Mark, you prompted me in a PM to reply to this discussion, so I have, but my indoor "chores" are taking priority over this forum.
mcrossley wrote: ↑
Wed 15 Jul 2020 4:52 pm
So some questions to start the discussion...
If MX treated Humidex as an index, it should stop using the C-->F conversion on it for users who use Fahrenheit?
(Canada used to use F, but the Humidex scale used then is not the same as the current scale converted using the normal C-->F - they just took the "Fahrenheit" version and multiplied it by 5/9)
(The "comfort" tables used to interpret Humidex all appear to use the "Celsius-fied" Humidex values)
However humidex looks like a temperature, and it appears it is commonly abused as a "feels like" temperature. So we treat it as a temperature?
Like Heat Index, Humidex is a scale index used for high temperatures, indeed the formula used breaks down below 7C. So I propose that below 10C that MX stops calculating Humidex and just returns the current temperature (in Celsius? - see Q1!)
Like Heat Index, low value records etc are fairly meaningless, we are interested in highest values only?
(For heat index MX does not produce low records)
Your views please.
1. Canadian Humidity Index (Humidex) is an index, you don't quote units. Please see viewtopic.php?f=14&t=150&p=1304&hilit=Humidex#p1304
, for original discussion on this, it even mentions the Fahrenheit question where Steve Loft actually says:
steve wrote: ↑
Sun 26 Oct 2008 5:36 pm
Yes - the idea is that it's just an index, unlike the other data, and as it was invented in Canada it uses degrees C to calculate it. It would be wrong to quote it with a degrees C unit, and even more wrong to provide a Fahrenheit equivalent.
2. The only people still using Fahrenheit are in USA, they do use Humidex although they have their own USA Heat Index which also does not have units. FWIW, if someone wants Humidex in o
F, then omit "5/9" multiplier, don't apply any 32 offset; i.e. go back to original Canadian formula. That is what my script shared at viewtopic.php?f=18&t=18096&p=142634#p142634
does, and what my aborted log editor did.
3a. As for USA Heat Index, that is only defined above 80 o
Fahrenheit (approx 27 o
C), so it hardly ever applies in UK, I actually coded it to appear above 25 o
C on my web pages written a decade ago
, as otherwise I never saw it. Just to make that clear, below my cut-off I treat Heat Index as null, i.e. not defined. I can't understand why Cumulus would choose to return air temperature for a dimensionless index; it is a computer programmer's aversion to null, that Steve Loft seemed to apply, just like Cumulus outputs valid values even when a sensor does not exist. I'm sure a meteorologist would prefer null (i.e. report not available
) when a statistic is not valid. It puzzles me, as Steve Loft reports "--" for monthly values on first day of month in some web tags.
3b. As a side issue, nobody ever requested Heat Index be added to Cumulus. Given how small a proportion of Cumulus users are in USA (and as far as I know only the USA bother to report Heat Index), it remains a mystery to me why [when Steve Loft added the WX gauges to Cumulus in 1.7.7 (21 Jun 2008)], he also added Heat Index. I suspect the only reason that Heat Index got added to Cumulus 1 was that Steve based a lot of his implementation on weatherlink, which is USA based. Anyway, apparently Steve Loft thought it useful enough that he revised the formula in 1.9.0 (11 Sept 2010) and implemented it as monthly, yearly and all-time derivatives.
4a. In my web pages, I don't display Wind Chill above 10 o
Celsius, I don't think replacing it by the air temperature is valid when Wind Chill is not defined. Similarly, if there is no wind (or wind less than 3 mph), there is no wind chill. I always thought it strange that UseWindChillCutOff=1
4b. At this point it is worth noting that Cumulus has only reported the greatest wind chill for daily and longer periods (i.e. highest wind and lowest temperature). That reflects that the least wind chill is obviously when it stops being defined at low wind speeds and high temperatures, so not reporting least wind chill is an extension of not bothering to report lowest wind speed.
5 So wind chill is for cold windy days. What about muggy days? Well my web pages report Absolute Humidity (related to dew point) rather than relative humidity because I want to record how muggy it feels, and Cumulus 1 (and MX) outputs monthly extremes for dew point, so I have a good starting point. But as described later, I also include the Canadian Humidity Index, although Cumulus, so far, makes that hard work.
6a. Because of Heat Index being so rarely quoted, and because Canada does seem to have a climate closer to UK's than the USA, I have been displaying Humidex on my web pages as it gives a better comfort index on those days when Humidity makes the air feel heavy (oppressive). Now obviously I use scripts to enable me to see humidex on my web pages. I have to add humidex to my standard log files, and calculate daily, monthly, and yearly extremes for myself (so I look forward to MX doing it for me).
6b. There was a user request for Humidex and the request came from TNT weather in USA! By the way, this is not the TNT weather channel that started in 2016, but the late Kevin Reed's user name before he died in Jan 2018. It is unclear to me why Steve Loft decided to pursue full implementation of the unrequested Heat Index (he said it was useless to him on Sanday), and not of the requested Humidex. It might interest some people to see viewtopic.php?f=14&t=195&p=1317
where a php function to describe Humidex in text is presented, a bit like Beaufort descriptions. Those with PHP web sites might want to implement that.
6c. As you say Humidex formula breaks down at low temperatures, so following my earlier points, I ignore it at low temperatures. Again, following my heat index points, I think it should be reported as null when it is out of range; a dimensionless index should not suddenly become a temperature.
7. Low extreme records. I notice that the files that were incorrectly released in 3.6.12 (build 3088) do have low humidex value and time in dayfile.txt editor, you copied your feels like temperature code. Humidex is not like feels like temperature which is defined for all temperatures. Humidex is just like wind and rain. There is a fixed lower-value cut-off, so the lowest all-time is known. Yes, sometimes there is interest in lowest humidex value. People may wonder what was the smallest rainfall rate (or smallest wind run) during a period when it rains (or is windy) every day. Put another way, the popular top ten script on your web site has Driest Months
, Lowest Max Wind Gust
and Min Daily Wind Run
. I can imagine Lowest Humidex appearing there in the future.
To sum up:
- I welcome addition of Humidex.
- Humidex and Heat index should continue to be treated as dimensionless
- Ideally, Humidex and Heat index should go to null instead of reporting air temperature when out of range.