Heat_Index calculation in my C program; question about formula

**Richard Heathfield** · Oct 25 '08, 05:45 AM

Re: Heat_Index calculation in my C program; question about formula

Barry L. Bond said:

>
Greetings!
>

<snip>

>
Here is the heat index formula:
>
<<>>
-42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 -
5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2 <<>>
>
And, here is the line in my C program:
>
<<>>
Heat_Index = (-42.379) + (2.04901523 * Temperature) +
(10.14333127 * Humidity) -
(0.22475541 * Temperature * Humidity) -
(6.83783 * powf (10.0, -3) * powf (Temperature, 2)) -
(5.481717 * powf (10.0, -2) * powf (Humidity, 2)) +
(1.22874 * powf (10.0, -3) * powf (Temperature, 2) *
Humidity) + (8.5282 * powf (10.0, -4) * Temperature *
powf (Humidity, 2)) - (1.99 * powf (10.0, -6) *
powf (Temperature, 2) * powf (Humidity, 2));

That looks to me like a reasonable translation of your given equation.

Although my system doesn't have a powf function, I presume it's a float
equivalent of pow. So I wrote my own float equivalent of pow, and compared
float calcs against double calcs, for temperatures in the range 0 to 30
(varying by 0.1) and humidities in the range 0-100 (varying by 1.0). Most
of the time, there was remarkably little difference between the two -
quite a few zeros after the point - but occasionally there were what I
would term minor discrepancies - e.g. T=5.2 H=83 gives a difference of
0.0000305175781 25 between a float calc and a double calc of the above
equation.

Temperature and Humidity are correct, as I've viewed it in gdb.

You might want to give some example data, and tell us what results the
weather station gets for those data and what results your program gets for
the same data.

It's
probably not surprising that I'm NOT getting what the weather station is
getting for the heat index. :-)

Is the equation generally accepted? Is your version an approximation? Is
theirs? Are the constants considered canonical, or are they merely
suggestions?

>
I'm not mathematically educated, so I'll bet there is something that
I'm not properly capturing from the above formula in my C code.
>
Especially where it says things like "1.99x10-6T2R2".
>
I am thinking the "times Temperature squared and times humidity
squared" is probably okay, but that "x10-6" part, I'm not sure that's
right. Since there is no variable there, why wouldn't they just modify
the number before the "x"?

No, that's fine - it's just a way of expressing numbers in a consistent
format. The idea is that any (real) number that can be expressed in a
finite number of decimals can also be expressed in this way:

optional sign
value in the range [1.0, 10.0)... the notation here means that 10.0 itself
isn't part of the range
multiplication sign
optional sign
exponent

This is so well-established that the C language has a notation for it.
Instead of writing (6.83783 * powf (10.0, -3)), for example, you can just
write (6.83783E-3), where - in this context - E stands for "times ten to
the power of".

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Paul** · Oct 25 '08, 06:15 AM

Re: Heat_Index calculation in my C program; question about formula

Barry L. Bond wrote:

Greetings!
>
I just got a new Peet Brothers Ultimeter 2100 Weather Station. This
new one has a way to display the heat index, if you press the "dew point"
key twice.
>
Being aware of all the controversy surrounding the exact calculation
of the heat index, I would like my own software (which I programmed in the
1990's, when I got their Ultimeter 2000 weather station, but that one
didn't show heat index) to display it. Living in Florida, when it's hot,
in particular, I wouldn't mind knowing what it is, even if I'm not close
to the actual weather station mounted in my office to press the "d.p." key
twice. (My software shows all the current and daily record weather
conditions, updated twice a minute.)
>
Here is the heat index formula:
>
<<>>
-42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2
<<>>
>
And, here is the line in my C program:
>
<<>>
Heat_Index = (-42.379) + (2.04901523 * Temperature) +
(10.14333127 * Humidity) -
(0.22475541 * Temperature * Humidity) -
(6.83783 * powf (10.0, -3) * powf (Temperature, 2)) -
(5.481717 * powf (10.0, -2) * powf (Humidity, 2)) +
(1.22874 * powf (10.0, -3) * powf (Temperature, 2) *
Humidity) + (8.5282 * powf (10.0, -4) * Temperature *
powf (Humidity, 2)) - (1.99 * powf (10.0, -6) *
powf (Temperature, 2) * powf (Humidity, 2));
<<>>
>
Don't laugh! :-D
>
Temperature and Humidity are correct, as I've viewed it in gdb. It's
probably not surprising that I'm NOT getting what the weather station is
getting for the heat index. :-)
>
I'm not mathematically educated, so I'll bet there is something that
I'm not properly capturing from the above formula in my C code.
>
Especially where it says things like "1.99x10-6T2R2".
>
I am thinking the "times Temperature squared and times humidity
squared" is probably okay, but that "x10-6" part, I'm not sure that's
right. Since there is no variable there, why wouldn't they just modify
the number before the "x"?
>
Anyway, if anyone doesn't mind talking with extreme patience to
someone who truly doesn't know whether what I have is right (but I know
it's not getting the correct amount), what should I do?
>
You don't have to rewrite the whole statement. If you could just tell
me how I handle things like the "1.99x10-6T2R2", I expect I would be able
to do the other "x" portions appropriate! :-)
>
Thank you!
>
Barry

Page 41 has the equation. Checked second doc for units for TEMP and HUMIDITY.

GIS Software for Mapping and Spatial Analytics | Esri

http://gis.esri.com/library/userconf/health07/docs/use_of_gis-based.pdf

Esri’s GIS software is the most powerful mapping & spatial analytics technology available. Learn about Esri’s geospatial mapping software for business and government.

http://www.srh.noaa.gov/ffc/html/studies/ta_htindx.PDF

Assuming no transcription errors, I read the equation this way.

-42.40 +
(2.04901523 * TEMP) +
(10.14333127 * HUMIDITY) -
(0.22475541 * TEMP * HUMIDITY) -
(0.00683783 * TEMP * TEMP) -
(0.05481717 * HUMIDITY * HUMIDITY) +
(0.00122874 * TEMP * TEMP * HUMIDITY) +
(0.00085282 * TEMP * HUMIDITY * HUMIDITY) -
(0.00000199 * TEMP * TEMP * HUMIDITY * HUMIDITY)

TEMP = ambient dry bulb temperature in degrees Farenheit
HUMIDITY = relative humidity as integer percentage 0..100

If it still doesn't work for you, include a sample (TEMP, HUMIDITY, HI)
result so there is something to debug with.

HTH,
Paul

**Barry L. Bond** · Oct 25 '08, 06:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi Richard!

Wow! Thank you for your fast reply!

>Although my system doesn't have a powf function, I presume it's a float
>equivalent of pow. So I wrote my own float equivalent of pow, and compared
>float calcs against double calcs, for temperatures in the range 0 to 30
>(varying by 0.1) and humidities in the range 0-100 (varying by 1.0). Most
>of the time, there was remarkably little difference between the two -
>quite a few zeros after the point - but occasionally there were what I
>would term minor discrepancies - e.g. T=5.2 H=83 gives a difference of
>0.000030517578 125 between a float calc and a double calc of the above
>equation.

Yes. I found a man page for powf, that's what it is.

>You might want to give some example data, and tell us what results the
>weather station gets for those data and what results your program gets for
>the same data.

Ah! Okay.

Running it just now, with 74.5999985 F as the temp and 88.5% as the
humidity, the Heat_Index is 73.6352844. The Weather Station currently shows
79 F.

(Actually, I just noticed I'm making a one decimal point placement
error in the displaying of it, so it actually isn't as far off as I
thought earlier! :-) But, it is still different from what the weather
station reports for Heat Index.)

>Is the equation generally accepted? Is your version an approximation? Is
>theirs? Are the constants considered canonical, or are they merely
>suggestions?

Yes. In fact, I was given three URLs from Peet Brothers, and one of
the URLs is the exact same page as the second link in Paul's reply to my
article. I printed that one side/both sides of paper, and I'm looking at
exactly that same text.

They said they used exactly the same calculation to calculate the
heat index in their Ultimeter 2100 as that very URL.

Merely suggestions, I'd say. But, I was rather wanting to see it,
again, especially when it was 95+ and high humidity, as we have a lot in
our summer months in Florida.

>No, that's fine - it's just a way of expressing numbers in a consistent
>format. The idea is that any (real) number that can be expressed in a
>finite number of decimals can also be expressed in this way:

>optional sign
>value in the range [1.0, 10.0)... the notation here means that 10.0 itself
>isn't part of the range
>multiplicati on sign
>optional sign
>exponent

>This is so well-established that the C language has a notation for it.
>Instead of writing (6.83783 * powf (10.0, -3)), for example, you can just
>write (6.83783E-3), where - in this context - E stands for "times ten to
>the power of".

Ah! That makes sense! Thank you for your kind and patient help! :-)

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

**Barry L. Bond** · Oct 25 '08, 06:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi Paul!

And, thank you for your fast reply, as well!

>Page 41 has the equation. Checked second doc for units for TEMP and HUMIDITY.

>http://gis.esri.com/library/userconf..._gis-based.pdf
>http://www.srh.noaa.gov/ffc/html/studies/ta_htindx.PDF

The second URL is exactly the same PDF file that Peet Brothers
emailed me, and they said they used the exact same calculation to get
their heat index.

>Assuming no transcription errors, I read the equation this way.

>-42.40 +
>(2.04901523 * TEMP) +
>(10.14333127 * HUMIDITY) -
>(0.22475541 * TEMP * HUMIDITY) -
>(0.00683783 * TEMP * TEMP) -
>(0.05481717 * HUMIDITY * HUMIDITY) +
>(0.00122874 * TEMP * TEMP * HUMIDITY) +
>(0.00085282 * TEMP * HUMIDITY * HUMIDITY) -
>(0.00000199 * TEMP * TEMP * HUMIDITY * HUMIDITY)

>TEMP = ambient dry bulb temperature in degrees Farenheit
>HUMIDITY = relative humidity as integer percentage 0..100

Yes, that's how I'm reading it, thanks to what Richard posted! :-)

>If it still doesn't work for you, include a sample (TEMP, HUMIDITY, HI)
>result so there is something to debug with.

Something I just gave Richard, Temperature 74.5999985 and Humidity
88.5% gives 73.6352844. The weather station gives 79. (It rounds to the
nearest degree in what it displays. But, for things that it gives to my
software, most things are one decimal point.)

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

**Gordon Burditt** · Oct 25 '08, 07:45 AM

Re: Heat_Index calculation in my C program; question about formula

I just got a new Peet Brothers Ultimeter 2100 Weather Station. This

>new one has a way to display the heat index, if you press the "dew point"
>key twice.
>
Being aware of all the controversy surrounding the exact calculation
>of the heat index, I would like my own software (which I programmed in the
>1990's, when I got their Ultimeter 2000 weather station, but that one
>didn't show heat index) to display it. Living in Florida, when it's hot,
>in particular, I wouldn't mind knowing what it is, even if I'm not close
>to the actual weather station mounted in my office to press the "d.p." key
>twice. (My software shows all the current and daily record weather
>conditions, updated twice a minute.)
>
Here is the heat index formula:
>
><<>>
>-42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 -
>5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2

There's a copy of this formula on
http://www.crh.noaa.gov/jkl/?n=heat_index_calculator which is written
in algebra (with exponents shown as superscripts) which is easier
to understand. It seems to match yours. Something like 8.5282x10-4TR2
means "8.5282 times ten to the -4 power times T times R squared".

There's also a heat index calculator on the web page which you can
use to check against your formula. I took your formula and put it
on an OpenOffice spreadsheet, and the two seem to match.

As a test data point from the calculator on the web page, 94F at
85% humidity gives 135F (I think the calculator only deals in whole
degrees. It also has a bunch of warnings built in for stupid input
values, like temperatures low enough you'd never worry about heat
stroke.

><<>>
Heat_Index = (-42.379) + (2.04901523 * Temperature) +
(10.14333127 * Humidity) -
(0.22475541 * Temperature * Humidity) -
(6.83783 * powf (10.0, -3) * powf (Temperature, 2)) -
(5.481717 * powf (10.0, -2) * powf (Humidity, 2)) +
(1.22874 * powf (10.0, -3) * powf (Temperature, 2) *
Humidity) + (8.5282 * powf (10.0, -4) * Temperature *
powf (Humidity, 2)) - (1.99 * powf (10.0, -6) *
powf (Temperature, 2) * powf (Humidity, 2));
><<>>

Some comments on your translation into C:

1. I hope Temperature and Humidity are declared float or double.
2. I would have calculated everything in doubles. Unless proved
by benchmarking, there isn't that much difference on modern
hardware.
3. ***READ THIS ONE ***
The second argument of powf() is a float, not an integer, yet you
pass it an integer. If you don't have a declaration of powf() in
scope (and didn't include <math.h>), you could get complete garbage.
If you do have a correct declaration of powf() in scope, you should
be OK.
4. At a quick glance, your translation of the formula into C matches
the one on the web page.
5. I would have manually calculated some of the powers of 10 in my
head and avoided calls to powf(). Also, Temperature*Tem perature
is probably clearer than powf(Temperatur e,2).

Don't laugh! :-D
>
Temperature and Humidity are correct, as I've viewed it in gdb. It's
>probably not surprising that I'm NOT getting what the weather station is
>getting for the heat index. :-)

Give us some test data.

One of the important things to know about a formula like this is
the units of measure. In this case, Temperature is in degrees F,
Humidity is a percentage (range 0-100), and Heat Index is in degrees F.

I'm not mathematically educated, so I'll bet there is something that
>I'm not properly capturing from the above formula in my C code.
>
Especially where it says things like "1.99x10-6T2R2".

It helps if you can read it in a form of text that supports exponents.

I am thinking the "times Temperature squared and times humidity
>squared" is probably okay, but that "x10-6" part, I'm not sure that's
>right. Since there is no variable there, why wouldn't they just modify
>the number before the "x"?

When you're copying numbers like .00000000000000 0005551212 around,
it's easy to mis-count the zeroes, so they use exponential notation.

Anyway, if anyone doesn't mind talking with extreme patience to
>someone who truly doesn't know whether what I have is right (but I know
>it's not getting the correct amount), what should I do?
>
You don't have to rewrite the whole statement. If you could just tell
>me how I handle things like the "1.99x10-6T2R2", I expect I would be able
>to do the other "x" portions appropriate! :-)

**Richard Heathfield** · Oct 25 '08, 08:35 AM

Re: Heat_Index calculation in my C program; question about formula

Barry L. Bond said:

<snip>

Running it just now, with 74.5999985 F as the temp and 88.5% as the
humidity, the Heat_Index is 73.6352844. The Weather Station currently
shows 79 F.

Using double, I get 73.63526917-. Using float, I get 73.63528442+, which
matches your result. The difference is 0.0000152587890 62.

Without knowing what the proper result should be, I don't see what other
help I can give.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**George** · Oct 25 '08, 09:15 AM

Re: Heat_Index calculation in my C program; question about formula

On Sat, 25 Oct 2008 08:31:41 +0000, Richard Heathfield wrote:

Barry L. Bond said:
>
<snip>
>

> Running it just now, with 74.5999985 F as the temp and 88.5% as the
>humidity, the Heat_Index is 73.6352844. The Weather Station currently
>shows 79 F.

>
Using double, I get 73.63526917-. Using float, I get 73.63528442+, which
matches your result. The difference is 0.0000152587890 62.
>
Without knowing what the proper result should be, I don't see what other
help I can give.

I'd be curious to see your source. I have a similar project.
--
George

The action we take and the decisions we make in this decade will have
consequences far into this century. If America shows weakness and
uncertainty, the world will drift toward tragedy. That will not happen on
my watch.
George W. Bush

**Keith Thompson** · Oct 25 '08, 04:05 PM

Re: Heat_Index calculation in my C program; question about formula

barry@barrycon. cfl.rr.com (Barry L. Bond) writes:
[...]

Here is the heat index formula:
>
<<>>
-42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2
<<>>
>
And, here is the line in my C program:
>
<<>>
Heat_Index = (-42.379) + (2.04901523 * Temperature) +
(10.14333127 * Humidity) -
(0.22475541 * Temperature * Humidity) -
(6.83783 * powf (10.0, -3) * powf (Temperature, 2)) -
(5.481717 * powf (10.0, -2) * powf (Humidity, 2)) +
(1.22874 * powf (10.0, -3) * powf (Temperature, 2) *
Humidity) + (8.5282 * powf (10.0, -4) * Temperature *
powf (Humidity, 2)) - (1.99 * powf (10.0, -6) *
powf (Temperature, 2) * powf (Humidity, 2));
<<>>

[...]

None of your calls to powf() are necessary. In 5 places, you use it
where a floating-point constant would be clearer:

6.83783 * powf (10.0, -3) should be 6.83783e-3
5.481717 * powf (10.0, -2) should be 5.481717e-2
and so forth

The other calls all use an exponent of 2, i.e., you're just computing
the square of a number. IMHO it would be clearer to write
"Humidity*Humid ity" rather than powf (Humidity, 2). And since both
Humidity*Humidi ty and Temperature*Tem perature are used several times,
I'd probably compute them separately:

const double Humidity_Square d = Humidity * Humidity;
const double Temperature_Squ ared = Temperature * Temperature;

Make sure Heat_Index, Humidity, and Temperature are all declared as
type double.

Here's my version of the formula. (Note that it matches the formula
given on the Wikipedia page.)

Heat_Index = -42.379
+ 2.04901523 * Temperature
+ 10.14333127 * Humidity
- 0.22475541 * Temperature * Humidity
- 6.83783e-3 * Temperature_Squ ared
- 5.481717e-2 * Humidity_Square d
+ 1.22874e-3 * Temperature_Squ ared * Humidity
+ 8.5282e-4 * Temperature * Humidity_Square d
- 1.99e-6 * Temperature_Squ ared * Humidity_Square d;

Note that arranged the expression so that each term is on a line by
itself. I've also dropped all the parentheses, taking advantage of
the fact that "*" binds more tightly than "+" and "-". Adding
parentheses to complicated expressions in C often adds clarity, but in
this case the precedence is sufficiently well known that parentheses
would just add clutter.

Here's another version of the same formula, which more closely follows
the mathematical form as given on the Wikipedia page; I find it
clearer than the above:

/* T is temperature in degrees Fahrenheit,
* R is relative humidity in percent
*/
double Heat_Index(doub le T, double R)
{
const double C1 = -42.379;
const double C2 = 2.04901523;
const double C3 = 10.14333127;
const double C4 = -0.22475541;
const double C5 = -6.83783e-3;
const double C6 = -5.481717e-2;
const double C7 = 1.22874e-3;
const double C8 = 8.5282e-4;
const double C9 = -1.99e-6;
return C1 + C2*T + C3*R + C4*T*R + C5*T*T + C6*R*R +
C7*T*T*R + C8*T*R*R + C9*T*T*R*R;
}

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Tim Prince** · Oct 25 '08, 05:35 PM

Re: Heat_Index calculation in my C program; question about formula

Keith Thompson wrote:

return C1 + C2*T + C3*R + C4*T*R + C5*T*T + C6*R*R +
C7*T*T*R + C8*T*R*R + C9*T*T*R*R;

more efficient would be a grouping of terms such as
C1 + T*(C2 + T*(C5 + R*(C7 + C9*R)) + R*(C4 + C8*R)) + R*(C3 + C6*R)

The lack of an exponentiation operator in C, the existence of Horner's
rule, and the explicit allowance for such regrouping in the standards
for certain other programming languages, all should point in this direction.

**Keith Thompson** · Oct 25 '08, 06:15 PM

Re: Heat_Index calculation in my C program; question about formula

Tim Prince <timothyprince@ sbcglobal.netwr ites:

Keith Thompson wrote:
>

> return C1 + C2*T + C3*R + C4*T*R + C5*T*T + C6*R*R +
> C7*T*T*R + C8*T*R*R + C9*T*T*R*R;

more efficient would be a grouping of terms such as
C1 + T*(C2 + T*(C5 + R*(C7 + C9*R)) + R*(C4 + C8*R)) + R*(C3 + C6*R)
>
The lack of an exponentiation operator in C, the existence of Horner's
rule, and the explicit allowance for such regrouping in the standards
for certain other programming languages, all should point in this
direction.

The greater complexity and obscurity of your version, the fact that
compilers are likely to perform common subexpression optimizations
(e.g., computing T*T and R*R only once) should point away from your
version and in the direction of greater clarity. Unless the
computation is time-critical, and this change improves performance
measurably.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Barry L. Bond** · Oct 26 '08, 01:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi again, Richard!

> Running it just now, with 74.5999985 F as the temp and 88.5% as the
>humidity, the Heat_Index is 73.6352844. The Weather Station currently
>shows 79 F.

>Using double, I get 73.63526917-. Using float, I get 73.63528442+, which
>matches your result. The difference is 0.0000152587890 62.

Thank you. I will be changing it, to use the E- notation,
etc. However it is good to know that you're getting the same thing, or
close enough!

>Without knowing what the proper result should be, I don't see what other
>help I can give.

Oh, I'm sorry.

Peet Brothers emailed me 3 URLs, and the second URL is the 2-page PDF
file (that perfectly matches Paul's second URL he posted). They said that
they used that exact calculation for the Ultimeter 2100. So, they (said
that they) use the exact same calculation.

I'm not sure what to think now... but at least my Heat_Index appears
to be "working", though I now know a couple of improvements!

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

**Barry L. Bond** · Oct 26 '08, 01:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi George!

>I'd be curious to see your source. I have a similar project.

Oh boy. :-) While I have many things I wouldn't mind other people
seeing the source code, this one isn't one of them! :-D

Still, though, it DOES work. (I was going to add some things and
improve it from writing it in the 1990's, but I just haven't gotten to
it.) :-O It DOES work, and I actually run it frequently, and especially
when there is any sort of "weather emergency" or situation that is like to
have some weather item different.

If you can give me an email address that would work for you, I'll
email it to you. I have no reason not to share it, since you explicitly
asked.

You can do whatever you want to "mess up" the email (so it can't be
grabbed by software scanning USENET posts) but still be usable to me, such
as the way I do my email address at the end.

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

**Barry L. Bond** · Oct 26 '08, 01:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi Gordon!

>There's also a heat index calculator on the web page which you can
>use to check against your formula. I took your formula and put it
>on an OpenOffice spreadsheet, and the two seem to match.

Ah! Great idea! Thank you! Yes, I'll do that!

Thank you for your comments! I mean it!

>1. I hope Temperature and Humidity are declared float or double.

They are declared float.

>2. I would have calculated everything in doubles. Unless proved
by benchmarking, there isn't that much difference on modern
hardware.

I actually agree and am more doing things that way, now. I am now
making the minor modification to this source code in styles and ways that
are NOT the way I do new source code now, but it was the way I did it a
decade and a half ago! :-)

>3. ***READ THIS ONE ***
The second argument of powf() is a float, not an integer, yet you
pass it an integer. If you don't have a declaration of powf() in
scope (and didn't include <math.h>), you could get complete garbage.
If you do have a correct declaration of powf() in scope, you should
be OK.

Thank you! I hadn't realized that!

I am including <math.hand I'm not getting garbage, but thank you
for bringing that to my attention.

(But, when I recode it, I'll no longer be using the powf function at
all.)

>4. At a quick glance, your translation of the formula into C matches
the one on the web page.

Thank you. I'm getting other comments that agree.

I failed to mention this in my original posting, but have mentioned
it in a few replies by now, but Peet Brothers emailed me that they were
using the exact same calculation in their calculation of heat index which
their Ultimeter 2100 does. So, when I say I'm not getting what they are
getting, that's among my reasons for wondering whether I was doing
something wrong.

>5. I would have manually calculated some of the powers of 10 in my
head and avoided calls to powf(). Also, Temperature*Tem perature
is probably clearer than powf(Temperatur e,2).

And, now that I know how (thanks to several postings already), I'll
be doing that, too! :-)

>Give us some test data.

Here's one I gave from my gdb debugging early this morning:

Running it just now, with 74.5999985 F as the temp and 88.5% as the
humidity, the Heat_Index is 73.6352844. The Weather Station currently
shows 79 F.

I've had other extremely helpful postings that indicate they're
getting pretty well what I am getting.

>When you're copying numbers like .00000000000000 0005551212 around,
>it's easy to mis-count the zeroes, so they use exponential notation.

Thank you! :-) I am seeing a couple of advantages to that notation,
now!

Thank you for your time and help!

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

**Barry L. Bond** · Oct 26 '08, 01:45 AM

Re: Heat_Index calculation in my C program; question about formula

Hi Keith!

Thank you VERY much for your time and clear help!

>None of your calls to powf() are necessary. In 5 places, you use it
>where a floating-point constant would be clearer:

6.83783 * powf (10.0, -3) should be 6.83783e-3
5.481717 * powf (10.0, -2) should be 5.481717e-2
and so forth

I agree with what you've posted, and when I make my next
modification, I'll be doing exactly what you indicate, above and what I am
not quoting in this reply! (Now that I know what they are! Other
wonderful postings as well as yours have shown me what that is!)

Thank you again!

Barry
--
Barry L. Bond | http://home.cfl.rr.com/os9barry/
Software Engineer, ITT Corporation | (My personal home web page, last
bbondATcfl.rr.c om | updated February 17, 2005)

Heat_Index calculation in my C program; question about formula

Heat_Index calculation in my C program; question about formula

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