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→‎If the derivative you want is not available in your Cumulus release
 
However, to do this would involve more complicated code, because MX would need to keep track of which row was last updated, keep track of when the clocks changed, and modify its search routine to find the correct row. Life is much easier if the Cumulus user makes allowance for any clock change when making their request.
 
=If the derivative you want is not available in your Cumulus release=
 
It was mentioned [[#Which weather values are stored?|earlier]] that more recent history web tag names have been added as Cumulus developed. In many cases, Cumulus has actually started calculating the derivative and using it for charts, in earlier releases than when it becomes available for others to use via web tags.
 
From 3.12.0, the availability of the database for viewing outside Cumulus, means that anyone who knows how to read a SQLite database can write a script, or program, to access any derivative that is in the database, but not available as a web tag, and can then pass the output to their web site by an appropriate technical process.
 
Alternatively, although Humidex, 'Apparent Temperature', 'Feels Like temperature', and other derivatives Cumulus can calculate, are not available in all releases, they can be calculated in a script from recent 'outside temperature', 'wind speed', and 'relative humidity' values (using the same time selection for all).
 
There are other derivatives that can be calculated similarly from a set of simultaneous values. Note that Cumulus 1 and MX do not always use identical formula, and although MX added Feels Like it has changed the formula a few times.
 
The relevant formulae using JavaScript, you need to adjust them for other languages, for some of these are shown below:
 
== Canadian Humidity Index ==
 
If you are in USA and use Fahrenheit instead of Celsius, you will need to omit the 5/9 term, but as the index is dimensionless no other conversion is needed. This example is for 3 hours ago, change the input modification parameter to suit your need.
 
Cumulus 1:
 
H = <#RecentOutsideTemp h=3> + 5/9 * (6.1094 * Math.exp(5417.753 *(1/273.16 - 1/ (273.16 + <#RecentDewPoint h=3> )))-10);
 
Cumulus MX:
 
svp = 6.112 * Math.exp((17.62 * <#RecentOutsideTemp h=3) / (243.12 + parseFloat(<#RecentOutsideTemp h=3)));
H = (5/9 * (<#RecentHumidity h=3> /100 * svp - 10)) + <#RecentOutsideTemp h=3;
 
== Apparent Temperature and Feels Like ==
 
Note this apparent temperature formula uses Celsius for temperature and '''metres per second''' for wind speed. You will need to do the appropriate conversions from the quoted recent history tags if you use different units. The Australian Apparent temperature formula is same for Cumulus 1 and MX:
 
var actualVaporPress = <#RecentHumidity h=3>/100) * 6.105 * Math.exp(17.27 * <#RecentOutsideTemp h=3>) / (237.7 + parseFloat(<#RecentOutsideTemp h=3>))));
var appTempDegC = parseFloat(<#RecentOutsideTemp h=3) + (0.33 * actualVaporPress) - (0.7 * <#RecentWindSpeed h=3>) - 4;
 
Feels Like was implemented as a recent history web tag at version 3.6.11 (see [[#Feels_Like|Feels Like section below Current condition web tags]]) for the gradual introduction of feels like elsewhere. For earlier MX versions, and if you are using Cumulus 1, you can calculate it:
 
The formulas below use Celsius for temperature and '''km per hour''' for wind speed. Again, you will need to do the appropriate conversions from the quoted recent history tags if you use different units.
 
Calculation from recent history tags is much more complicated because there are 3 different calculations: Feels Like reports exactly same as wind chill for temperatures '''below''' 10°C or 50°F so the WC here should equal <#RecentWindChill h=3>:
<pre>if(<#RecentWindSpeed h=3> < 4.828) WC = <#RecentOutsideTemp h=3>;
else{
wind_pow = Math.pow(<#RecentWindSpeed h=3>, 0.16);
WC = (13.12 + 0.1625 * <#RecentOutsideTemp h=3>) - (11.37 * wind_pow) + (0.3965 * <#RecentOutsideTemp h=3> * wind_pow);// Brackets used to ensure "+" is interpreted as addition not concatenation
} </pre>
 
For temperatures '''above''' 20°C or 68°F Feels Like uses a different way to calculate apparent temperature that it uses at these higher temperatures (this formula only used for 3.6.10 onwards):
<pre>var actualVaporPress = <#RecentHumidity h=3>/100) * 6.112* Math.exp((17.62 * <#RecentOutsideTemp h=3>)/(243.12 + <#RecentOutsideTemp h=3>)) / 10.0; // Not same as at build 3084
/* uses kilometres per hour for wind speed */
/* What Cumulus MX will use to calculate apparent temperature for feels like is changed very slightly */
if(<#RecentWindSpeed h=3> > 72) <#RecentWindSpeed h=3> =72;
AT= (1.04 * <#RecentOutsideTemp h=3>) + (2 * actualVaporPress) - (0.1805553 * <#RecentWindSpeed h=3>) - 2.7;</pre>
 
For in-between temperatures it uses a more complicated merge of the two formulas for AT and WC as defined above:
<pre>app_temp_mult = (<#RecentOutsideTemp h=3> - 10) / 10;
wind_chill_mult = 1 - app_temp_mult;
 
FL= AT * app_temp_mult + WC * wind_chill_mult;</pre>
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