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.