HEAT INDEX- may not be as useful as we thought

[wdrag]

Let's discard this topic if it's been posted elsewhere.  Thanks. 

https://www.natlawreview.com/article/judge-finds-no-scientific-basis-nws-heat-index-chart-used-osha-heat-stress-cases?fbclid=IwAR3GqCKx7x0of72DYREF9X-mU7e34H2ECMQfyr71R0tmeJE_XTG9NtwkkdY

May be replaced by WBGT (Wet Bulb Globe Temperature- 82 might be a threshold). I'm not studied on this and I'll try to get more info and add to this topic and if there is any predictive data to view.

[rclab]

47 minutes ago, wdrag said:

Let's discard this topic if it's been posted elsewhere.  Thanks. 

https://www.natlawreview.com/article/judge-finds-no-scientific-basis-nws-heat-index-chart-used-osha-heat-stress-cases?fbclid=IwAR3GqCKx7x0of72DYREF9X-mU7e34H2ECMQfyr71R0tmeJE_XTG9NtwkkdY

May be replaced by WBGT (Wet Bulb Globe Temperature- 82 might be a threshold). I'm not studied on this and I'll try to get more info and add to this topic and if there is any predictive data to view.

An article was sent to me recently about the heat related death of a USPS mounted delivery carrier on July 6th, 2018. The late carriers name was Peggy Frank. The incident took place in the Los Angela’s area during a 3 digit heat wave. At that time much of the aging fleet of vehicles had no air conditioning. The main plants and associate offices are air conditioned, for the most part. Not sure at this time if vehicleS have been upgraded. I always viewed the heat and cold index as a common sense guide. Thank you for the article. I hope an acceptable guideline can be reached soon. As always ....

[wdrag]

I'd heard of that near LA and absolutely-common sense! Have no idea of NWS plans. I think the military has been using WBGT as their guide for 50+ years. Posted this article because of the Law relating to HI litigation. This may accelerate research-and any subsequent change and comments.  I've drifted away from descriptive adjectives spicing up the weather and just letting the numbers dictate local action.

[LibertyBell]

On 7/18/2020 at 9:41 AM, wdrag said:

Let's discard this topic if it's been posted elsewhere.  Thanks. 

https://www.natlawreview.com/article/judge-finds-no-scientific-basis-nws-heat-index-chart-used-osha-heat-stress-cases?fbclid=IwAR3GqCKx7x0of72DYREF9X-mU7e34H2ECMQfyr71R0tmeJE_XTG9NtwkkdY

May be replaced by WBGT (Wet Bulb Globe Temperature- 82 might be a threshold). I'm not studied on this and I'll try to get more info and add to this topic and if there is any predictive data to view.

I like the European heat index better, it accounts for sun angle, wind speed and other variables.

Question- what is the difference between wet bulb temperature and dew point?

 

[wdrag]

3 hours ago, LibertyBell said:

I like the European heat index better, it accounts for sun angle, wind speed and other variables.

Question- what is the difference between wet bulb temperature and dew point?

 

Difficult for me to easily explain in lay persons language. Temp/dew of 35/32 should yield a wet bulb of about 33, cold enough to snow. Need that wet bulb at or below about 33.5F to have a chance of snow in a non convective situation. Dewpoint is the commonly referred indicator of discomfort (or lack of) in warm season. 

 

Wet Bulb Temperature - T_wb

The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.

The wet bulb temperature is always lower than the dry bulb temperature but will be identical with 100% relative humidity (the air is at the saturation line).

Combining the dry bulb and wet bulb temperature in a psychrometric diagram or Mollier chart, gives the state of the humid air. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the Psychrometric Chart.

 

Dew Point Temperature - T_dp

The Dew Point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Above this temperature the moisture will stay in the air.

If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low.

If moisture condensates on a cold bottle from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator.

The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.

The Dew Point is given by the saturation line in the psychrometric chart.

[mayjawintastawm]

I like this explanation. It provides a reason why one often does not find coasters on a table in Denver, as the DP is more often than not < the temp of the beer. On the other hand, maybe we just drink it too fast for significant moisture to condense and ruin the table.  

I also remember breaking more than one thermometer when I was a teenage weather geek trying to use a sling psychrometer to measure the WB temp and RH.

I never found the heat index to be very useful, as the individual human variables that go into heat stress vary too much for some objective combination of heat and humidity to predict heat stress with any accuracy. If HI was truly predictive, we wouldn't have nearly as much to argue about during conversations about where to set the air conditioning.