBGT above 82F results in some sort of restrictions on activities. At WBGT of 90F or above, moderate activities require 10 minutes of rest per hour, "active" activities requre 40 minutes rest per hour and strenuous activities (which includes the arena show mobilization) requires alternate plans.

    According to, the average, real (not indexed) temperature for the end of July is 88F. How's this going to work?

    Hal, how many days have you had in the past month that weren't above 90?

  • #2
    Heat index is simply how hot it feels to an individual based on the air temp and the relative humidity. The higher the humidity, the hotter it actually feels above the air temp. For example, if the air temp is 100F and the humidity is 60% (not unheard of in DC in the middle of summer), the heat index value is roughly 130F.

    I bow my head in shame. Why? 'Cause I have two degrees in meteorology and I HAVE NEVER HEARD OF THE "WET BULB GLOBE TEMPERATURE" (WBGT). Provided it's been 15 years since I finished the second degree and I have never successfully found employment in the weather field, but that's beside the point.

    Researching it online, I now see why. It appears to have been developed by the Marine Corps.

    Here's the formula:

    TW is "natural wet bulb temperature"
    TG is "globe thermometer temperature"
    TD is "Dry Bulb Temperature"

    The first and last are easy. TD is the air temp measured by a regular thermometer.

    TW is also easy if you have the proper instrument such as a sling Psychcrometer--it's a thermometer mounted on a pivot you can spin. There's a piece of muslin cloth sock over the bulb of the thermometer. You wet the sock and then spin the thermometer around. As the water evaporates, it cools the thermometer and you get a lower reading, aka the "wet-bulb temperature". A low wet-bulb temp is a good thing. The lower this number is, the better water (or human sweat) will evaporate. Unfortunately, suburbia DC in the summer won't have much of a difference between TD and TW. There are formulas (or tables if you have the proper reference book) to calculate relative humidity if you have TD and TW. Quick example: If TD is 100F and TW is 95F, your RH is 83%.

    TG, I've never heard of before this. A Globe thermometer measures solar radiation.

    Here's the wiki article on it with links to other sites: message has been edited by moxieman--I wouldn't have to edit it if I could see the whole message at once as it will appear before it posts. I always forget to check that option.)(This message has been edited by moxieman)


    • #3
      So why not just use the heat index that is simple to monitor and understandable by everyone?

      Either way, it's going to be hot. Hydration will be the key to health.


      • #4
        The Wet Bulb Globe is a manner of measuring heat index and setting standards of work/rest cycles based on levels of activity. This is something that the military has been using for many years and it works. THere is a standard that the military follows, and I would guess since they are using a military installation, they are going to use a measuring method that has worked. It is used all around the world and in multiple climates.

        It may seem complicated, but it works.

        here is a link to the chart that the military uses for heat index using relative humidity and ambient temperature.

        It also shows the different levels of danger in the heat index.


        • #5
          Regardless of whether you use the heat index or WBGT, you need to set a tone of preparedness for your contingent. Let the participants know that it will be VERY hot and muggy from the get-go, and make sure that they are ALL briefed on first aid for heat-related emergencies.


          • #6
            SctDad - The issue is that Scouting isn't the military. The NWS heat index is something that almost everyone understands and is easily explained to those who don't. Why not use it as the baseline?


            • #7
              My guess is that since jambo is on a base with it's own weatehr service, the BSA is using its resources and following base precedents.

              I just hope they don't red flag you in perfectly good football weather


              • #8
                My guess is that the WBGT takes into account the amount of sunshine that may occur that day. An overcast, hot and humid day might not feel as hot due to the lack of direct solar radiation.


                • #9
                  That's right, WBGT attempts to improve accuracy in sunny environments by adding consideration for direct solar heating. Since it can also be computed through a simple process, it is more suitable to getting location-specific information. which also makes it more useful to the military.

                  Even if you and your scouts are accustomed to the summer weather in Virginia, you need to continually watch for heat stress. Remind your group to reapply sunscreen frequently as well.


                  • #10
                    Right, got all that.

                    But my real question is when the jambo guidelines say they will "black flag" activities at 90F WBGT, how does that translate to what us Average Joes know about the temperature? Is that equivalent to a 90 heat index (which seems low).

                    What has me scratching my head is going "black flag" at 90 degrees. It's always 90 degrees here in the summer.


                    • #11
                      At the last jambo, the military tried to convince the BSA that holding the opening ceremony as scheduled was a giant health issue. The BSA, who seemed to be more interested in getting the free publicity of the president's speech with a group of Scout's behind him than in the health of the boys, chafed at that and went forward as planned. When approximately 600 scouts were evacuated from A.P. Hill and another 1500 treated for various heat related health issues, they decided that maybe they would listen this year. Not sure if it was because of the bad publicity from 2005 or concern for health issues this time around.

                      IMHO, it helps to have a "tripping point" going in so that the "heat" of the moment doesn't impair judgment. One can argue the details now - 90F, 95F, etc. but decide now and enforce during Jambo.


                      • #12
                        It's not Florida! I'm used to the Florida heat & humidity.


                        • #13
                          "Not sure if it was because of the bad publicity from 2005 or concern for health issues this time around."

                          Probably the latter, and the lawsuits that might result. National has not been concerned with bad publicity or bad messaging for quite some time.


                          • #14
                            Not arguing, Acco. I think you hit the nail on the head, in fact.

                            What I'm looking for is an idea of what this really means. Right now, outside it is 92 degrees, 36% humidity, bright sunshine. The regular, NWS head index is 91. I understand what that means and can walk outside and experience it. What would the WBGT be? How does this compare with a WBGT of 90 degrees?


                            • #15
                              Twocubdad's questions prove my point. I understand that it's held on a military installation and that these devices are already there. But the majority of Scouts and Scouters will have never heard of it, and thus won't be able to explain why it's important.