Tuesday, July 19, 2011

How Legitimate Are All These Heat Index Readings?

How Reliable Are Weather Stations?
With higher and higher heat index reports coming in from parts of the state (mostly western MN) like competing bids at a Sotheby auction, we wondered how reliable such readings might be. We asked a National Weather Service meteorologist for an opinion on the legitimacy of such readings and how often the NWS might review or approve weather stations beyond the larger ones such as the Minneapolis/St. Paul airport. Here's what he had to say:
The first-order stations with ASOS (Automated Surface Observing System) instruments (such as the MSP airport) are rigorously monitored and maintained by NWS technicians. They undergo at least routine maintenance, and when errors are detected non-routine maintenance is performed. As for all the other stations out there -- FAA, AWOS (Automated Weather Observing System), MNDOT system, etc. -- I am not aware of the maintenance they undergo, but based on my experience I believe it is less rigorous than the NWS maintained stations.

If a record high dew point is observed in the state (which may have happened I believe) there will be a much closer look done, likely by the state climate office consulting with the local NWS office and possibly some other folks at national climate centers.

Why Isn't Wind Factored Into the Heat Index Calculation?
If you're like us (which is to say weather enthusiasts with very little scientific knowledge), you might wonder why wind conditions don't factor into the heat index. At some level, it would seem that wind -- even a warm, humid wind -- would slow the effect of heat on the body. Of course a scientific explanation trumps a misinformed hunch. Thanks to Nick Benson (@Ottergoose on Twitter), we received a well-reasoned explanation:
The heat index doesn't take wind into effect because heat loss at high temperatures comes from perspiration, not convection. Only air cooler than you can actually cool you down; sweating only works if the air has capacity to absorb your perspiration. Think of air as a mop; if it's saturated, the floor will stay damp no matter how quickly you scrub it. Likewise, saturated air blowing over you will have a minimal impact on absorbing perspiration (cooling).
And so ends a Simply Science version of TMF.


  1. Good points brought up there. But how can he say sweating even in a warm breeze doesn't cool the body down? Not only did I sweat less yesterday when the temperature was 98 and heat index was in the 110s, but i could stand there and actually dry up when the wind was blowing 20 mph. That wasn't happening when the wind wasn't blowing though in cooler conditions. By his explanation, sitting outside next to a fan in this weather will not cool you down....

  2. I have to strongly disagree with Nick's comments. Even with some of these insanely high dewpoints, the highest relative humidities I've seen during the day have been around 70%, meaning that there is still plenty of evaporative cooling capacity available.

    It is a well established scientific principle that forced convection (wind) will increase mass transfer (evaporation) regardless of the temperature of the air and the temperature of the body, as long as the air is not saturated with water. Since we are not at 100% humidity (seen any fog lately?), then the wind, regardless of temperature, will increase evaporation.

    The challenge, which Nick was getting at, is that forced convection (wind) will also increase heat transfer, so if the air temperature of the wind is higher than your temperature of your skin, it will warm you up.

    I think they ignore the effects of wind as it is just too complicated to include effectively.

    I have a Ph.D. in chemical engineering, although what I mentioned here was largely covered in my undergraduate education. Heat transfer problems can be very challenging at times. Thank goodness we are ignoring radiation effects as that can quickly make the problem real ugly and complicated. Radiation is greatly overlooked except as a final explanation (google the "Pioneer Anomaly" to see what has left NASA and other rocket scientists perplexed until recently).

  3. While ASOS stations may have more maintenance or be "higher quality", remember that most of those stations are at airports. This may create a temperature bias. Airports attract commerce, buildings, traffic, pavement, etc. and may not always be representative of the larger area.

    This is why larger networks, like COOP, state climatology networks, and volunteer networks are important.

    So while any given reading from any specific observation station should always be treated suspiciously (even ASOS, I have some stories to tell about that!), other networks beyond ASOS are very important to observing and understanding the weather.

  4. Yesterday on KARE-11 at 5pm, weather actress Belinda Jensen made an off-hand remark that the dew point "can't be as high" at 100F (forecast for Wednesday) as it was at 97F (high on Tuesday). My bullshit alarm went nuts. I couldn't believe the nonsense she was spouting.

  5. John hits the nail on the head with his comment; unless the RH is 100%, your perspiration will evaporate, and a breeze will aid that effort. In other words, my mop analogy only makes sense when it's foggy.

    To clarify my original comment, if the air is warmer than your skin (95 F), the convectional component of this process will be working to warm you up, instead of cooling you down.

    Check out the table here:


    The results below 95 degrees make sense, I think, but, once the air gets warmer than your skin, things get a little strange (dryer air actually cools you less effectively than humid air, at least insofar as the numbers there go).

    The problem with that table, of course, is that the heat index is only supposed to be used for temperatures above 80 F with an RH of greater than 40%, which is a pretty small region on that table. Within that region, even with a blustery wind of 33 MPH, it's impact on perceived temperature is pretty minimal.

    In other words, the wind isn't taken into account because it doesn't make a whole lot of difference unless it's really, really windy, and even then its impact can be positive or negative.

  6. Nick,

    Thanks for the link. It's quite interesting, but raises still another question.

    Why is 5.6 mph wind speed built into the original tables? The difference between 0 and 5.6 mph may not seem like much, but that first little bit of wind convection really makes a big difference. (That's why you need a really calm night to get fog and frost.) The comparison then between 5.6 mph and 33.6 mph is then small potatoes (as you pointed out).

    The other interesting trend I see in the chart is that at low RH's and high ambients, the wind increases the heat to a person. That says that the rate of convective heat transfer from the wind is greater than the rate of cooling produced by the mass transfer. I didn't expect that.

    (So how long before Bill shuts the comments section down for being too longwinded and over technical?)

  7. @John This is good stuff. Keep it coming! One of the fun things about doing this blog is finding the various pockets of interest and how they relate to weather....

  8. By the way, the next topic of scientific debate ... how much further does a baseball travel (i.e., a 490-foot home run from Jim Thome) at an 80-degree dew point than a 50-degree dew point (all other things being equal, of course)?

  9. @Disco, I don't really think the Belinda Jensen criticism is legit. If we hit 100 degrees today it will likely be because the dew points aren't as high. Sure, dew points can (in theory) be as high at 100 as they are at a humid 97, but the fact is that in MN, they almost never are.

    We all know that moist air takes longer to heat up. That moist air has caused extreme heat index readings, but it has also caused our actual air temperatures to hover in the 80s-90s. If we were getting this heat sans humidity, we'd be facing a repeat of June 7th, when the air temperature was 103, but there was no heat index.

    Dew points and humidity are both slightly lower today than yesterday, which is thus far giving us lower heat index readings than we've seen the past two days. Because of that, there is more potential to heat up. If we DO hit 100 (I'm doubtful), it will probably be because the humidity and dew points are lower than they've been the past few days.

  10. @Disco, Must say I got a bit of a chuckle out of the "weather actress" label. She doesn't come across as overly weather knowledgeable (but perfect for the "chat" shows they do).

  11. @John, here's a link to the original paper that lays out how and why the heat index is calculated:


    I've just glanced through it briefly; there's plenty to review. Table 3, if I'm reading correctly, indicates that radiation and convection only account for a small fraction of heat transfer, and that assumed impact of clothing type/coverage makes a much bigger difference than wind velocity does.

  12. Heres what NWS Grand Forks has to say about the Moorhead dew point.


  13. @CWY2190 Great find. That record just doesn't smell right.

  14. @Bill They didn't say it was wrong, they stated that the dew point of 88 is likely good, but only representative of that small, specific area.

    It gets into what are you trying to sample, the synoptic/mesoscale weather or microscale climates? I would argue that, since people live, work, exist in microclimates that the data are just as valid and maybe even more representative of the conditions that impact parts of our society at the small, human scale.

  15. @P Points well taken.

    I guess the only real issue is when comparing historical records, since it seems that most official records have been taken at airports.

  16. Since everyone is busy talking about dew points you might have missed that a pretty good severe out break is heading this way on Saturday,somewhere in MN(likely southern part)will get damaging storms.

  17. Nick,

    Thanks for the link to the paper. Wow, that is a very extensive approach to the issue. I don't have the time to really go over it with a fine-toothed comb, but it seems like it might be pretty authoritative, or at least serve as a model for future research.

    I still don't like the 5.6 mph wind speed, but if I had the time, I could recalculate all the results.

    My biggest concerns are with radiation: First,
    it might be an issue that the emissivity of skin and clothing were taken as the same (I don't know for sure). Similarly, absorptivity was ignored, or more correctly, addressed in the assumptions that the entire environment is isothermal. This clearly then implies that the person is not exposed to the sun and is in the shade.

    None of these assumptions are poor by any means, but they should be more clearly understood and communicated. If this report is indeed the basis for the heat index, then we need to tell all that it will be worse in the sun. I don't know that I've ever heard any weather forecaster or public health official state that.

  18. Here's an article about the origins of the heat index from The Washington Post: http://www.washingtonpost.com/blogs/capital-weather-gang/post/the-origins-of-the-heat-index-and-why-its-important/2011/07/21/gIQASKrnRI_blog.html

  19. @Bill,

    Nice article. It does appear that Steadman's work is the basis for the heat index.

    I also see that the NWS states that sunshine will increase the heat index. Good, but that should be more widely broadcast. I've never seen it before (or is it just me?).

  20. @John I guess I'd think the fact that sunshine would increase the heat index would be common sense and not have to be stated or broadcast. Then again the way reminders to hydrate have been all over the media, maybe nothing is common sense.

  21. Re: sunshine increasing heat index - I for one am pretty used to weathermen saying stay in the shade and drinks lots of fluids. I mean it's pretty common knowledge that shade is cooler than the sun. Even plants understand that concept lol

  22. If it makes you feel any better, one of if not the best TV meteorologists in the country, James Spann has stated many times on his weather podcast how much he considers the "heat tips" insulting to the viewer but its usually a producer's call.

  23. Speaking of heat index, it looks like it might turn oppresive again. I am not looking forward to it.

  24. ok Saturday has potential to get nasty around here, if any one wants i will post my threat area, the way I see it.

  25. Dave in WoodburyJuly 22, 2011 at 5:43 AM

    Randy - please post your threat area - I'd be interested in seeing it.

  26. @Bill and others,

    My point wasn't that the sultriness is worse in the sun, but that the heat index was calculated for the shade. If you asked the average person on the street if the heat index is for being in the sun or in the shade, do you think they would know? Did you know, I mean really know (you read the research), until this discussion came up?

  27. One last comment: I am now able to quantitatively justify my earlier statement about the wind speed assumption. It's actually worse than that for the Steadman research, as it also shows how bad it was to assume a constant wind speed.

    I posted the thoughts over at my blog.

  28. randyinchamplin said...
    ok Saturday has potential to get nasty around here, if any one wants i will post my threat area, the way I see it.

    OK...the stars are starting to align with what Randy is seeing, and the SPC is agreeing.

    Are there any more thoughts out there?!

  29. Just posted a new blog entry for possible severe Saturday storms.

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